Among land plants, we can distinguish two contrasting patterns of growth. In one pattern, the main plant stems (rhizomes) creep through the soil or along a tree trunk, branching occasionally and extending the territory of the plant colony (see Figs. 4.8, 4.10). This is the pattern of typical ferns, club mosses, and horse- tails, as well as most bryophytes. Such colonies can live for thousands of years,
Figure 4.11 An ancient sphenophyte, Sphenophyllum, had whorls of fan-shaped leaves at
each node. Sporangia were borne in branched clusters on top of scalelike leaves. Drawings
units (B) that hide a ring of sporangia underneath. The shield-like units evolved from branched structures (C) in which the sporangia were bent back toward the stalk; spores of horsetails (D, E) have elongate appendages called elaters that twist and turn with changing humidity, pushing the spores out of the sporangia. Drawings from Kerner & Oliver 1895 (A, B, D, E) and Haupt 1953 (C).
Figure 4.13 Psilotum (A) has forking stems and lacks roots and leaves; the sporangia (B)
occur in fused clusters of three and are attached along the sides of the stem rather than
being solitary at the tip as in the Rhyniophytes; Tmesipteris (C) is a cousin of Psilotum
though they will fragment through decay of older sections. Leaves may be spread out along the horizontal stem individually or clustered in short vertical shoots. Plants that “move about” by creeping rhizomes have been compared by Halle (2002) to bilaterally (two-sided) symmetrical animals such as centipedes. A fern rhizome does grow at one end (the “head”) and decay at the other (the “tail”), producing roots (the “legs”) along the way, so moves slowly forward, but the anal- ogy is complicated by the by the fact that rhizomes branch and remain connected to one another.
In the other pattern, exhibited by bird’s-nest ferns of the genus Asplenium (Fig. 4.14), territorial conquest and immortality are sacrificed for permanent anchorage at a single favorable spot. These plants face upwards, and their leaves form a compact rosette (roundish cluster resembling a rose blossom) around the terminal bud. Halle compares these vertical plants to radially (wheel-like) sym- metrical animals such as sea anemones. One could theoretically rotate a radially symmetrical organism around its central axis without affecting its orientation to the environment, but rotating a creeping fern rhizome around its axis would plunge leaves into the ground and lift roots into the air. Bilateral and radial
Figure 4.14 Some ferns, such as this Asplenium or bird’s-nest fern, have adopted an
symmetries are not always as precisely defined in plants as they are in animals, but the analogy is useful for distinguishing the different growth strategies in plants. We will later see radially and bilaterally symmetrical flowers, which are adapted for different pollination strategies.
If the stem of an upward-facing rosette were to gradually increase in height, we would have something we could call a tree. That is exactly what happens in tree ferns, which are a common sight in moist tropical or subtropical forests, par- ticularly in Australia, New Zealand, and other parts of the Pacific region. Tree ferns, which superficially resemble palm trees, have large leaves, divided into many small leaflets and supported by strong cords of fibers in the leaf stalks (peti- oles). The unbranched trunks of tree ferns consist of a slender core of vascular tis- sues and fibers, covered from top to bottom with a mass of short, absorptive roots (Fig. 4.15). There is, however, no wood in a tree fern—no way to increase its thick- ness over time.
The club mosses and horsetails of today survive today as modest, creeping plants, but each had ancient relatives that attained treelike proportions, with trunks thickened with wood. Wood consists of layers of the water-conducting
Figure 4.15 A tree fern, of the genus Alsophila, exhibits upright growth of a fibrous,
vascular tissue, xylem, which builds up over time. As it builds up, this second- ary xylem also provides the physical support necessary to support the increasing weight of the branching crown of the tree.
Wood was made possible by a new invention, the vascular cambium, which is a third type of meristem (after apical and intercalary). The vascular cambium consists of a more-or-less cylindrical array of embryonic cells wrapped around the core of the stem (and in seed plants, of the roots as well), which divide to produce new vascular tissues. In modern trees, the cambium produces new layers of xylem to the inside and new layers of phloem to the outside (Fig. 4.16). The accumula- tion of wood beneath it causes the vascular cambium to expand over time.
Giant horsetails and club mosses were rather limited as trees go, however, because their vascular cambia could produce only layers of xylem, not phloem. Phloem is the tissue required to transport photosynthetic product from the leaves to other parts of the plant. When the original phloem wore out, the trees declined and died, and so they were probably relatively short lived.
Trees also require a root system that can provide support for a massive trunk and crown. The giant club mosses, such as Lepidodendron and Sigillaria (Fig. 4.17A,B), sat on pedestals of specialized spreading stems at the base of the tree, and from them small roots, or modified leaves (Raven & Edwards, 2001), emerged to absorb water and nutrients. Giant horsetails, such as Calamites (Fig. 4.17C), on the other hand, had massive rhizomes from which their large, bamboo-like upright shoots arose. Like bamboos today, the trunks of these giant horsetails most likely extended rapidly upward through the synchronized elongation of their internodes. Giant club mosses and horsetails formed the basis of the first forests, which appeared toward the end of the Devonian Period. They flourished in extensive swamp forests throughout the Carboniferous Period, building up the massive coal deposits that modern society is now so rapidly turning back into carbon dioxide.
As big as they got, club moss and horsetail trees were short lived. Modern trees live for many years, centuries even, not just because they can continually produce new layers of wood, but because they can also produce new layers of phloem. The key to this was a vascular cambium that could alternately push new cells to the inside and to the outside. Those pushed to the outside became new phloem tis- sues and those pushed to the inside became xylem. Such a two-faced (bifacial) cambium is characteristic of woody seed plants, but it first appeared in a group of seedless vascular plants called progymnosperms, the most famous of which was the massive Archaeopteris (Fig. 4.18). This more versatile cambium arose inde- pendently from those in club mosses and horsetails. These stronger, more durable trees came to dominate in the middle to late Devonian period. Their deep roots and leaf litter contributed to the development of the first thick layers of soil and the major ecological changes mentioned at the beginning of the chapter.
The wood of progymnosperms and the early seed plants that followed consisted mostly of the simple tracheids laid down in concentric growth rings. A trunk full
Figure 4.17 Two treelike lycopods from the Coal Age were Lepidodendron (A) and Sigillaria (B), which had spreading rhizome branches to form a basal pedestal. Calamites
(C) was apparently more like a bamboo, with upright shoots arising from horizontal rhizomes. Drawings from Smith 1935.
Vascular cambium
New phloem
Figure 4.16 In a modern vascular cambium, the cells of the meristematic layer divide so as to produce new cells alternately to the inside (xylem) and to the outside (phloem). Drawing (A) from Ganong 1916.
of densely packed tracheids creates millions of tiny capillary passageways for the upward movement of water. The cohesion of water molecules plus the adhesion of those molecules to the sides of the tracheids allows them to continually move upward as evaporation at the top of the tree drains them away. Thus, movement of water up even the tallest of trees is passive and is due to strictly physical processes. The maximum height of a tall tree represents a balance point between the upward pull of transpiration and the cumulative weight of the water in the tree.