230
S. Gleissberg et al.
are unifacial, giving rise to three cross zones at their respective borders to the bifacial blade and sheath. The phenotypes of phan and lbll mutants, in which ectopic
lamina ridges are formed at the borders of abaxial and adaxial tissue sectors, parallel the development of cross zones at the borders between bifacial and unifacialleaf
segments.
The adaxially fused margins at the borders of bifacial and unifacialleaf sectors
would seem to provide morphogenetic potential to enhance diversity during evolution. Cross zones between leaf sheath and petiole may develop median (instead of
paired lateral) stipules, as in Bergenia crassifolia. From the cross zone located at
the base of a blade adjacent to a unifacial petiole, median leaflets can arise (Rodgersia
aesculifolia), or, if this region is included in surface growth of the blade, peltate
leaves can form (Tropaeolum majus). Morphologically more derived forms such as
ascidiate leaves found in the carnivorous plants Sarracenia or Nepenthes are also
based on peltate leaves.
The reason why the course of the leaf margin is stressed by many authors in
judging the unifaciality syndrome is that it is frequently only partly reflected in the
configuration of tissues. In those cases, unifacialleaf sectors retain the open bundle
arc and xylem orientation typical for bifacialleaves (petiole of Acer mono). On the
othcr hand, many bifacial leaves develop identical epidermis and mesophyll on
both sides (equifacialleaves, Eucalyptlls globosa) or show an inverse arrangement
with palisade parenchyma on the abaxial side (Alstroemeria pelegrina). This indicates that the determination of adaxial and abaxial identities may have an early
morphogenetic role (in designating the course of the leaf margin which in turn
allows for oriented surface development) and a later histogenetic role in defining
tissue identities in the respective domains.
5 The Ontogenetic Origin of Unifaciality
The point in time after initiation that a leaf acquires ab-adaxial symmetry has been
the subject of controversy. Although some authors viewed leaves as initially radial
protuberances that acquire flattening only after emergence (Cutter 1958; Steeves
and Sussex 1989), others believe that leaf primordia are initially subjected to adaxialabaxial polarity resulting from their position in a polar field at the flanks of the
shoot apex (Wardlaw 1968; Hagemann 1970). Hence, unifacialleaf parts that lack
transverse asymmetry would arise, in principle, postgenitally. The latter point of
view is also indicated by the bifacial leaf base present in all unifacial leaves.
It has been observed that increased thickening growth of the leaf primordium
c~n ultimately produce a rounded, unifacial segment (e.g., petiole, forerunner tip).
At the same time, development of the margin is suppressed so that lamina outgrowth does not take place. The adaxial bundles of the vascular circle in such
unifacial sectors differentiate from tissue produced by the adaxial thickening meristem. However, both adaxially and abaxially located tissue of the primordium contribute to thickening (Roth 1949; Kaplan 1975).
S. Gleissberg et al.
are unifacial, giving rise to three cross zones at their respective borders to the bifacial blade and sheath. The phenotypes of phan and lbll mutants, in which ectopic
lamina ridges are formed at the borders of abaxial and adaxial tissue sectors, parallel the development of cross zones at the borders between bifacial and unifacialleaf
segments.
The adaxially fused margins at the borders of bifacial and unifacialleaf sectors
would seem to provide morphogenetic potential to enhance diversity during evolution. Cross zones between leaf sheath and petiole may develop median (instead of
paired lateral) stipules, as in Bergenia crassifolia. From the cross zone located at
the base of a blade adjacent to a unifacial petiole, median leaflets can arise (Rodgersia
aesculifolia), or, if this region is included in surface growth of the blade, peltate
leaves can form (Tropaeolum majus). Morphologically more derived forms such as
ascidiate leaves found in the carnivorous plants Sarracenia or Nepenthes are also
based on peltate leaves.
The reason why the course of the leaf margin is stressed by many authors in
judging the unifaciality syndrome is that it is frequently only partly reflected in the
configuration of tissues. In those cases, unifacialleaf sectors retain the open bundle
arc and xylem orientation typical for bifacialleaves (petiole of Acer mono). On the
othcr hand, many bifacial leaves develop identical epidermis and mesophyll on
both sides (equifacialleaves, Eucalyptlls globosa) or show an inverse arrangement
with palisade parenchyma on the abaxial side (Alstroemeria pelegrina). This indicates that the determination of adaxial and abaxial identities may have an early
morphogenetic role (in designating the course of the leaf margin which in turn
allows for oriented surface development) and a later histogenetic role in defining
tissue identities in the respective domains.
5 The Ontogenetic Origin of Unifaciality
The point in time after initiation that a leaf acquires ab-adaxial symmetry has been
the subject of controversy. Although some authors viewed leaves as initially radial
protuberances that acquire flattening only after emergence (Cutter 1958; Steeves
and Sussex 1989), others believe that leaf primordia are initially subjected to adaxialabaxial polarity resulting from their position in a polar field at the flanks of the
shoot apex (Wardlaw 1968; Hagemann 1970). Hence, unifacialleaf parts that lack
transverse asymmetry would arise, in principle, postgenitally. The latter point of
view is also indicated by the bifacial leaf base present in all unifacial leaves.
It has been observed that increased thickening growth of the leaf primordium
c~n ultimately produce a rounded, unifacial segment (e.g., petiole, forerunner tip).
At the same time, development of the margin is suppressed so that lamina outgrowth does not take place. The adaxial bundles of the vascular circle in such
unifacial sectors differentiate from tissue produced by the adaxial thickening meristem. However, both adaxially and abaxially located tissue of the primordium contribute to thickening (Roth 1949; Kaplan 1975).