Structure and Form

About 25 species of horsetails (the name usually applied to branching forms that look a little like a horse's tail) and scouring rushes (unbranched forms) (Fig. 21.10) are scattered throughout all continents, including Australia, where they are weeds. They usually grow less than 1.3 meters (4 feet) tall, but some in the tropics and coastal redwood forests of California exceed 4.6 meters (15 feet) in height. Where branches occur,

406 Chapter 21

Paleobotany
Figure 21. 9 A small portion of the surface of the fossil lycopod, Lepidodendron, showing leaf (microphyll) bases similar to those seen in modern lycopods. (Specimen courtesy University of Illinois Paleobotany Laboratory)

they are normally in whorls at regular intervals along the jointed stems. Both branched and unbranched species have tiny scalelike leaves (microphylls) in whorls at the nodes. These leaves are fused together at their bases, forming a collar. They are green when they first appear, but they soon wither and bleach, and virtually all photosynthesis occurs in the stems.

The stems are distinctly ribbed and have obvious nodes and internodes; there are numerous stomata in the grooves between the ribs. A cross section of a stem (Fig. 21.11) reveals that the pith breaks down at maturity, leaving a hollow central canal. There are two cylinders of smaller canals outside of the pith. The inner cylinder consists of water-conducting carinal canals that are aligned opposite the ribs of the stem. Each canal has a patch of xylem and phloem to the outside. The canals of the outer cylinder, called vallecular canals, contain air. They are larger than the carinal canals and are aligned opposite the "valleys" between the ribs.

The aerial stems develop from horizontal rhizomes, which also have regular nodes, internodes, and ribs. In some species, the rhizomes have adventitious roots and may form extensive branching systems as much as 2 meters (6.5 feet)

Figure 21. 10 Horsetails (Equisetum). A. An unbranched species. B. A branched species.
Equisetum Stem Cross Section

Figure 21.11 Ahorsetail (Equisetum) stem in cross section. (Photomicrograph by G. S. Ellmore)

epidermis (silica is deposited mostly on the inner walls of epidermal cells)

chlorenchyma cortex sclerenchyma endodermis carinal canal pith central canal vallecular canal

Figure 21.11 Ahorsetail (Equisetum) stem in cross section. (Photomicrograph by G. S. Ellmore)

below the surface. Both internal stem structure and external features help distinguish the various species of horsetails from one another.

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