Long before there was any real appreciation of the diversity of fossil plants, the French botanist Octave Lignier (1908) advanced a theory about the morphological changes necessary during the move onto the land and the evolution of roots. His hypothesis used an algal ancestor with a three-dimensional, dichotomously branched system that was periodically desiccated during fluctuations of available water. According to Lignier's scenario, one segment of the branching system became covered with substrate and over time assumed the function of an anchoring and absorbing organ, much like a root. Thus, the "root" of this early land plant would be homologous with an aerial branch system, differing only in function. Although Lignier's scenario began with an alga that we now know is not closely related to land plants, his hypothesis of a morphological model is strengthened by the occurrence of some early land plants with no organ differentiation between aerial stem and prostrate rhizome (e.g., Aglaophyton major, see Chapter 8). These permineralized axes possess the same complement of cells and tissue systems in both the aboveground aerial axes and the rhizomes. Additionally, the rhizomes produced tufts of rhizoids only on those portions in direct contact with the substrate and stomata can also be found on the rhizome, indicating that these parts of the plant may have been photosynthetic.
Lignier's model addresses a fundamental need for land plants—some way to anchor themselves to the substrate. In addition, unless a plant has a flat, thalloid plant body, the underground portions must also serve to anchor upright axes. The early land plants and the modern vascular cryptogams have no true roots, that is, with the specialized anatomy and morphology of roots (Chapter 7). Their anchoring organ is generally a rhizome, a horizontal stem which is either below ground level or on the surface of the substrate. These early plants produced rhizoids, small, usually unicellular hair-like structures, on the rhizomes, which absorbed water and minerals from the substrate.
Once an early land plant was anchored in the substrate, some method of moving water and nutrients from the substrate to the rest of the plant was needed. The ancestral aquatic alga would have been suspended in water and water could easily enter the organism by diffusion and osmosis; the terrestrial realm, however, was a hostile and desiccating environment. One of the most important structural adaptations of the plant body was the evolution of mechanisms and structures to both obtain and conserve water. A plant growing on land has some water in the surrounding air, for example as rain, fog, or dew, but potentially more water in the substrate in which it is anchored. Thus, early land plants had to have a system to absorb water from the air and/or the substrate, as well as effective mechanisms to prevent water loss (discussed later) in order to survive periods of drought. Extant land plants overcome these obstacles in two different ways. Bryophytes are poikilohydric, that is they have no specialized mechanism to prevent desiccation, but many can tolerate desiccation and rehydrate later. Vascular plants, however, have evolved homoiohydry, that is the capacity to remain hydrated internally. This adaptation, however, except in a few rare cases (e.g., Selaginella lepidophylla, the resurrection plant), is coupled with vegetative intolerance of desiccation (Proctor and Tuba, 2002; Proctor et al., 2007). Water uptake by bryophytes occurs in the form of simple diffusion and osmosis, either internally or externally. Some extant liverworts, for example certain members of the Calobryales and Metzgeriales, contain endohydric conduits (those on the inside of the thallus), and some mosses, for example in the Bryales and Polytrichales, possess hydroids and leptoids that are functionally equivalent to the xylem and phloem of vascular plants, although the hydroids are structurally very different from tracheids (Hebant, 1977; Ligrone et al., 2000). In contrast, vascular land plants use a variety of specialized subterranean and aerial absorbing structures, which allow them to live in almost any environment on Earth.
As noted earlier, a successful transition to land required structural modification for upright support of the plant. It is impossible to separate a discussion of support in early, upright land plants from water transport, since in most living vascular plants, the vascular tissues (xylem and phloem) are involved in both support and conduction (Chapter 7). In many early land plants, however, it appears that the central strand initially functioned primarily in conduction and that the plant stood erect as a result of turgor pressure in the parenchymatous cells of the axis (Speck and Vogellehner, 1988; Niklas, 1990). As land plants continued to evolve and grew larger, vascular tissue also took over the role of support, as it does in modern vascular plants. Tracheary elements in the xylem and fibers in the phloem (Chapter 7) have secondary cell walls impregnated with lignin, a polyphenolic polymer which provides structural support and flexural stiffness to the plant organ. Many of the early land plants, however, exhibit a central strand of conducting elements, but these do not have the secondary wall thickenings that are characteristic of xylem tracheids (Kenrick and Crane, 1991). Instead, the central strand is made up of a series of tubes, some with internal or external bands that superficially resemble tra-cheid thickenings (Chapter 8). These bands, however, are made of primary cell wall material and do not appear to be lignified, but based on their location and structure they must have served in conducting water throughout the plant. The discovery of these interesting cell types in the central strand of plants that were once thought to contain vascular tissue necessitates a continued reexamination of all early land plant cell types, especially those involved in conduction.
protection against desiccation and radiation
In addition to structural support and conduction, evolving land plants also required a method to retain water in a desiccating environment. It is believed that these organisms existed for at least a portion of their life history in a terrestrial, desiccating environment where uncontrolled transpiration, and thus water loss, presented a major physiological problem. Many of the enigmatic organisms discussed earlier in this chapter possess a non-cellular outer envelope that has been referred to as a cuticle or cuticle-like layer, and may have been effective as a boundary layer against excessive transpiration. The presence of sporopollenin in the spore wall represents a similar adaptation to prevent desiccation of reproductive propagules.
At the same time, the cuticle or cuticle-like layer may have been effective in the attenuation of UV radiation, including the especially dangerous UV-B (Raven, 2000). As noted earlier, this would have been a particularly important function of the cuticle in terrestrial habitats. It is interesting to note that many of the enigmatic Silurian-Devonian organisms, such as Orestovia and some early embryophytes, are characterized by a massive cuticle, which exceeds in thickness that of most plants found in geologically younger rocks.
Once early terrestrial plants had developed a cuticle, they would then need some means for gas exchange (Raven, 2002), as the cuticle is only very weakly permeable to gases. Although the process of photosynthesis functions in both aquatic algae and terrestrial plants, the source of carbon dioxide for each system is quite different. In algae, carbon dioxide dissolved in the water is available to chloroplast-containing cells through osmosis, whereas in most land plants carbon dioxide enters the plant through specialized openings termed sto-mata (Chapter 7). The regulation of these openings provides for a physiological balance within the plant (Hetherington and Woodward, 2003), a system that is regulated by other means in algae. Stomata have been identified on the naked aerial axes of many early land plants (Edwards et al., 1998a; Habgood et al., 2002); they have also been reported as occurring on the axes of the free-living gametophyte generation of several Rhynie chert plants (Kerp et al., 2004) (Chapter 8). Less specialized pores in the cuticle (FIG. 6.14) occur on other presumably terrestrial organisms, such as Spongiophyton and Orestovia, suggesting that these plants also may have carried out some level of gas exchange (Chaloner et al., 1974). Like the pores in extant liverworts, however, these unspecialized pores do not appear to include a mechanism to regulate their opening and closing, as do the stomata of vascular plants.
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