Megaphylls
These are leaves associated with stems that have either a siphonostele or eustele, and are attached to the stem by a petiole (Figure 4.7b). Their evolution is thought to be closely linked to the three-dimensional vegetative branching pattern (i.e. branches with no sporangia on them) of the earliest vascular plants
- (a) Microphylls (0
Figure 4.7 (a) Formation of microphylis according to (i) the 'Enation theory' (Bower, 1935), which proposed that these 'stem-hugging leaves' evolved from the outgrowth of vascular tissue into the spines on the stems, and (ii) the 'Telome theory', which suggested that they were the end product of an evolutionary series of reduction from once-forked side branches, (b) Formation of megaphylls according to 'Telome' theory' (Zimmerman, 1952), which proposed that these leaves evolved by a fusion of non-fertile branches, with undifferentiated tissue forming webbing between the branches. (Redrawn from Stewart and Rothwell, 1993.)
Figure 4.7 (a) Formation of microphylis according to (i) the 'Enation theory' (Bower, 1935), which proposed that these 'stem-hugging leaves' evolved from the outgrowth of vascular tissue into the spines on the stems, and (ii) the 'Telome theory', which suggested that they were the end product of an evolutionary series of reduction from once-forked side branches, (b) Formation of megaphylls according to 'Telome' theory' (Zimmerman, 1952), which proposed that these leaves evolved by a fusion of non-fertile branches, with undifferentiated tissue forming webbing between the branches. (Redrawn from Stewart and Rothwell, 1993.)
(e.g. Psilophyton dawsonii, Figure 3.17). According to the Telome theory (Zimmerman, 1952), megaphylls are thought to have evolved by a fusion of these non-fertile branches, with undifferentiated tissue forming webbing between the branches. The former branches would thus provide vascular tissue into the megaphyll leaf, resulting in the evolution of a broad leaf containing many vascular strands (Figure 4.7b). Early and extant filicopsids (ferns), a number of extinct sphenopsids (horsetails), and all flowering plants possess megaphylls.
Recent studies based on morphological observations and modelling of the biophysical principles of plant physiology suggest that evolution of the megaphyll leaf may have occurred in response to the massive reduction in atmospheric C02 during the late Devonian and early Carboniferous (Robinson, 1991; Beerling et al., 2001). It is suggested that in the high C02 world of the early and mid Devonian (~417-370 Ma), the evolution of a megaphyll leaf would have been detrimental to plant survival, as leaf temperatures would have exceeded lethal threshold limits. It was not, therefore, until the late Devoninan/ early Carboniferous, when C02 levels plummetted, that increased leaf size would have been advantageous to land plants (Beerling et al., 2001).
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