15. The Regulation of Dorsiventral Symmetry in Plants
227
3 Leaf-like Shoots/Flattened Stems
Leaves are not the only plant organs to exhibit a flattened or dorsi ventral organization. Entire shoots of some flowering plant genera (and a single gymnosperm genus) show morphological modifications which result in strikingly flattened and
often leaflike structures (Troll 1937).
Troll (1937) recognized two main types of flattened shoots: platyclades and
phylloclades. Platyclades, such as Homalocladium platycladium (Polygonaceae),
are flattened shoots which develop a number of conspicuous nodes and internodes.
Platyclades exhibit indeterminate growth and often show heteroblastic leaf development. Anatomy in the distal regions of the Homalocladium platyclade shows
chlorenchyma adjacent to both flattened epidermal surfaces and sclerenchyma at
the margins of the flattened stem (Kaussmann 1955).
Phylloclades, on the other hand, are determinate and distinctly leaflike in form.
The aerial shoots of Ruscus spp., one of the best known examples, bear lateral
branches which resemble leaves in shape, anatomy and orientation. Nonetheless,
the lateral phylloclades of Ruscus are initiated in the axil of the subtending scale
leaf and show early development typical of other axillary shoots (Hirsch 1977).
Phylloclades of Ruscus may be either fertile or sterile. Sterile phylloclades have no
nodes and bear no leaves. Fertile phylloclades, on the other hand, bear a small
inflorescence on the upper surface, subtended by a small bract. The inflorescence
and the bract are situated at or near the center of the phylloclade surface (l-Iirsch
1977). Phylloclade anatomy is also strikingly leaflike, with paratiel veins converging at the phylloclade tip and chlorenchyma at both adaxial and abaxial surfaces
(Cooney-Sovetts and Sattler 1986).
4 Transsectional SymmetryCorrespondence 0' Mutant Phenotypes and
Unifacial Leaves in Angiosperms
In leaves, usually, two distinct planes of asymmetry can be found in most plants: 1)
basal-apical (longitudinal) asymmetry and 2) adaxial-abaxial (transsectional or transverse) asymmetry. Left-right (lateral) asymmetry can also be found in some species, i.e., Ulmus. How these three asymmetric aspects of leaves are controlled genetically is largely unknown. Several interesting mutants and genes responsible for
the asymmetry of leaves have been reported recently and may allow us in the future
to understand the generation and function of asymmetry in leaves and other organs.
The phantastica mutant in Antirrhinum majus has radially symmetric leaves. In
phantastica leaves, adaxial cell fate is replaced by abaxial cell fate, suggesting that
PHANT ASTICA plays a role in establishing adaxial cell fate in leaf primordia
(Fig. 1A and B; Waites et al. 1998). The loss of PHANTASTICA function in the
227
3 Leaf-like Shoots/Flattened Stems
Leaves are not the only plant organs to exhibit a flattened or dorsi ventral organization. Entire shoots of some flowering plant genera (and a single gymnosperm genus) show morphological modifications which result in strikingly flattened and
often leaflike structures (Troll 1937).
Troll (1937) recognized two main types of flattened shoots: platyclades and
phylloclades. Platyclades, such as Homalocladium platycladium (Polygonaceae),
are flattened shoots which develop a number of conspicuous nodes and internodes.
Platyclades exhibit indeterminate growth and often show heteroblastic leaf development. Anatomy in the distal regions of the Homalocladium platyclade shows
chlorenchyma adjacent to both flattened epidermal surfaces and sclerenchyma at
the margins of the flattened stem (Kaussmann 1955).
Phylloclades, on the other hand, are determinate and distinctly leaflike in form.
The aerial shoots of Ruscus spp., one of the best known examples, bear lateral
branches which resemble leaves in shape, anatomy and orientation. Nonetheless,
the lateral phylloclades of Ruscus are initiated in the axil of the subtending scale
leaf and show early development typical of other axillary shoots (Hirsch 1977).
Phylloclades of Ruscus may be either fertile or sterile. Sterile phylloclades have no
nodes and bear no leaves. Fertile phylloclades, on the other hand, bear a small
inflorescence on the upper surface, subtended by a small bract. The inflorescence
and the bract are situated at or near the center of the phylloclade surface (l-Iirsch
1977). Phylloclade anatomy is also strikingly leaflike, with paratiel veins converging at the phylloclade tip and chlorenchyma at both adaxial and abaxial surfaces
(Cooney-Sovetts and Sattler 1986).
4 Transsectional SymmetryCorrespondence 0' Mutant Phenotypes and
Unifacial Leaves in Angiosperms
In leaves, usually, two distinct planes of asymmetry can be found in most plants: 1)
basal-apical (longitudinal) asymmetry and 2) adaxial-abaxial (transsectional or transverse) asymmetry. Left-right (lateral) asymmetry can also be found in some species, i.e., Ulmus. How these three asymmetric aspects of leaves are controlled genetically is largely unknown. Several interesting mutants and genes responsible for
the asymmetry of leaves have been reported recently and may allow us in the future
to understand the generation and function of asymmetry in leaves and other organs.
The phantastica mutant in Antirrhinum majus has radially symmetric leaves. In
phantastica leaves, adaxial cell fate is replaced by abaxial cell fate, suggesting that
PHANT ASTICA plays a role in establishing adaxial cell fate in leaf primordia
(Fig. 1A and B; Waites et al. 1998). The loss of PHANTASTICA function in the