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The Anatomy of Arborescent Plant Life Through Time |
| Collectors of petrified wood focus on permineralized plant
material related to arborescent (tree-like) plant life. Evidence
for the first fossil forest occurs in the Devonian. Fossil forest
composition changes through geologic time, reflecting variety
in evolutionary strategies for constructing a tree form. It is
helpful and informative to study the anatomy of various trunk
designs.
Evolutionary adaptations for trunk structure can be recognized by the arrangement 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 system. Ground tissues including parenchyma, collenchyma and sclerenchyma are involved in photosynthesis, storage, secretion, transport, and structure. Parenchyma tissue produces 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 make 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 vessels are enlongated cells that lack protoplasm at maturity and have secondary walls strengthened with lignin. Vessels are lager in diameter than tracheids and are an adaptation of flowering 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 the leaves pulls columns of water enclosed within these stacked, tube-like cells up the plant. Phloem tissue 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 types 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 throughout the plant. 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. The cells that make up these structures are produced from clusters of dividing cells called meristems. Apical meristems occur at the tips of roots and shoots. Lateral meristems such as the vascular cambium and cork cambium (phellogen) produce secondary growth, increasing the girth of stems. The vascular cambium produces secondary xylem to the inside and secondary phloem to the outside. The cork cambium produces 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). As we explore the anatomy of arborescent plant life we will discover that a variety of tissues and organs have been co-opted as strengthening elements. Permineralized 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. Arboreal
horsetails contributed to the canopy of Paleozoic
forests. The trunk of Calamites grew in
a telescoping fashion from one node to another
(Selmeier, 1996, p.
139). In cross-section
the stem consists of pith and or a medullary cavity
surrounded by Ferns Tree Ferns Arborescent
ferns possess a kind of buttressed or braced trunk and evolved
during the Carboniferous and Lower Permian. Psaronius was
the largest arborescent fern found in the coal measure swamps
(Willis & McElvain, 2002, p. 108). Psaronius was
up to 10 m tall and occupied the drier areas of the swamps.
Ground Cover Osmundaceae, the Royal Fern family, makes its appearance in the Permian. Sixteen living species are recognized along with 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 family have rhizomes that grow upright and produce closely spaced fronds (leaves). Not unlike Psaronius, the fossil osmundales also possessed
leaf-root-trunk stems (click on picture to the left). The
stem is composed of persistent leaf bases and rootlets
(Tidwell, 1998). It is the wonderful pattern of leaf traces,
petiole cross-sections and rootlets surrounding a central
stele (pith, xylem and phloem) in the permineralized stem,
which
attracts the interest of the fossil wood collector. The typical
anatomy of the osmundaceous stem in cross-section starts
with a centralized pith surrounded by a circle of horseshoe-shaped
xylem strands. Phloem tissue is just outside the central
xylem
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
to the
right). Some of
these
petioles
are
outlined
with stipular wings.Seed Ferns Seed ferns (Pteridospermatophyta) flourished from the Carboniferous to the Lower Permian. Pteridosperms had fern-like foliage, but reproduced with seeds (Selmeier, 1996, p. 142). Seed ferns exhibited both vine-like and arborescent forms. The stems and trunks of many seed ferns consisted of separate  vascular
segments in the form of wedges (polystele) (click
on picture to the right). In some species wedges produced
secondary xylem (wood) only to
the
inside, others produced wood towards the inside and
outside,
and still others produced wood all the way around
the wedge. There is a distinct evolutionary trend in the
number
of vascular bundles embedded in the pith along with
the
position
of peripheral vascular bundles. Over
 time,
a single, deeply divided vascular bundle evolved into
three or more. Outer vascular bundles became fused,
forming
rings of secondary
wood by the Lower Permian (Jung, 1996, p. 158). Glossopteris is
a seed fern with a eustele vascular bundle (concentric
vascular bundles with enclosed pith), which is characteristic
of conifers and angiosperms (click on picture to
the left). Both the polystele and eustele wood of
seed fern
stems were composed of conifer-like wood. The vascular
cambium of these plants produced both secondary xylem
and secondary phloem. Cork cambium produced periderm
on the exterior. The solid xylem making up the trunks
of seed ferns made them
strong and
resistant to buckling. The adaptation of a woody
cylinder stem would also develop in arborescent gymnosperms
and
later angiosperm dicots. Solid, woody trunks can
support profuse branching and large crowns.Paleozoic flora was dominated by ferns and clubmosses (Paleophytic flora). Paleophytic flora would 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 spore producers to seed producers. Conifers, cycads, and ginkgoes diversified during this time and dominated the landscape (Kenrick & Davis, 2004, p. 143). Gymnosperms Gymnosperms ("naked-seeds") include plants that usually bear their seeds in cone-like structures as opposed to the angiosperms (flowering plants) that have seeds enclosed in an ovary. Gymnosperms range from the Carboniferous to recent times. Gymnosperms include the following extant divisions: Pinophyta, Ginkgophyta, Cycadophyta, and Gnetopyta. Conifers (Division Pinophyta) are by far the most abundant and widespread of the living gymnosperms. In most modern trees, trunks are woody cylinders. In modern day  gymnosperms
secondary growth begins early. The vascular cambium
produces secondary
xylem to
the inside and secondary phloem to the outside. The
cork cambium produces pelloderm to the inside and phellen
(cork) to the
outside. All of the tissues produced outside the vascular
cambium comprise
the
bark (click on picture to the right). Thus,
the typical gymnosperm stem is a eustele with central
pith surrounded by substantial amounts of secondary xylem,
which
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). New, living
wood produced
by the vascular cambium towards 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 the leaves. However,
the tube-like xylem cells only become fully functional
water conducting cells after they lose their protoplasm
and die. The older wood in the center of the stem
is termed heartwood. The transition from sapwood to heartwood
marks the area where xylem tissue shuts down and dies.
In some species tracheids making up the heartwood act
to
store waste
products called extractives as they age and no longer
conduct water. Rays are ribbon-like tissues that
cross the growth rings
at right angles. Medullary rays connect the cental pith
to the cambium. Rays are made primarily of living parenchyma
cells and
function
to carry sap radially through the plant. The rays of conifers
are usually very thin, being one to two cells wide. Overall,
the rays of softwoods (conifers) account for 8% of the
woods
volume
(Raven, Evert, & Curtis, 1981, p. 492). Trunks formed
from solid xylem tissue are very strong and resistant to
buckling
(Kenrick
&
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 experience life 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,
resulting
in
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
damage
or boring insects. Resin canals can be found in four genera
of the family Pinaceae (Pinus, Picea, Larix,
and Pseudotsuga).
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.
The
resin
canals
are
relatively
small
and often occur in tangential groups of two to several (click
on  picture
to the left). Some growth rings may lack resin canals. The
epithelial cells surrounding the resin canals are thick-walled.
Traumatic
resin
canals may also form as a result of environmental stress in
both species
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). Although still successful today, gymnosperms dominated the world's Mesophytic flora from the Triassic to the Early Cretaceous. Flowering plants first emerge during the Early Cretaceous and undergo a great adaptive radiation during the Middle Cretaceous. Flowering plants quickly became a major constituent of species diversity and the world entered the third great age 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. Gymnosperms and their relatives relied mostly on wind pollination and bore naked seeds clustered in cones or on the end of stocks. Flowering plants coevolved with animal pollinators, underwent double fertilization, and encased seeds in a fleshy ovary that encouraged seed dispersal. Our modern plant world is a continuation of the Cenophytic age of plants. Angiosperms Dicots Woody
dicots possess eustele stems. A eustele stem is composed
of a central pith surrounded by secondary wood and bark
(see picture above). The woody stems of arborescent dicots
are strong and resistant to buckling. Vascular
cambium produces secondary xylem to the inside and secondary
phloem
to the
outside. Most
angiosperms have cell types that are distinctly different in
size making up their xylem tissue (wood). Vessels are larger
diameter water conducting cells.
Monocots show no secondary woody growth. However, some species, like the palms, produce fibrous, wood-like stems. The apical meristems of palm trees produce leaves at the top of the tree. The base of alternating leaves expand to form new trunk material. The trunk does not increase in girth as the palm lacks a lateral meristem (vascular cambium). In  cross-section
monocot fiber is fairly uniform yielding little specific taxonomic
information. In cross-section fibrous monocots possess scattered
vascular bundles embedded in a ground mass of parenchyma tissue
(click on picture to the left). In the center of the stem vascular
bundles are spaced far apart.
Towards the periphery of the stem the vascular bundles become
more numerous and crowded.
The scattered vascular bundles making up the palm trunk form a fibrous composite,
a design strategy that evolved earlier in the tree ferns.
Convergent
Evolution |
Click on the word anatomy for a revised, printable version of our article. |
Bibliography |
Arens,
N.C. Lab V Lycophytes, UCMP Berkley: Hoadley,
B.R. (1990). Identifying Wood: Accurate Results with
Simple Tools. Newton, Connecticut: Taunton Books & Videos. |
