Early Paleozoic Trends

Despite these difficulties and limitations, the plant fossil record indicates that the most ancient floras were dominated by chamaephyte-like species and that these floras were replaced within a geologically short time by phaner-ophyte-dominated floras during the Early Paleozoic. The chameophyte-to-phanerophyte transition is evident in terms of broad evolutionary trends during the Early Paleozoic (Figure 3).

Specifically, the first land plants were nonvascular life-forms with a sexual life cycle in which a large, long-lived gametophyte generation alternated with a small, short-lived sporophyte generation. In contrast, the first vascular land plants had a life cycle dominated by a larger, long-lived sporophyte alternating with a small, short-lived gameto-phyte (e.g., Cooksonia and Steganotheca). The branching and stature of the sporophyte generation increased over the course of land plant evolution and the phyletic increase in the size of the sporophyte was attended by a transition from

Figure 3 Character transformations describing life-form evolutionary trends of ancient land plants during the Early Paleozoic (arrows indicate the direction of each transformation): (1) life cycle with a dominant (indeterminate growth) gametophyte to one with a dominant sporophyte, (2) unbranched to branched sporophyte generation, (3) nonvascular to vascular (xylem and phloem) sporophytes, (4) equally to unequally branched sporophytes, (5) evolution of secondary tissues, and (6) a transition from a nonseed habit to a seed habit.

Figure 3 Character transformations describing life-form evolutionary trends of ancient land plants during the Early Paleozoic (arrows indicate the direction of each transformation): (1) life cycle with a dominant (indeterminate growth) gametophyte to one with a dominant sporophyte, (2) unbranched to branched sporophyte generation, (3) nonvascular to vascular (xylem and phloem) sporophytes, (4) equally to unequally branched sporophytes, (5) evolution of secondary tissues, and (6) a transition from a nonseed habit to a seed habit.

near-equal (dichotomous) branching to overtopped branching patterns (e.g., Rhynia and Psilophyton, respectively). By the end of the Devonian, this trend culminated in arborescent life-forms within each of the major vascular plant lineages (e.g., Lepidodendron, Psaronius, Calamities, and Archaeopteris). The stems of progressively taller species contained varying quantities of xylary conducting elements. By the end of the Devonian, the tallest representatives in each of the major plant lineages possessed the capacity for secondary growth and reached heights comparable to the tallest extant species (e.g., arborescent lycopods, horsetails, and progymnosperms). Thus, over the course of early land plant history, the renewal buds (apical meristems) of the first land plants were progressively elevated above ground on the tallest representatives of each major vascular plant lineage in successively younger floras.

The advent of the seed habit by the end of the Devonian undoubtedly played a major role in the diversification of habitats suitable for plant growth. The retention of egg-bearing gametophytes within sporophytic tissues eliminated the dependency on the availability of free-standing water for sexual reproduction, which characterizes the life cycles of extinct and extant pteridophytes. Species with a seed habit could thus inhabit drier habitats than their pter-idophytic ancestors provided that sufficient water was available for vegetative growth.

The use of Raunkiaerian terminology to describe the evolutionary transition from ancient chamaephyte-like floras to more derived phanerophyte-like floras does not imply major changes from a hydric to a more xeric world climate. Significant climatic changes did occur during this time interval, but the evolutionary increase in maximum plant stature during the Early Paleozoic is 'transitional' and 'directional' only in the sense that the most ancient land plants lacked the anatomical and morphological features required to elevate apical meristems significant distances above ground. They also possessed a life cycle that required free-standing water to complete sexual reproduction. In contrast, some geologically younger des-cendents attained vegetative and reproductive features that permitted the adoption of a phanerophytic life-form, perhaps under selection regimes that conferred greater fitness to plants capable of elevating photosynthetic and reproductive organs higher than their neighbors.

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