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S. Gleissberg et al.
could have had a two-dimensional organization, as seen in the thalli of liverworts
and fern prothalli. From this perspective, radial shoot axes would have arisen later
in phylogeny as specialized structures. Hagemann (1976) noted that some extant
ferns (e.g., Hypolepidaceae) still exhibit transversely flattened shoots that bear leaves
in the same plane of symmetry. According to Sattler (1998), some primitive land
plants (Cooksonia, Zosterophyllum) already had both radial and flattened organization.
Despite the probable differences in evolutionary origin and mature form and
structure, enations, microphylls and megaphylls have many features in common.
Among these are their determinate growth, multicellular origin, and initiation at
the shoot apical meristem by cell divisions in the surface and underlying layers.
Wardlaw (1957) reported that leaf inception is generally comparable in all vascular
plants, regardless of leaf type. According to Wardlaw, the critical difference lies in
the inherent growth potentials of enation, microphyll and megaphyll primordia. In
megaphylls, prolongation of meristematic activity of leaf apical initials, especially
in the tangential plane, eventually results in the formation of leaf marginal meristems. Marginal meristems contribute to the formation of the lamina and its venation, both of which may be quite elaborate. In contrast, the apical initials of a microphyll or enation primordium (e.g., Lycopodium or Psi/otum, respectively) persist for a time as a group of distal meristematic cells without formation of marginal
meristems. The meristematic properties of the initial cells eventually disappear
after, depending on the duration, having given rise to a small lamina with a single
vein (e.g., Lycopodium) or a non-laminate, non-vascularized scale (e.g., Psi/otum)
(Wardlaw 1957). Although microphylls and megaphylls apparently arose independently, they both still exhibit ab-adaxiality. This suggests that "intrinsic" features
- the placement of primordia within the apical-basal gradient at the shoot tip -
may be necessary to achieve transversal symmetry of a leaf. Gene products might
have acquired the function to interpret this gradient and establish ab-adaxial polarity. It will be interesting to know if the same or different genes play this role in
microphyllous and megaphyllous plant groups. However, ultimately radial symmetric organs like axillary meristems also form near the shoot apex, so interpretation of the gradient must be different in the various appendages of the shoot apical
meristem.
A still unresolved question is at what point is a derivative organ on a shoot apex
determined as a bilateral leaf or a radial axillary bud? Cutter (1958) suggested that
there are at least two sets of factors controlling the placement and fate of lateral
organs. The phyllotactic factors determine organ placement while the organogenic
factors determine organ fate. When these two factors are separated in time it is
possible to convert the fate of an organ from bilateral to radial (as is seen in ferns).
However, when the two factors operate in an almost coincident fashion the resultant organ is almost always a bilaterally symmetric leaf. This occurs at position 12 or
earlier - as soon as its location is defined, the organ is also determined to become
a bilaterally symmetric leaf.
S. Gleissberg et al.
could have had a two-dimensional organization, as seen in the thalli of liverworts
and fern prothalli. From this perspective, radial shoot axes would have arisen later
in phylogeny as specialized structures. Hagemann (1976) noted that some extant
ferns (e.g., Hypolepidaceae) still exhibit transversely flattened shoots that bear leaves
in the same plane of symmetry. According to Sattler (1998), some primitive land
plants (Cooksonia, Zosterophyllum) already had both radial and flattened organization.
Despite the probable differences in evolutionary origin and mature form and
structure, enations, microphylls and megaphylls have many features in common.
Among these are their determinate growth, multicellular origin, and initiation at
the shoot apical meristem by cell divisions in the surface and underlying layers.
Wardlaw (1957) reported that leaf inception is generally comparable in all vascular
plants, regardless of leaf type. According to Wardlaw, the critical difference lies in
the inherent growth potentials of enation, microphyll and megaphyll primordia. In
megaphylls, prolongation of meristematic activity of leaf apical initials, especially
in the tangential plane, eventually results in the formation of leaf marginal meristems. Marginal meristems contribute to the formation of the lamina and its venation, both of which may be quite elaborate. In contrast, the apical initials of a microphyll or enation primordium (e.g., Lycopodium or Psi/otum, respectively) persist for a time as a group of distal meristematic cells without formation of marginal
meristems. The meristematic properties of the initial cells eventually disappear
after, depending on the duration, having given rise to a small lamina with a single
vein (e.g., Lycopodium) or a non-laminate, non-vascularized scale (e.g., Psi/otum)
(Wardlaw 1957). Although microphylls and megaphylls apparently arose independently, they both still exhibit ab-adaxiality. This suggests that "intrinsic" features
- the placement of primordia within the apical-basal gradient at the shoot tip -
may be necessary to achieve transversal symmetry of a leaf. Gene products might
have acquired the function to interpret this gradient and establish ab-adaxial polarity. It will be interesting to know if the same or different genes play this role in
microphyllous and megaphyllous plant groups. However, ultimately radial symmetric organs like axillary meristems also form near the shoot apex, so interpretation of the gradient must be different in the various appendages of the shoot apical
meristem.
A still unresolved question is at what point is a derivative organ on a shoot apex
determined as a bilateral leaf or a radial axillary bud? Cutter (1958) suggested that
there are at least two sets of factors controlling the placement and fate of lateral
organs. The phyllotactic factors determine organ placement while the organogenic
factors determine organ fate. When these two factors are separated in time it is
possible to convert the fate of an organ from bilateral to radial (as is seen in ferns).
However, when the two factors operate in an almost coincident fashion the resultant organ is almost always a bilaterally symmetric leaf. This occurs at position 12 or
earlier - as soon as its location is defined, the organ is also determined to become
a bilaterally symmetric leaf.