15. The Regulation of Dorsiventral Symmetry in Plants
225
Selaginella spp.) bear an enigmatic structure known as a ligule on the adaxial
surface, and others are associated with sporangia in the axils or on the adaxial
surfaces and are referred to as sporophylls. Thus, microphylJs exhibit ab-adaxial
asymmetry, some with varying degrees of anatomical and functional specialization.
The most common leaf type in the vascular plants is termed a megaphyll.
Megaphylls are flattened structures with a relatively complex system of veins; they
may be simple or compound. Megaphylls of the type represented by fern fronds are
hypothesized to have evolved from lower Devonian to lower Carboniferous "prefern" ancestors possessing a three-dimensional branching system with small "sterile appendages." The flattened highly compound fronds of ancient and modern
ferns can be derived by invoking the hypothetical processes of overtopping, planation and webbing of Zimmermann's "telome theory" (Zimmermann 1965). Accordingly, the phylogenetic and morphological homology of fern fronds would be
to the branch systems of the earliest vascular plants.
The typical fern leaf is dorsi ventral, with stomata generally occurring on the
abaxial side. The mesophyll may consist of homogeneous parenchyma with chloroplasts (chlorenchyma) or be organized into definite adaxial palisade and abaxial
spongy parenchyma layers (Bower 1935; Ogura 1972; Gifford and Foster 1989).
Sporangia may be borne singly, in clusters, in a row on or near the margin of the
leaf, or more commonly, on the abaxial surface.
The megaphylls of modern seed plants (cycads, Ginkgo, conifers, gnetophytes,
and angiosperms) are now thought likely to have had a different evolutionary origin than fern megaphylls, although some of the same elementary processes of the
telome theory are invoked. Even within the gymnospermous seed plants, megaphylls
of the cycadophyte line and of the coniferophyte line also may have had different
evolutionary origins. According to Beck's interpretation, the compound cycadophyte leaf was derived from an entire lateral branch system of a pro gymnosperm
ancestor, while only the ultimate segments of the lateral branch became a leaf in
the coniferophyte line (Beck 1976, 1981; Stewart and Rothwell 1993). In both lines,
planation and webbing of ultimate segments on a system of dichotomously branching axes can be envisioned to produce a compound or a simple leaf with branched
venation.
The needlelike leaves of many modern conifer genera (e.g., Pinus, Abies) were
considered by Fiorin (1950, 1951) to have resulted from simplification and reduction from a dichotomously branched appendage. In spite of their simple morphology, conifer leaves possess an intricate and complicated anatomy, including (in
different genera), unifacial and bifacial mesophyll, specialized mesophyll parenchyma ("folded parenchyma"), and accessory conducting cells (transfusion tissue)
and internal barriers (endodermis).
However, the theoretical derivations assumed by the telome theory are problematic, since they do not represent observable morphogenetic processes (Sattler 1998).
Moreover, it has been questioned that radial organ symmetry was the primitive
condition for vascular plants. According to Hagemann (1976), ancestral land plants
225
Selaginella spp.) bear an enigmatic structure known as a ligule on the adaxial
surface, and others are associated with sporangia in the axils or on the adaxial
surfaces and are referred to as sporophylls. Thus, microphylJs exhibit ab-adaxial
asymmetry, some with varying degrees of anatomical and functional specialization.
The most common leaf type in the vascular plants is termed a megaphyll.
Megaphylls are flattened structures with a relatively complex system of veins; they
may be simple or compound. Megaphylls of the type represented by fern fronds are
hypothesized to have evolved from lower Devonian to lower Carboniferous "prefern" ancestors possessing a three-dimensional branching system with small "sterile appendages." The flattened highly compound fronds of ancient and modern
ferns can be derived by invoking the hypothetical processes of overtopping, planation and webbing of Zimmermann's "telome theory" (Zimmermann 1965). Accordingly, the phylogenetic and morphological homology of fern fronds would be
to the branch systems of the earliest vascular plants.
The typical fern leaf is dorsi ventral, with stomata generally occurring on the
abaxial side. The mesophyll may consist of homogeneous parenchyma with chloroplasts (chlorenchyma) or be organized into definite adaxial palisade and abaxial
spongy parenchyma layers (Bower 1935; Ogura 1972; Gifford and Foster 1989).
Sporangia may be borne singly, in clusters, in a row on or near the margin of the
leaf, or more commonly, on the abaxial surface.
The megaphylls of modern seed plants (cycads, Ginkgo, conifers, gnetophytes,
and angiosperms) are now thought likely to have had a different evolutionary origin than fern megaphylls, although some of the same elementary processes of the
telome theory are invoked. Even within the gymnospermous seed plants, megaphylls
of the cycadophyte line and of the coniferophyte line also may have had different
evolutionary origins. According to Beck's interpretation, the compound cycadophyte leaf was derived from an entire lateral branch system of a pro gymnosperm
ancestor, while only the ultimate segments of the lateral branch became a leaf in
the coniferophyte line (Beck 1976, 1981; Stewart and Rothwell 1993). In both lines,
planation and webbing of ultimate segments on a system of dichotomously branching axes can be envisioned to produce a compound or a simple leaf with branched
venation.
The needlelike leaves of many modern conifer genera (e.g., Pinus, Abies) were
considered by Fiorin (1950, 1951) to have resulted from simplification and reduction from a dichotomously branched appendage. In spite of their simple morphology, conifer leaves possess an intricate and complicated anatomy, including (in
different genera), unifacial and bifacial mesophyll, specialized mesophyll parenchyma ("folded parenchyma"), and accessory conducting cells (transfusion tissue)
and internal barriers (endodermis).
However, the theoretical derivations assumed by the telome theory are problematic, since they do not represent observable morphogenetic processes (Sattler 1998).
Moreover, it has been questioned that radial organ symmetry was the primitive
condition for vascular plants. According to Hagemann (1976), ancestral land plants