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. Evolutionary adaptations for trunk structure can be recognized by the arrangement of tissues. 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 primary and secondary xylem; bark tissue covered the exterior (click on picture to the right). As the tree grew, secondary xylem added to the girth of the trunk, while the central pith area formed hollow interior chambers. The hollow trunk forms a kind of reinforced tube. These trees were fast growing, but 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 picture to the left). Xylem tissue consists of an interconnected system of tube-like cells (tracheids) that conduct water throughout the plant. The tracheids that make up the xylem of arboreal horsetails have cell walls strengthened with lignin. Paleozoic horsetail trees reached heights of 30 m, attained diameters of 1 m, and were anchored to the ground with prostrate rhizomes.

Ferns

Ferns occupied many habitats in the Carboniferous. Fern species grew as epiphytes, ground cover, understory and canopy in these ancient forests. Isolated, but intertwined roots and leaf petioles formed an important part of the stems of ferns with tree and shrub-like forms.

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. The trunk had no secondary wood for strength. In cross section, tree fern stems consisted of a narrow cylinder composed of vascular tissue. Enclosing this cylinder was 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. The adventitious roots making up this 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 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 to absorb and retain rainwater; however, the intertwined strengthening elements were isolated, making the structure prone to bending from perpendicular forces such as wind (Kenrick & Davis, 2004, p. 71). Click on the picture to the right to learn more about root structure.

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.

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

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. Phloem is the living tissue in plants that transports sugars made from photosynthesis to the rest of the plant. 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). Xylem tissue is composed of tracheids that function as an interconnected system of tubes for conducting water throughout the plant (click on picture to the left). Tracheid cell walls are reinforced with the polymer lignin. The tube-like cells only become functional after they die and lose their protoplasm. New wood produced by the vascular cambium towards the inside of the trunk is termed sapwood. The older wood in the center of the stem is termed heartwood. The transition from sapwood to heartwood marks the area where tracheids shut down and die. 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. 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; whereas latewood consists of tracheids with smaller diameters. 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

Flowering plants or angiosperms (phylum Magnoliophyta) range from the Cretaceous to recent times. Traditionally angiosperms are divided into the monocotyledons and dicotyledons. Today angiosperms are divided into the monocots, eudicots, and magnoliids. Monocots and eudicots are monophyletic groups. Eudicots contain most of the dicots. It is useful to known the major differences in stem structure between monocots and dicots (eudicots & magnoliids) when studying both extinct and extant plants.

Woody dicots possess eustele stems, which is a central pith surrounded by secondary wood and bark (click on picture above). 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. Smaller, less numerous, water conducting cells called tracheids along with abundant fibers can also be viewed in cross-section (click on picture to the left). The rays making up the hardwood of dicots can be from one to 30 cells wide depending upon the 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 make identification easier in some instances.

The 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. Oak, elm, and hickory are typical ring-porous woods. Semi-ring-porous wood possesses relatively large early wood pores. Pores in the growth ring gradually reduce in size from early to late wood. Live oak, tanoak, and walnut are typical semi-ring-porous woods. Diffuse porous woods possess vessels of equal size from early to latewood. Beech, sycamore, maple, and cherry are diffuse-porous woods (Hoadley, 1990, pp. 99-100).

Monocots

Monocots usually show no woody growth. However, some species, like the palms, produce fibrous, wood-like stems. 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.

Convergent Evolution

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. The student of permineralized wood can learn to identify these different evolutionary strategies and enjoy their unique structures.

Arens, N.C. Lab V Lycophytes, UCMP Berkley:
http://www.ucmp.berkeley.edu/IB181/VPL
/Lyco/Lyco2.html#arborescent

Arens, N.C. Lab III Plant Fossils & Their Preservation: Virtual Gallery
http://www.ucmp.berkeley.edu/IB181/VPL/Pres/PresVG.html

Hoadley, B.R. (1990). Identifying Wood: Accurate Results with Simple Tools. Newton, Connecticut: Taunton Books & Videos.

Kenrick, P. and Davis, P. (2004). Fossil Plants. Smithsonian Books: Washington.

Miller, C.N.Jr. (1971). Evolution of the Fern Family Osmundaceae Based on Anatomical Studies. Contributions From the Museum of Paleontology The University of Michigan, Vol 23, No.8, pg 105-169.

Selmeier, A. (1996). Identification of Petrified Wood Made Easy. In Dernbach, U. Petrified Forest: The World's 31 Most Beautiful Petrified Forests (pp. 136-147). Germany: D’ORO Publishers.

Raven, P.H., Evert, R.F., & Curtis, H. (1981). Biology of Plants [3rd Ed]. New York: Worth Publishers, Inc.

Tidwell, W.D. (2002). The Osmundaceae: A Very Ancient Group of Ferns. In Dernbach, U. & Tidwell, W.D. Secrets of Petrified Plants: Fascination from Millions of Years (pp. 135-147). Germany: D’ORO Publishers.

Willis, K.J. & McElwain, J.C. (2002). The Evolution of Plants. New York: Oxford Univeristy Press.