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S. Gleissberg et al.
tion of polarity along the ab-adaxial (dorsiventral) axis. Considerable confusion
exists in the current literature about the terms dorsal and ventral. Differences between adaxial and abaxial leaf sides have often been described using dorsal and
ventral that compare them to the upper and lower sides of animals. However, while
a leaf's abaxial (lower) side was traditionally denominated its "dorsal" and the
adaxial (upper) its "ventral" side (Goebel 1928; Napp-Zinn 1973), the use of these
terms in just the opposite way in recent publications has created terminological
confusion. We will, therefore, use instead the more unequivocal terms adaxial (oriented towards the shoot tip) and abaxial (pointing to the base of the shoot) in the
description of transsectional asymmetry. Recently, several interesting studies have
revealed possible genetic regulation of symmetry in leaves and other plant organs.
Here, we review these results in light of data about the evolutionary and ontogenetic origin of abadaxiality and of related symmetry phenomena in other organs
and organisms.
2 Evolutionary History of Leaves
Some of the earliest known vascular plants, dating from the mid-Silurian to lower
Devonian, 395-420 million years ago, lacked leaves. These usually diminutive plants
(Cooksonia, Rhynia) had cylindrical photosynthetic stems which branched dichotomously several to many times. However, in a relatively short time by evolutionary
standards, a diversity of multicellular emergences and flattened appendages appearcd on stcms. Some of these are now recognized as early leaves.
Thc modern taxon Psilotum, of uncertain evolutionary affinity, bears small scalclikc structures helically arranged on the upper part of the acrial stem. Internally,
the appendage consists of photosynthetic parcnchyma cells which are continuous
with similar tissue in the stem. There is no vascular tissue in the appendages, although in P. comp/anatum, a vascular strand tcrminates at the basc of the foliar
structure. This lattcr arrangement, the termination of "leaf traces" at the bases of
vcinlcss enations, was also found in extinct lower Devonian lycopsids such as
Asteroxylon. An increase in the sizc of the foliar appendage is correlated with
cxtension of a vascular bundle into the lamina. Indeed, Bower (1935) proposed that
some of the earliest leaves evolved from nonvascularlized cpidermal outgrowths by
extension of the trace into the lamina of the enation. Fossil and modern members of
the Lycophyta are characterized by a particular type of leaf called a microphyll, in
which a single leaf trace extends as a (typically) unbranched midvein into each of
the blades. By the lower to middle Devonian, microphylls were distinctly dorsiventral and arranged in a definite phyllotactic pattern. Stomata were found on one or
both epidermal surfaces. Anatomical differentiation into palisade and spongy mesophyll layers is prescnt in some (e.g., Selaginella) but not all (e.g., Lycopodium)
extant microphyllous taxa (Gifford and Foster 1989). Microphylls range in size
from a length of a few millimeters in modern Selaginella spp. up to 1 meter in
length in Sigillaria, an extinct Carboniferous lepidodendrid. Some microphylls (e.g.,
S. Gleissberg et al.
tion of polarity along the ab-adaxial (dorsiventral) axis. Considerable confusion
exists in the current literature about the terms dorsal and ventral. Differences between adaxial and abaxial leaf sides have often been described using dorsal and
ventral that compare them to the upper and lower sides of animals. However, while
a leaf's abaxial (lower) side was traditionally denominated its "dorsal" and the
adaxial (upper) its "ventral" side (Goebel 1928; Napp-Zinn 1973), the use of these
terms in just the opposite way in recent publications has created terminological
confusion. We will, therefore, use instead the more unequivocal terms adaxial (oriented towards the shoot tip) and abaxial (pointing to the base of the shoot) in the
description of transsectional asymmetry. Recently, several interesting studies have
revealed possible genetic regulation of symmetry in leaves and other plant organs.
Here, we review these results in light of data about the evolutionary and ontogenetic origin of abadaxiality and of related symmetry phenomena in other organs
and organisms.
2 Evolutionary History of Leaves
Some of the earliest known vascular plants, dating from the mid-Silurian to lower
Devonian, 395-420 million years ago, lacked leaves. These usually diminutive plants
(Cooksonia, Rhynia) had cylindrical photosynthetic stems which branched dichotomously several to many times. However, in a relatively short time by evolutionary
standards, a diversity of multicellular emergences and flattened appendages appearcd on stcms. Some of these are now recognized as early leaves.
Thc modern taxon Psilotum, of uncertain evolutionary affinity, bears small scalclikc structures helically arranged on the upper part of the acrial stem. Internally,
the appendage consists of photosynthetic parcnchyma cells which are continuous
with similar tissue in the stem. There is no vascular tissue in the appendages, although in P. comp/anatum, a vascular strand tcrminates at the basc of the foliar
structure. This lattcr arrangement, the termination of "leaf traces" at the bases of
vcinlcss enations, was also found in extinct lower Devonian lycopsids such as
Asteroxylon. An increase in the sizc of the foliar appendage is correlated with
cxtension of a vascular bundle into the lamina. Indeed, Bower (1935) proposed that
some of the earliest leaves evolved from nonvascularlized cpidermal outgrowths by
extension of the trace into the lamina of the enation. Fossil and modern members of
the Lycophyta are characterized by a particular type of leaf called a microphyll, in
which a single leaf trace extends as a (typically) unbranched midvein into each of
the blades. By the lower to middle Devonian, microphylls were distinctly dorsiventral and arranged in a definite phyllotactic pattern. Stomata were found on one or
both epidermal surfaces. Anatomical differentiation into palisade and spongy mesophyll layers is prescnt in some (e.g., Selaginella) but not all (e.g., Lycopodium)
extant microphyllous taxa (Gifford and Foster 1989). Microphylls range in size
from a length of a few millimeters in modern Selaginella spp. up to 1 meter in
length in Sigillaria, an extinct Carboniferous lepidodendrid. Some microphylls (e.g.,