Collectors of petrified wood focus on permineralized plant material
related to arborescent (tree-like) plant life. Fascination with
fossil wood may be related to human reverence for living trees.
Trees provide humans and other organisms with shelter and food.
We plant trees near our homes and in our communities to enrich
the environment. Trees define many biomes. Trees help moderate
Earth’s atmosphere, sequestering carbon and releasing oxygen.
Trees are one of the first plant categories a child learns. Asking
a person to identify a plant as a tree may seem like “child’s
play”; however, defining a tree can be difficult.
United States Forest Service defines a tree as a woody
plant at least 13 feet (4 m) tall with
a single trunk at least 3 inches (7.62 cm) in diameter at breast
height (4.5 ft; 150
cm) (Petrides, 1993, p. 4). This definition fits well with
many people’s concept of a tree being a large, columnar,
woody, long-lived organism. However, many trees are not constructed
from secondary growth (wood), such as palms and tree ferns.
Some species, such as black willow, are multi-trunked. Size
can also be problematic as an Engelmann spruce growing at tree
line may be small compared to one growing at a lower elevation.
Some species, such as the juniper, can grow as shrubs or trees.
The Japanese art of bonsai demonstrates how environment can
effect tree growth to extremes. We will adopt a more encompassing
definition of the tree form. A tree is a perennial plant often
having a single trunk supporting secondary branches with leaves
or a crown of leaves.
for the first fossil trees and forests occur in the Devonian.
The oldest arborescent plant, Eospermatopteris,
is related to ferns (Taylor, Taylor, and Krings, 2009,
p. 479). Fossil
forest composition changes through geologic time, reflecting
in evolutionary strategies for constructing a tree form.
It is helpful and informative to study the anatomy of
Organs & Tissues
for trunk structure can be recognized by the
of tissues and organs. A quick survey
of plant organs and tissues will enhance our discussion of
the various evolutionary strategies for constructing a tree
form. Plants are made of four types of organs: roots, stems,
leaves, and reproductive structures. In turn, these organs
are composed of three basic tissue systems: the ground tissue
system, the vascular tissue system, and the dermal tissue
Ground tissues including
parenchyma, collenchyma and sclerenchyma are involved in
photosynthesis, storage, secretion, transport,
and structure. Parenchyma tissue generates all other tissues.
Living parenchyma cells are involved in photosynthesis, storage,
secretion, regeneration and in the movement of water and
food. Parenchyma cells are typically spherical to cube shaped.
Collenchyma tissue provides structural support for young
growing organs. Living collenchyma cells are elongated cylinders
and help to make up the familiar string-like material in
celery stalks and leaf petioles. Sclerenchyma tissue provides
support for primary and secondary plant bodies. Sclerenchyma
cells often have lignified secondary walls and lack protoplasm
at maturity. Elongated slender sclerenchyma cells known as
fibers make up well known fiberous material such as hemp,
jute, and flax. Shorter sclerenchyma cells known as sclereids
up seed coats, the shells of nuts, and account for the gritty
texture of pears.
The vascular tissue
system is represented by the water conducting tissue xylem
and the food conducting tissue phloem. Xylem
tissue is made primarily of parenchyma cells, fibers, and
tracheary elements. Tracheary elements are represented by
tracheids and vessel elements. Tracheids and vessel elements
are enlongated cells that lack protoplasm at maturity and
secondary walls strengthened with lignin. Vessel elements
are lager in diameter than tracheids and are an adaptation
plants. Tracheary elements form an interconnected system
of overlapping, leaky tubes that conduct water and minerals
from the roots to the rest of the plant. Transpiration
of water from leaves pulls columns of water enclosed
these stacked, tube-like cells up the plant.
is made primarily of sieve elements, parenchyma cells and
fibers. Sieve elements are represented by sieve cells and
sieve-tube members. Sieve cells and sieve-tube members
are elongated cells that are living at maturity. Both cell
are closely associated with parenchyma cells. Sieve-tube
members possess larger pores and are an adaptation of flowering
plants. Sieve elements form an interconnected system of
tubes that transport the food products of photosynthesis
made within leaves throughout
The dermal tissue
system forms a protective outer covering including the
epidermis and periderm. The epidermis forms
the outer most layer of the primary plant body. During secondary
growth the periderm replaces the epidermis. The periderm
consists of protective dead cork tissue, the cork cambium,
and phelloderm, a living parenchyma tissue.
that make up plant organs are produced from clusters
of actively dividing
parenchyma cells called meristems. Cells produced from
meristems differentiate into the specialized cells making
up tissues. Apical meristems occur at the tips of roots
and shoots. Apical
are responsible for primary growth or increase in length.
Lateral meristems such as the vascular cambium and cork
produce secondary growth, increasing the girth of stems.
Typically, the vascular cambium is bifacial, producing
secondary xylem to the inside and secondary phloem
to the outside.
Secondary xylem makes up the wood of a stem, which
will be explored in more detail when looking at modern
conifers and angiosperms. The cork
phelloderm to the inside and phellem (cork) to the outside.
Together, the phelloderm, cork cambium, and cork make up
the periderm, a protective layer made by secondary growth
(Raven, Evert, & Curtis, 1981, pp 417-429). The term
bark refers to all tissues outside the vascular cambium.
Knowledge of how
primary vascular tissues are organized within the central
cylinder of stems and roots can aid in identification.
of a sprout axis, which includes vascular tissue and associated
ground tissue, is called a stele. Structurally, one can
think of three basic ways in which the stele of stems and
roots are organized based upon tissue found in the center
of the axis. The center of a stele can be made of solid
xylem, pith or vascular tissue embedded in
A protostele has no
pith; instead the center is a solid core of xylem. Most
primitive plants and roots are protostelic. Three types
of protostele are recognized by the shape of the xylem
core in cross-section. In a haplostele, cylinder-shaped
xylem is surrounded by a ring of phloem. Actinosteles have
projections surrounded externally by phloem. Plectosteles
possess plate-like regions of xylem surrounded by phloem.
The plates look like they are separate but in fact are
a pith surrounded by vascular tissue. Siphonosteles can have
phloem on both sides of the xylem (amphiphloic) or only
external to the xylem (ectophloic). Two variations of
this structure can be
recognized based upon how many interruptions in the vascular
ring occur in any one cross-section due to the formation
These interruptions or breaks in the vascular ring are
called leaf gaps. Solenosteles exhibit no more than
one leaf gap in a cross-section. Dictyosteles exhibit multiple
leaf gaps in cross-section. Solenosteles and dictyosteles
are found in ferns.
All seed plants have
primary vascular tissue organized into bundles either surrouding
a pith or scattered within ground tissue. Gymnosperms and
dicotyledonous angiosperms have vascular strands arranged
in a ring around the pith, called a eustele. A variation
of the eustele, called the atactostele is found in angiosperm
monocots in which the vascular bundles are scattered throughout
the ground tissue. The roots of monocots are arranged as
a eustele, while in gymnosperms and dicots, roots are protosteles.
The eustele was once thought to be derived from the siphonostele,
it is derived from the protostele (Taylor, Taylor & Krings,
2009, p. 220).
arborescent vegetation represent vascular plants
with true roots, stems, and leaves. As
we explore the anatomy of arborescent plant life
we will discover
that a variety of tissues and organs have been co-opted
elements. Several basic trunk structures can be recognized
by the arrangement of strengthening elements: solid woody
cylinders, fibrous cylinders composed of isolated, intertwined
elements and reinforced tube-like cylinders with hollow
or soft centers.
plant material is often cut and polished in the cross-sectional
or transverse plane to reveal the
anatomy perpendicular to a trunk, stem, or root axis. Familiarity
with the anatomy may even allow one to identify the taxon
to which a specimen belongs; however, for many specimens
radial and tangential sections of the stem must also be studied.
In this article, we will focus only on stems in cross-section.
You can obtain some general information about major plant
groups by visiting the Science Olympiad section of our
lycopods or clubmosses (phylum Lycopodiophyta) range
from the Silurian to recent times. Lycopods are not mosses.
Clubmosses are in fact more closely
related to ferns and conifers (Kenrick & Davis, 2004,
p. 32). Today 1,500
species of small herbaceous clubmosses represent a
small fraction of modern flora. However, clubmosses have
a rich evolutionary history extending
years, which includes tree-like forms.
extinct clubmoss trees or arborescent lycopods dominated
the canopy of Carboniferous forests and went extinct
during the Permian. Paleozoic tree clubmosses could reach
heights of 40 m and attain diameters of 2 m. The
best cross-sections of lycopsid stems and trunks
from coal ball cellulose
acetate peels. Evidence from coal ball fossils has
allowed paleobotanists to reconstruct the growth and structure
of these unusual trees.
tree stems start out as protosteles with solid xylem cylinders.
As the tree grew
a pith developed
making the upper trunk a siphonostele. In cross-section
the upper lycopod
trunk anatomy consists
of a small central pith surrounded by a xylem ring
with medullary rays. The vascular cambium produced
xylem to the inside but, unlike modern trees did
not produce secondary phloem to the outside. Although
phloem is found outside the secondary xylem lycopsid
apparently did not produce secondary phloem (Taylor,
Taylor, and Krings, 2009, pp. 286-287). The core
of water conducting woody tissue (xylem ring) was
in diameter. This relatively small xylem ring was
encased in a wide area of living parenchyma cells
tissue). The cork cambium produced less phellem
(cork tissue) to the outside than modern trees.
the bark had a hard, lignified outer layer. This
non-water conducting outer layer provided the structural
for the tree (Kenrick & Davis, 2004, p. 70).
Although not hollow, the lycopsid trunk acted as
structure. In cross-section, 98% of the massive
lycopsid trunk was
periderm, making the term "bark stem" appropriate
for these trees (Selmeier, 1996, p. 139). Not only was the structure
of these trees very different from modern trees their growth
was as well.
branches grew at the top of the tree they once again became
protosteles with very small primary vascular
Thus, without secondary tissues these branches
lost their ability to grow larger. The tree literally
grew itself out (Taylor, Taylor, & Krings, 2009,
& 289). This determinate growth contrasts with
most woody plants, which have indeterminate growth.
horsetails (phylum Sphenophyta or Equisetophyta) range
from the Devonian to recent times. Equisetum is
the only extant (living) genus (Willis & McElvain,
2002, p. 104). Extant species representing this genus are
all small herbaceous plants. Horsetails have jointed stems
with vertical ribbing. All the branches, leaves and cones
are borne on whorls (Kenrick & Davis, 2004, p. 89).
Horsetails (sphenopsids) are closely related to ferns.
Some taxonomists place the sphenopsids
in the fern division Pteridophyta. Like the lycopsids, sphenopsids
also have a rich evolutionary history that includes tree
horsetails contributed to the canopy of Paleozoic
forests. Calamites is the most well known
arboreal horsetail. The trunk of Calamites grew
in a telescoping fashion from one node to another
(Selmeier, 1996, p.
139). Like the lycopsids arboreal horsetails exhibited
determinate growth. The stems and roots of horsetail
trees are siphonostelic. In cross-section the stem
consists of pith and or
a medullary cavity
surrounded by primary
and secondary xylem (click on picture to the right).
The vascular cambium produced secondary xylem toward
secondary phloem to the outside as inferred by
preserved root structures (Taylor, Taylor & Krings,
2009, p. 354). The cork cambium produced small
of periderm on the exterior. As
the tree grew,
added to the girth of the
trunk, while the central pith area formed
hollow interior chambers. The trunk was mostly
a cylinder of secondary xylem. The hollow trunk
a kind of reinforced tube. These
sensitive to local buckling (Kenrick & Davis,
2004, p. 70). The xylem or wood of these
trees was made primarily of tracheid cells that
are very similar to modern gymnosperms (click on
horsetail trees reached heights of 30 m, attained
1 m, and were anchored to the ground with prostrate
rhizomes. The tube-like trunk of the lycopsid and
horsetail tree supported small, sparsely branched
(division Pteridophyta or Filicophyta) range from
the Devonian to recent times. Extant ferns are
represented by over 12,000 species. While many
ferns are smaller herbaceous plants, larger
exist. A type of tree related to ferns was present
in the oldest known forest.
Devonian fossil trunks from Gilboa, New York provide
into the earliest forests (click on picture to
the right). Stumps with roots stretching out into
a paleosol (fossil
have been preserved as casts at a site known as
Riverside Quarry. The casts reveal only the outer
structure of these tree stumps. From the 1870's
and until recently the stumps have been assigned
to various plant groups including Psaronius (the
tree fern), Eospermatopteris (the
accepted name) and a progymnosperm (Nudds & Selden,
2008, pp. 98 & 99). The mystery of Eospermatopteris identity was
not solved until recently. In 2007 Stein et al.
described the discovery of fossil trees from Shoharie
County, New York uniting the crown of Wattieza with
the trunk of Eospermatopteris. Wattieza is
a genus of prehistoric tree that belongs to the
class Cladoxylopsida. This class is currently placed
within the division Pteridophyta. So, fossil stumps
at Gilboa are cladoxylopsid trees related to ferns.
diversified and flourished during the Carboniferous,
occupying many habitats. Fern species grew as epiphytes,
cover, understory and canopy in these
ancient forests. We will examine several groups
of ferns that evolved tree forms. Isolated, but intertwined
roots and leaf petioles formed an important
part of the
of ferns with tree and
in the order Marattiales range from the Carboniferous
to recent times. This was the first modern group
of ferns to evolve a structure that we think
of as a real tree fern. The Carboniferous arborescent
possessed a kind
or braced trunk.
the largest arborescent fern found in the coal measure
swamps (Willis & McElvain, 2002, p. 108). Psaronius was
up to 10 m tall with a 1 m wide trunk. These tree ferns
occupied the drier areas of the swamps and by the
end of the Carboniferous replaced arborescent lycopods
as the dominant trees in the swamp forests (Cleal
& Thomas, 2009, p. 111). Psaronius trees
were unbranched trunks supported by prop roots
and crowned with large fronds. The trunk
had no secondary wood for strength. In cross
section, tree fern stems consisted of a narrow cylinder
vascular tissue. Enclosing this cylinder
a mantle of
petioles and aerial roots, which created a fibrous,
tough, lightweight structure (Kenrick & Davis,
2004, p. 71) (click on picture to the left). The root
mantle was thickest towards the base of the trunk.
start out as protosteles but as they mature the
central xylem strands become embedded in a
ground tissue making them a dictyostele. The
root mantle became narrower
towards the top of the tree, while the stem
diameter actually increased. Thus, the stem was an
inverted cone supported by a thick mantle of adventitious
roots, which acted
as guy ropes, tethering the tree to the
ground (Willis & McElwain, 2002, p. 89). The
fibrous nature of the tree fern trunk allowed it
and retain rainwater;
however, the intertwined strengthening elements were
isolated, making the structure prone to bending from
such as wind (Kenrick & Davis, 2004, p. 71).
In general tree ferns were small and did not support
the stem of another marattialean tree fern from
the Permian of Brazil. In cross-section the two
tree ferns have the same basic structure, but they
can be distinguished. Whereas the vascular tissue
is in sinuous strands at the center
the Psaronius stem
the vascular tissue in the center of Tietea is
found as round, ovoid, or C-shaped bundles. Tietea
singularis makes up close to 90% of some fossil
assemblages in Brazil. Click on the pictures above
and to the left to view cross-sections of
and aerial root as well as a comparison of Psaronius with Tietea.
representing the order Osmundales range from the
Permian to recent times. Osmundales is a primitive
group that may have important evolutionary ties
to many modern fern families. Two families within
order had species that reached
is an extinct family of ferns that ranges from
to the Triassic. At this time the family is known
from four genera of fossil tree ferns. Members
of Guaireaceae possess leaf-root-trunk stems. The
vascular tissue of a Guairea stem
is a dictyostele. In cross-section the stem is
composed of pith, a vascular
cortex with leaf traces that are C, V, or
Y-shaped, and adventitous roots (Tidwell, 2002,
p. 150). The structure of the cortex and petiole
bases set this family apart from Osmundaceae. The
cortex of Guairea is not divided into two zones
and the petiole bases lack sclerotic rings. The
mantle seems to be made exclusively of roots with
no persistent petiole bases. Click on the
to look closer at the permineralized
specimen of Guairea carnieri from Paraguay.
the Royal Fern family, makes its appearance in
the Permian. Sixteen living species are recognized
nearly a hundred fossil forms (Miller, 1971). Osmundaceae
is the best-represented family of ferns in the
fossil record and
is known from foliage, stems, roots and reproductive
structures. The family diversified and was widespread
during the Mesozic
era, but decreased in numbers and geographic range
during the Tertiary (Tidwell, 2002, p. 135). The
ferns in this
have rhizomes that grow upright and produce closely
Not unlike Psaronius, the fossil osmundales
leaf-root-trunk stems (click on picture to the left).
The stem is composed of persistent leaf bases and
(Tidwell, 1998). It is the wonderful pattern of leaf
traces, petiole cross-sections and rootlets surrounding
stele (pith, xylem and phloem) in the permineralized
attracts the interest of the fossil wood collector.
The vascular tissue (stele) of the Osmunda stem
is somwhat dictyostelic. The typical anatomy of the
osmundaceous stem in cross-section
with a centralized pith surrounded by a circle of
horseshoe-shaped xylem strands. Phloem tissue is
just outside the central
and may be just inside as well in
some species. A mantle of material surrounds this
central stele that is composed of leaf
shoots that contain C-shaped xylem strands. As one
moves outwards from the center the C-shaped
xylem strands become enclosed in a ring of supportive
tissue, denoting cross-sections of frond petioles
(click on picture
right). In some specimens one can see that the base
with stipular wings.
(Polypodiales) is the largest order of ferns today
with roughly 9,000 extant species. Filicales range
from the Carboniferous to recent. Several families
within this group are represented by excellent
a tree fern genus from the Permian (click on the
picture to the left). It is currently placed in
the order Filicales. However, the permineralized
stem has characteristics that suggest it is related
to Osmundaceae and may represent an early evolutionary
link to this family (Taylor, Taylor and Krings,
2009, p. 451; Stewart and Rothwell, 1993, p. 251;
Dernbach, Noll, and Robler, 2002, p. 86). The stele
of Grammatopteris is either a protostele
or a mixed protostele with parenchyma. In cross-section
the Grammatopteris stem consists of a
protostele surrounded by a three zoned cortex with
leaf traces. The stem is surrounded by a dense
mantle of petiole bases and roots. Near the base
of the tree the mantle expands into an outer region
made exclusively of roots. The mantle of adventitious
aerial roots buttressed the base of the trunk (Robler & Galtier,
2002, pp. 205 & 228).
fossil forests in Brazil have proved to be a great source
of permineralized fossil fern trees. In the past
decade several new finds have been made including
a new small tree fern named Dernbachia brasiliensis (click
on picture to the right). The affinity of this
fern is not certain at this time. It may be related
to early ferns or the order Filicales. In cross-section Dernbachia is
somewhat unusual for a late Paleozoic tree fern.
The central vascular tissue is in a "star
shape", botanically known as an actinostele.
The vascular bundle is surrounded by a narrow cortex
extending into petiole bases. The exterior consists
of a mantle of adventitious roots and petiole bases
(Taylor, Taylor, and Krings, 2009, p. 409).
tree ferns employ a mantle of roots
around a vascular cylinder to form a
trunk. A very unique tree fern, Tempskya,
employed a very different strategy for constructing
a fibrous trunk. Tempskyaceae
are an extinct family of Mesozoic ferns, in the
order Filicales, represented by the single genus Tempskya (Tidwell,
2002, p. 153).Tempskya occurs
as the silicified false trunk of a Cretaceous aged
tree fern. Tempskya is referred to as
a false trunk because
internally it is composed of not one, but numerous
small branching stems and petioles embedded in
a mat of adventitious roots (Brown, 1936, p. 48). The
stems making up the false trunk appear as circular
or lobe-shaped structures measuring roughly 1 cm
in diameter. Each small stem is a solenostele.
In cross-section the stem has a central pith surrounded
by a ring of xylem bounded on both sides by phloem.
The outer phloem is surrounded by a pericycle,
endodermis and cortex with two to three layers.
The roots are mostly circular and measure around
the left). Roots are protosteles with their
xylem elements in a characteristic cross-shape.
Phloem, pericyle, endodermis and a cortex encloses
the xylem. The
ropelike mass of the false trunk is often club-shaped,
straight or conical when found intact. The mass
of intertwined roots gives the exterior a rope
stems initiated the growth of the tree fern and
branched dichotomously in a uniform profuse manner
throughout life, producing both the apical and
lateral growth of the false trunk. Roots, emerging
from the sides of stems, branched profusely filling
in voids, tying the mass of stems together forming
the false trunk. Roots, greatly outnumbering the
stems provided the structural support for the false
growing down ruptured stems and petioles. Upper
portions of stems continued to be nourished by
emerging adventitious roots. Consequently, transverse
sections near the base of the false trunk have
few stems and many roots while transverse sections
towards the apex of the false trunk have many stems
embedded among the roots.
suggests that Tempskya was a short to medium
tree fern with diameter of up to 30 cm and heights
reaching up to 3 meters. The tree bore small leaves
on its crown and for a considerable distance downward
from the apex (Tidwell, 1998, p. 190; Andrews, 1943,
p. 136; Andrews & Kerns, 1947, p. 155). To learn
more, read our article on Tempskya.
in the family Cyatheaceae, which ranges from the
Jurassic to recent. Cyathodendron represents
the trunk of an arborescent fern from Eocene strata
Texas (Tidwell, 1998, p. 192; Stewart & Rothwell,
1993, p. 256; Arnold, 1945, p. 18). The small, upright
trunks have spirally arranged leaf
from the exterior, click on picture to the right.
an irregular shaped solenostele with numerous medullary
bundles. In cross-section one can see numerous vascular
bundles embedded within the central pith. Vascular
bundles within the pith area are called medullary
bundles. Ribbon-like vascular tissue surrounds the
pith. The ribbon-like vascular tissue and the medullary
bundles contribute to the origination of leaf traces.
Adventitious roots arise from the leaf traces.
In cross-section petioles exhibit numerous vascular
strands. The mantle making up the exterior of Cyathodendron is composed of leaf
bases and multicellular epidermal hairs (Tidwell,
1998, pp. 192 & 193; Taylor, Taylor & Krings,
2009, p. 466).
not an arborescent plant, the beautifully permineralized
petioles of the giant fern Acrostichum are worthy of
mention. Acrostichum is an extant genus of the leatherferns
within the family Pteridaceae. Acrostichum is
composed of erect rhizomes bearing very large fronds,
up to 3.5
meters in living species. Today these ferns are often
found in marsh environments. Evidence for this same type
of environment is found for fossil forms as well (Tayolon,
Taylor & Krings, 2009, p. 471).
the permineralized fern petioles found in southwestern
Wyoming were given
the name Eorachis, but are now known to be
the petioles of Acrostichum. In cross-section
the petioles of Acrostichum are
rounded or angular with many having a distinct adaxial
groove (click on picture to the left). Internally,
the petiole consists of numerous
vascular bundles in an omega-shaped configuration surrounded
by a sclerenchymatous sheath (Tidwell,
1998, pp. 194 & 195). Progymnosperms
(Division Progymnospermophyta) range from the Devonian
to the Carboniferous and are thought to represent
the plant group from which all seed plants evolved.
Progymnosperm trees reached heights
of 8 meters and diameters up to 1.5 meters (Willis & McElwain,
2005, p. 110). Progymnosperms are believed to link
ferns to gymnosperms.
1960 the American Paleobotanist Charles Beck published
between the foliage known as Archaeopteris and
the wood Callixylon. Archaeopteris was
believed to be a fern, while Callixylon was
thought to represent a gymnosperm. The work of
Beck demonstrated that the foliage known as Archaeopteris and
wood known as Callixylon belonged to
the same plant. Archaeopteris is
a true missing link between fern-like plants
conifer-like plants (Kenrick and Davis, 2004,
pp. 41-43). The trunk (Callixylon)
of this tree was constructed of conifer-like
wood, while the branches were adorned with fern-like
fronds (Archaeopteris). The underside
of the fronds had sac-like sori that contained
spores for reproduction.
are the earliest trees with modern wood anatomy
and growth habit (Taylor, Taylor, and Krings,
2009, p. 479). Progymnosperms stems are eusteles.
In cross-section the trunk of Archaeopteris (Callixylon)
was very much like modern conifers consisting
of a small central pith surrounded by secondary
and bark. The solid cylinder construction could
support profuse branching. Archaeopteris made
up a significant portion of the canopy of early
we continue to
a tree form it
is helpful to
how plants reproduce.
the make-up of
the life cycle of plants a gametophyte (haploid)
generation alternates with a sporophyte
(diploid) generation. The gametophyte is a plant
that represents the sexual phase of plant reproduction.
The sporophyte is a plant that represents the asexual
phase of reproduction. All land plants possess
sexual and asexual phases.
vascular plants such as lycopods, sphenopsids, ferns,
and progymnosperms possess structures
on the sporophyte plant that produce spores dispersed
by wind. The spores grow into small, inconspicuous
possess one set of genetic instructions in their
cells (haploid). Gametophyte plants have structures that
or eggs. Water is required for the sperm to swim
to and fertilize eggs, forming zygotes. Zygotes
with two sets of genetic instructions (diploid) grow
into the large
sporophyte plants. The sporophyte plant will once
again produce spores continuing an alternation
of generations. The stems we have explored thus far
the sporophyte plant. Wet,
humid environments are needed to complete the life
cycles of spore producing
plants that produce seeds (gymnosperms and angiosperms)
the two phases are bound into a single individual.
Gymnosperms produce very small
gametophytes within male and female
cones or on specialized branches or leaves. The male
gametophyte is the pollen, while the female gametophyte
consists of eggs in the ovule. The
sperm of gymnosperms do not swim through water to
reach the ovules (Ginkgo biloba and cycads
are the exception, see Taylor, Taylor, and Krings,
2009, p. 744). Wind carried pollen transports the
sperm. When pollen lands near an ovule a pollen tube
grows towards the ovule. Sperm travel down the pollen
tube to fertilize the eggs in the ovule. Seeds
provide many advantages for both plants and animmals.
encase the developing plant embryos, providing them
the surrounding environment. Seeds allow embryos
to remain dormant during unfavorable conditions.
Pollen frees gymnosperms from the need to have
water for fertilization. Seeds allow gymnosperms
germination until favorable conditions exist. These
reproductive strategies gave gymnosperms a distinct
advantage over the spore producing plants in dry
environments. The seeds also became a valuable source
of food for animals.
include plants that usually bear their seeds
in cone-like structures
to the angiosperms (flowering plants) that
have seeds enclosed
in an ovary. Gymnosperms range from the Carboniferous
to recent times. Modern gymnosperms include
the following extant divisions: Pinophyta,
and Gnetopyta. Conifers (Division Pinophyta)
are by far the most abundant and widespread
of the living
gymnosperms. There are over 700 species of
extant gymnosperms. Trunk structures representing
solid cylinders and
cylinders can be found among both extinct and
ferns (Pteridospermatophyta) flourished from
the Carboniferous to the Lower Permian. Pteridosperms
had fern-like foliage,
but reproduced with seeds (Selmeier, 1996,
Seed ferns exhibited both vine-like and arborescent
forms. The term pteridosperm is descriptive but,
misleading as seed ferns are actually
early gymnosperms (Cleal & Thomas,
2009, p. 139).
stems and trunks of many seed ferns are polyvascular,
that is they consist of separate vascular
segments in the form of wedges
(click on picture to the right). In some species wedges
secondary xylem (wood) only to
inside, others produced wood towards the
inside and outside,
and still others produced wood all the way
around the wedge. In the past these stems were referred
to as polystele. It was thought that each vascular
bundle represented a separate stele. The stems are
now regarded as a single stele (Taylor, Taylor & Krings,
2009, pp. 566 & 769).
Corystospermales) is the permineralized stem of a
Mesozoic seed fern found in South America, South
Africa, and Antarctica. In cross-section Rhexoxylon consists
of a central pith surrounded by a ring of primary
vascular bundles, click on image to the left. The
primary bundles are separated by wide
pith rays. Secondary growth produces wedges of
xylem both internally (centrifugally) and externally
(centripetally) to the primary bundles. The rays
are continued during secondary growth giving the
wood a segmented appearance. In older stems wedges
of secondary xylem separated by segments of parenchyma
develop in the outer cortex (Taylor, 1991, pp. 183
& 184). Secondary wood and sclerotic nests develop
in the pith and
exhibits this kind of growth today.
Glossopteridales) is a seed fern with a eustele
vascular bundle (concentric vascular bundles with
pith), which is
characteristic of conifers and dicot
angiosperms (click on picture to
the right). Seed
fern stems were composed of conifer-like wood.
The vascular cambium of these plants produced
xylem and secondary phloem. Cork cambium
on the exterior. The solid xylem making
up the trunks of these early arborescent
them strong and
resistant to buckling. The adaptation
of a woody cylinder stem would
appear later in angiosperm dicots. Solid,
woody trunks can support profuse branching
included progymnosperms and gymnosperms; however, ferns
and clubmosses dominated (Paleophytic flora). As the
climate became drier, paleophytic
give way to a new Mesophytic flora during
the Triassic period.
Woody seed-bearing plants and their relatives
would come to dominate the Mesophytic
flora. Thus, the change from
Paleophytic to Mesophytic represented
a change in reproductive strategy; from
to seed producers. Conifers,
cycads, and ginkgoes diversified during
this time and dominated the landscape
(Kenrick & Davis,
2004, p. 143).
Schilderia is a distinctive gymnosperm genus
found in Triassic aged
deposits. Schilderia was
a large tree with a solid woody cylinder. This
genus can be easily recognized by its large rays,
which are clearly visible to the naked eye (click
on picture to the right). Under magnification
the cell arrangement within the
rays can be seen to take on a chevron pattern.
The stem and ray anatomy is like that of Ephedra
or Morman Tea. Schilderia seems to be a likely
candidate for the ephedracious pollen found in
the Chinle Formation. It has been suggested that
Schilderia has affinities with Gnetphyta
(Tidwell, 1998, p. 216; Wright, 2002, pp. 130 & 131).
the genus of an extinct gymnosperm stem found in
Jurassic aged deposits of Colorado and Utah. The
genus is also represented by one Late Cretaceous
from the Aken Formation collected in the Bingeberg-Flöeg
sandpit in Hauset, Belgium (Knoll, 2010, pp. 181-185).
stems of Hermanophyton range from 3 to 22.5
cm in diameter and up to 18 m in length. In
cross-section the stem consists of a central pith
surrounded by 9 to 15 petal-shaped
segments of secondary xylem (click on picture to
the left). Wide primary rays radiate out from the
pith inbetween the xylem
cambium added secondary xylem to the woody wedges
and secondary phloem toward the outside. The wedges
making up the xylem cylinder are surrounded by
a layer of cortex and ramentum.
most likely a small to medium narrow stemmed
tree reaching heights of 18 meters and crowned with
small leaves which dropped off as the plant grew
leaf bases (Tidwell & Ash, 1990, pp. 87 & 88).
Fossil stems of Hermanophyton maintain
a fairly consistent diameter over substantial
lengths. The stem acted
as a solid cylinder; however, there is
of branching. No leaves or reproductive structures
have been associated with Hermanophyton so
its affinity with other gymnosperms is uncertain
(Tidwell & Ash,
1990). To learn more, read our article on Hermanophyton.
Cycads & Cycadeoids
(division Cycadophyta) made up a significant portion
of the Mesophytic flora. The Mesozoic is
to as the "age of cycads". Cycadophytes
are gymnosperms that have a superficial
resemblance to the flowering palms. Cydadophyte
trunks range from short and squat to tall
and columnar or tree-like. Stems are covered
with a protective layer of persistent leaf
Leaves can be scale-like or in the form
of fern-like fronds. One living and one
extinct order are recognized within this
both living and extinct belong to the order
Cycadales. Cycads range from the Permian
to recent times. The
extinct order Cycadeoidales or Bennettitales
range from the Triassic to the late Cretaceous.
In the U.S. this order is referred to as Cycadeoidales,
while in Europe it is known as Bennettitales.
In cross-section both cycad and cycadeoid stems
are eusteles consisting
of a large pith surrounded by a ring of vascular
tissue (click on picture to the right). The ring
of vascular tissue is composed of secondary xylem
rays dissect the vascular ring. Cortex tissue
surrounds the vascular ring. Finally, a ramentum
provides a strong outer covering (Stewart & Rothwell,
1993, pp. 358 & 359). Leaf scars on the exterior
are diamond shaped. In a broad sense, the
stem is a variation of the tube-like structure.
Cycads and cycadeoids can be distinguished by
of leaf traces and leaf stomata. The cones of
cycads are found at the apex of the plant while
in cycadeiods they are found embedded among the
Charmorgia and Lyssoxylon are
two important fossils found in the Chinle
formation of Arizona that belong to Cycadales
(Tidwell, 1998, pp 196-197). Many Mesozoic
species had slender branching forms. Extant
short, squat cycad forms did not appear
until the Tertiary (Willis & McElwain,
2002, p. 135). One
group of cycadeoids had a short barrel-like trunk
crowned with large fronds. A second group was
more shrub or tree-like. Perhaps the most well
known cycadeoid genus is Williamsonia. Williamsonia resembled
a shrub or tree and grew to a height of 3 meters. Williamsonia reached
its greatest diversity during the Jurassic.
cones of cycadeoids are remenescent of
flowers. Some paleotologists have suggested
that the cycadeoids may have a close evolutionary
relationship with angiosperms (flowering
plants), although current evidence makes
this tie unlikely. Both Cycadales and Cycadeoidales
most likely evolved from medullosan seed
ferns (Willis & McElwain, 2002, pp.
Maidenhair tree or Ginkgo biloba is
the only living species representing
the division Ginkgophyta. Representatives
of the order Ginkgoales date back to
the Permian, but the genus Ginkgo makes
its first appearance in the Jurassic. In
fact, Ginkgophytes reached their greatest
diversity during the Jurassic. Fossil
evidence indicates that at least 16 genera
of ginkgophytes made up a significant
part of the Mesozoic vegetation (Willis & McElwain,
2002, p. 139). Ginkgos
declined in the Paleogene and Neogene,
becoming nearly extinct (Tidwell, 1998,
trees have a constellation of characteristics,
making their origin difficult to discern.
The Ginkgo tree grows to 30 m. The fan
shaped leaves of Ginkgo remind one of
a deciduous flowering plant. The reproductive
structures of the Ginkgo are more like
that of a cycad. The roots and stems
of Ginkgo are conifer-like.
In cross-section the Ginkgo tree
is a solid woody
cylinder with a small pith, secondary
xylem and bark. Ginkgo wood
is very conifer-like; however, in cross-section
the small tracheids are highly variable
in size, which somewhat disrupts the
arrangement (click on picture to the
left). Ginkgo also
has large parenchyma cells scattered
Conifer tracheids in cross-section are
of nearly equal
size, except for growth rings and exhibit
an orderly radial arrangement. Fossil
leaves of ginkgos are common, while reproductive
structures and wood are rare.
more commonly known as conifers, range
to recent times. Cordaitales and Voltziales
are well-known extinct orders of the division Pinophyta.
The order Pinales has both
extinct and extant genera. Pinales includes
such familiar plants as pine, spruce, Douglas-firs,
cypresses, cedars, junipers, larches, sequoias,
and yews . Two primitive conifer orders have important evolutionary
ties to modern day conifers in the order
(order Cordaitales) are conifer like
plants that range from the Pennsylvanian
to the Permian. Cordaites grew as shrubs
and trees. Cordaite trees possessed a
slender trunk adorned with a branching
crown of leaves. Cordaite stems were
eusteles that possessed a rather large
pith. As the tree grew the pith broke
down to create a hollow area. Artisia is
the pith cast of Cordaites. The stem
was composed of secondary wood surrounded
by bark. In a broad sense the mature
trunk was a variation upon the tube-like
structure. Cordaites had long leathery
straplike leaves with
The leaves were spirally arranged around
(order Voltziales), commonly known as
walchias, range from the Carboniferous
to the Triassic. Voltziales
were large trees that possessed needle-like
leaves. The trunk was a solid woody cylinder
with pith, secondary xylem and bark.
are traditionally viewed as an evolutionary
transition between Cordaites and modern
families of Conifers (order Pinales)
(Bhatnagar & Moitra, 1996, p. 167;
Taylor, Taylor, & Krings, 2009, p.
from members of Voltziales, which in turn
have their origins within the Cordaitales
(Willis & McElwain, 2002, p. 150).
The fossil record of modern Pinales families
dates back to the Triassic (Taylor, Taylor, & Krings,
2009, p. 870). Several conifers, representing the family
Araucariaceae, from the Triassic and Jurassic
are sought by
you acquire a conifer from a Mesozoic
wood deposit that is composed of moderate
sized tracheids, uniseriate
rays with no resin canals or wood parenchyma
it is a good bet that it will have been
identified as Araucarioxylon.
Perhaps the most well known fossil wood
specimens assigned to this genus are the
permineralized trunks of the Petrified
monument in Arizona. If a specimen with
the same characteristics of Araucarioxylon is
found in older, Paleozoic deposits it will
be referred to as Dadoxylon. It
has become increasingly clear that the
wood ascribed to Dadoxylon and Araucarioxylon represent
a wide array of Paleozoic and Mesozoic
plants (Stewart & Rothwell, 1993, p.
416; Savidge, 2007, p. 324).
that the pith structure, as seen in cross-section,
can be used to distinguish between Dadoxylon and Araucarioxylon,
making the age of the material irrelevant.
The pith of Dadoxylon is variable
in size but large, while the pith of Araucarioxylon is
small and visually non discernable (Wright,
2002, pp. 129). A precise identification
of these early conifers would require
transverse, radial, and tangent sections.
tracheids making up the secondary wood
of Woodworthia is
araucarioid; however, in cross-section
one can see short-shoot
traces connecting the pith to short-shoot
scars or knots on the exterior (Click
on picture to the right). In life the tree
was graced with many thin
bearing clusters of leaves. Based upon
the size of the short-shoots and exterior
scars it seems clear that multiple species
of Woodworthia existed in the
members of the order Pinales can be found
in just about any environment
in the world.
In some biomes, such as the Taiga, conifers
are the dominant plant. Modern
day conifers grow as shrubs or trees.
Some arborescent conifers exhibit a pyramidal growth
form. Leaves are usually needle-like, but
also show a broad flat shape. Conifers
bear seeds in woody cones, except for
junipers and yews,
which have a berry-like structure.
modern day conifers
secondary growth begins early.
The vascular cambium produces secondary
the inside and secondary
phloem to the outside. The cork cambium produces
to the inside and
(cork) to the
outside. All of the tissues produced
outside the vascular cambium comprise
bark (click on picture to the
right). Thus, the typical gymnosperm stem is a
with central pith surrounded
by substantial amounts of secondary
in turn is enclosed by bark
(Raven, Evert, & Curtis,
1981, pp. 340-343). The
water conducting cells of gymnosperms
are tracheids (click on picture
to the left).
Tracheids are closed tubes
with pits along their length.
Water zigzags through paired
pits of overlapping cells up
through the plant. New, living
wood produced by the
the inside of the trunk is
termed sapwood. The main
function of sapwood is
to conduct water and minerals
throughout the plant. Sapwood
also stores reserves made within
leaves. However, the tube-like
xylem cells only become
water conducting cells after
they lose their protoplasm
and die. The older wood
center of the stem
is termed heartwood. The transition
from sapwood to heartwood marks
the area where
xylem tissue shuts
In some species tracheids making
up the heartwood act to
products called extractives
as they age and no longer conduct
water. Rays are
cross the growth rings
at right angles. Medullary rays connect
the cental pith to the cambium.
Rays are made
primarily of living
to carry sap radially through
the plant. The rays of conifers are
one to two
cells wide. Overall,
the rays of softwoods (conifers)
account for 8% of the woods
(Raven, Evert, & Curtis,
1981, p. 492). Trunks formed
from solid xylem tissue are
very strong and resistant to
& Davis, 2004, pp. 69-70).
As long as the tree is alive, vascular cambium, just beneath
the bark, continues to produce xylem, increasing the girth
of the stem, layer by layer. Trees growing in temperate regions
cycles that include
seasons of growth and dormancy. Seasonal growth results in
growth rings or annual rings (click on picture to the right).
Earlywood consists of tracheids that have a wide diameter
with thin walls; whereas latewood consists of tracheids with
smaller diameters and thick walls. In tropical climates,
growth may be consistent year around,
wood that does not have growth rings (Hoadley, 1990, p. 10).
Some conifers possess tubular passages in their wood
called resin canals.
Resin canals are intercellular
spaces lined with epithelial cells (click on picture
to the left). The epithelial cells
exude pitch or resin, which
functions to seal wounds caused by mechanical
or boring insects. Resin canals
can be found in four genera of the family Pinaceae (Pinus, Picea, Larix,
The presence of resin canals
helps to separate species within
these genera from all other
conifers (Hoadley, 1990, p.
20). Pines have
relatively large resin canals.
The resin canals in pines can
be found distributed somewhat
evenly throughout the growth
ring. Resin canals in pines
are usually found singly.
The epithelial cells surrounding
the resin canals in pines are
thin-walled (click on picture
to the right). In spruces,
larches, and Douglas-fir resin
canals are distributed unevenly.
and often occur in tangential
groups of two to several (click
to the left). Some growth rings
may lack resin canals. The
epithelial cells surrounding
the resin canals are thick-walled.
canals may also form as a result
of environmental stress in
that have resin canals as well
as species that do not normally
have them. Traumatic resin
canals appear in cross-section
as a single continuous line extending
for some distance along
a growth ring. Traumatic resin
canals are usually only slightly
larger than tracheids (Hoadley,
1990, pp. 20-21).
still successful today, gymnosperms
dominated the world's Mesophytic flora from the Triassic
to the Early Cretaceous. Flowering plants first
the Early Cretaceous
a great adaptive radiation
during the Middle Cretaceous. Flowering plants quickly
became a major
of species diversity
and the world entered the third
of plant life known as the Cenophytic
by the Late Cretaceous (Kenrick & Davis,
2004, p. 143). The
transition from Mesophytic
to Cenophytic represents a change in reproductive strategy.
relied mostly on wind pollination
bore naked seeds clustered in cones, on leaves,
or on the end of stocks. Flowering
with animal pollinators, underwent
double fertilization, and encased seeds in a fleshy
ovary (fruit) that encouraged
Our modern plant world is a
continuation of the Cenophytic age of plants.
reproductive strategies helped angiosperms
become a great success and diversify into
the forms we know today. In angiosperms
male and female structures develop within
flowers. The pistil is the central, female
organ of the flower and typically consists
of an ovary with ovules, a style and stigma.
The stamen is the male part of a flower
and typically consists of a filament or
stalk topped with pollen producing anthers.
pollen comes into contact with a flower's
stigma the growth of a pollen tube is activated.
Each pollen grain carries two sperm. One
sperm fertilizes an egg in the ovule; the
other sperm unites with two haploid cells
in the same ovule. This process is known
as double fertilization and is an important
adaptation found in angiosperms.
fertilized egg will undergo cell division
to become a zygote and then an embryo.
The second fertilization results not in
offspring, but rather the development of
endosperm, which acts as a nutrient for
the embryo. Cells in the endosperm have
three sets of chromosomes (triploid). Endosperm
not only serves as an important food source
for the embryos of flowering plants it
also is important to animals. Humans
depend upon the endosperm of rice, wheat,
seed is formed when the endosperm and the
embryo become enveloped in a part of the
ovule that hardens into the seed coat.
The ovary or other parts of the flower
in angiosperms develop into a fleshy fruit
surrounding the seeds. Many organisms such
as birds, bats, and insects have coevolved
to help pollinate angiosperms. The fleshy
fruits of angiosperms are an adaptation
for seed dispersal. Many animals use the
fruit as a food source, which results in
the dispersal of seeds encapsulated within
a natural fertilizer!
addition to new reproductive strategies
flowering plants also exhibit adaptations
to their vascular tissues. Sieve-tube members
in the phloem and vessel elements in the
xylem are found in most flowering plants.
the presence of vessels in fossil wood
is usually a good indication
that it is an angiosperm. However, one
must keep in mind there are vesseless dicot
families, such as Trochodendraceae and
gymnosperms that possess vessels in the
order Gnetales. Still, vessels are vital
in fossil wood identification.
plants or angiosperms (phylum Magnoliophyta) range from
the Cretaceous to recent times. Extant angiosperms are
represented by over 300,000 species. Traditionally angiosperms
are divided into the monocotyledons and dicotyledons. Today
angiosperms are divided into the monocots, eudicots, and
and eudicots are monophyletic groups. Eudicots contain
most of the dicots. It is useful to known the major differences
structure between monocots and dicots (eudicots & magnoliids)
when studying both extinct and extant plants.
dicots possess eustele stems, a
central pith surrounded by secondary wood and bark
(see picture above). The woody stems of arborescent
are strong and resistant to buckling. Vascular
cambium produces secondary xylem to the inside and
angiosperms have cell types that are distinctly different
in size making up their xylem tissue (wood). Vessel
elements are larger
diameter water conducting cells. Smaller,
less numerous, water conducting cells called tracheids
along with abundant fibers
in cross-section (click on picture to the left).
Vessel elements have pits along their sides just like
tracheids. However, vessel elements also have perforations
or holes, which can be at both ends and along their length.
Vessel elements are joined into long continuous columns
to form vessels or pores. Vessel elements are thought
to be much more efficient at conducting water up the
tracheids. The rays making up the hardwood of dicots
can be from one
to 30 cells
species. Oak rays can be 30 cells wide and hundreds
of cells high, making
them visible to the naked eye. Rays make up, on average,
17% of the volume of wood in hardwoods (Raven,
Evert, & Curtis, 1981, p. 492).
The wood of angiosperm dicots is more complex than
that of the conifers. This complexity can actually
easier in some instances.
distribution of vessels in cross-section aids in
hardwood identification (click on picture to the
right). Three types
of vessel distribution are recognized. Ring-porous
woods are characterized by a row or rows of relatively
large earlywood vessel or pores. Vessels throughout
the rest of the growth ring are much smaller in size.
elm, and hickory are typical ring-porous woods. Semi-ring-porous
relatively large early wood pores. Pores in the growth
ring gradually reduce in size from early to late
are typical semi-ring-porous woods. Diffuse porous woods
possess vessels of equal size from early to latewood.
maple, and cherry are diffuse-porous woods (Hoadley,
1990, pp. 99-100). The roots of angiosperm dicots
are protosteles. The absence of a pith can be helpful
in identifying fossil wood as a root.
Monocots usually lack secondary
woody growth. However, there is at least one arborescent
fossil monocot exhibiting secondary growth. More
are species, like the palms, which produce fibrous,
wood-like stems to construct a tree form using
only primary growth.
most common arborescent fossil monocot is the palm
tree. In palms (family Arecaceae) and the Screw Pine
or Pandanus (family Pandanaceae) leaf formation
and stem thickening occurs at the growing tip (apex).
The apical meristem produces a crown of leaves at
the top of the growing tree. Nodes mark areas of
leaf attachments. Internodes make up the space between
leaf attachments (click on picture to the left).
The primary thickening meristem (PTM), which is derived
produces cells that expand laterally more than they
do vertically. In this way stems reach their full
thickness in the initial growth at the tip or apex.
The stems diameter changes very little afterwards.
In fact, early in their growth, palm stems undergo
massive thickening as parenchyma cells enlarge and
divide. Nodes are closely spaced at this time, while
internodes remain short. The stem reaches adult diameter
at ground level during early establishment growth
and attains its maximum conducting capacity (Mauseth,
2014, p. 193). From this point the trunk does not
increase in girth as the palm lacks a lateral meristem
that can produce secondary growth. In many palms
growth in height occurs by means of longer internodes
(Esau, 1977, p. 287)
monocot fiber is fairly uniform yielding little
specific taxonomic information. Palm
stems are a variation of a eustele, called an atactostle. In
cross-section fibrous monocots possess scattered
vascular bundles embedded in a ground mass of parenchyma
tissue (click on picture to the right). The vascular
bundles can give a spotted appearance to the palm
In the center of the stem vascular bundles are
Towards the periphery of the stem the vascular
bundles become more numerous and crowded.
cuts reveal that the vascular bundles form rod-like
roots of palm are eusteles. Fossil palm
roots are known by the form genus Rhizoplamoxylon.
The scattered vascular bundles making up the palm
trunk and adventitious roots form a fibrous
in the tree ferns. If you like palms, you may want to
read our article on Palmoxylon of
the Catahoula Formation.
some members of the family Asparagaceae, such as Dragon
trees (genus Dracaena), Cordyline and
the Joshua tree (Yucca brevifolia) a secondary
thickening meristem (STM) develops in the cortex. The
STM extends down
the sides of the stem and produces parenchyma cells
internally. Some of the rapidly dividing parenchyma
cells differentiate into secondary vascular bundles.
Other parenchyma cells form a secondary ground tissue.
Thus, the STM produces a cambium-like cylinder composed
of vascular bundles in a parenchyma matrix. This unusual
secondary growth increases the conducting capacity
and strength of the stem. Monocots with this type of
secondary growth do increase in diameter with age (Mauseth,
2014, p. 193; Walters & Keil, 1996, p. 397).
Protoyucca shadishii is represented by permineralized
stems form Middle Miocene strata in northwestern Nevada.
This rare fossil has the distinction of being the first
recognized permineralized fossil monocot with secondary
growth. P. shadishii is
thought to be an ancestor of the present day Joshua tree, Yucca
brevifolia. The Joshua tree was thought to be a giant
member of the lily family (Liliaceae), but DNA evidence
has placed it in the family Asparagaceae.
cross-section the stems of P. shadishii consist
of a primary cylinder surrounded by a secondary cylinder.
Secondary cortex and periderm form the exterior (click
on picture to the left). The primary cylinder is
composed of vascular bundles randomly distributed
among parenchymatous tissue. Some of the tissue in
the primary cylinder disintegrates, resulting in
radiating spoke-like spaces. The secondary cylinder
is made up of radially aligned secondary bundles,
produced by a secondary meristem. The arrangement
of the bundles gives the false appearance of growth
rings. Each bundle is composed mostly of secondary
xylem and some phloem. The stems of P. shadishii grew
to 60 cm or more in diameter (Tidwell & Parker,
1990, pp. 81 & 82; Tidwell, 1998, pp. 246 & 247).
The Protoyucca stem is a variation of the
Growing taller than surrounding plants can be of real
value when competing for sunlight; the tree form
affords the adaptive advantage
of height. The value of the arborescent form is reflected
in the convergent evolution represented by the various
tree forms we have examined.
Several strategies for trunk designs have evolved multiple
times. Arborescent lycopsids, horsetails, cordaites,
cycadophytes and at least one moncot with secondary
growth utilized tube-like structures for their
monocots (palms) employed individual elements to form a
trunk. Finally, progymnosperms, many gymnosperms and
angiosperm dicots constructed trunks of solid xylem cylinders.
trunk designs co-opted a variety of plant tissues and organs
as strengthening elements. The student of permineralized
tree forms can learn
to identify these different evolutionary strategies and
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