Evolution of the Microphyll

One of the synapomorphies of the lycophytes is the presence of microphylls. Microphylls are true leaves, and as such, are borne in a definite pattern (phyllotaxy) on the stem, but they have an evolutionary history separate from the leaves of other vascular plants (megaphylls, see discussion in Chapter 11). Extant microphylls are small, although not all fossil microphylls are small. They usually contain a single vascular bundle (vein) and there is no leaf gap formed in the stele during the production of leaf traces (Chapter 7). Microphylls are believed to have evolved from enations, which as noted in the previous chapter are small, unvascularized flaps of tissue that do not have a phyllotaxy. There are several fossils that could be used to illustrate the stages in the evolution of microphylls. Beginning with a naked, vascularized axis (FIG. 9.1 A), the first laterals produced were small, scattered spines, as seen in many zos-terophylls, for example Sawdonia (FIG. 9.1B). Asteroxylon illustrates an intermediate stage (FIG. 9.1C ) , in which the laterals (enations) are more leaf-like, but vascular tissue extends only to the base of each enation. Finally, the laterals

(microphylls) become vascularized and are borne in a regular pattern on the axis, for example as seen in Leclercqia (FIG. 9.1D ) . There are several fossil plants that represent intermediates between stages B, C, and D, and some of these are included in the Drepanophycales. Kenrick and Crane (1997a) suggested several hypotheses on the origin of the lycophyte sporophyll, including reduction from a lateral branch (FIG. 9.2A) , fusion of a sporangium to an enation-microphyll (FIG. 9.2B), and sterilization of a second sporangium (FIG. 9.2C).

A study of stelar and microphyll vasculature in two extant Lycopodium species (Gola et al., 2007) confirms the unique nature of microphylls, especially when compared to mega-phylls, the leaf type found in other vascular plants (see section "Evolution of Megaphyll" in Chapter 11). This study reveals that the production of microphylls and the architecture of the stele are not closely connected developmentally, as they are in seed plants. Based on these data, the authors concluded that the origin of the microphyll and the development of vasculature to supply the leaf in the lycopsids may have evolved independently, a theory which is supported by many of the plants discussed in the next section.

Enation Theory Images

figure 9.1 Hypothesized stages in the evolution of the microphyll. A. Vascularized axis (shaded area) lacking epidermal appendages (e.g., Rhynia). B. Axis with epidermal appendages in the form of spines or enations (e.g., Sawdonia). C. Axis with vascular traces extending to the base of the enation (e.g., Asteroxylon). D. Axis with vascularized microphylls (e.g., Leclercqia). (From Taylor and Taylor, 1993.)

figure 9.1 Hypothesized stages in the evolution of the microphyll. A. Vascularized axis (shaded area) lacking epidermal appendages (e.g., Rhynia). B. Axis with epidermal appendages in the form of spines or enations (e.g., Sawdonia). C. Axis with vascular traces extending to the base of the enation (e.g., Asteroxylon). D. Axis with vascularized microphylls (e.g., Leclercqia). (From Taylor and Taylor, 1993.)

figure 9.2 Suggested transformational stages in the evolution of the microphyll with sporangium in lycopsids. A. Reduction from a lateral branch system. B. Origin from a sterile stem appendage (enation theory). C. Origin from a sterilized sporangium. (Modified from Kenrick and Crane, 1997a.)

figure 9.2 Suggested transformational stages in the evolution of the microphyll with sporangium in lycopsids. A. Reduction from a lateral branch system. B. Origin from a sterile stem appendage (enation theory). C. Origin from a sterilized sporangium. (Modified from Kenrick and Crane, 1997a.)

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