The median dorsal strand is the first to differentiate in Cocos, Phoenix, and
Caryota. It traverses the terminal abaxial ridge of the plications, to end at the apex of
the non-plicate margin of the lamina wing. The series of strands that afterwards
differentiate tangentially on either side of it form the first vasculature of the adaxial
ridges of the plications, and are termed "primary strands". In Borassus, the median
dorsal strand differentiates only after 5-10 pairs of primary strands are differentiated.
In Cocos and Borassus, each primary strand traverses one adaxial ridge; hence
the primary strands are more or less equal in number to the pinnae in Cocos and the
segments in Borassus. In Phoenix, each primary strand executes an almost right-angled,
adaxial curve at its tip and branches dichotomously into two to four branches, each
traversing a line parallel to an adaxial ridge, in the "haut" formed by the fusion of the
adaxial ridges with each other. Therefore the primary strands are characteristically fewer
than the pinnae. In addition to the branches that vascularize the haut, the primary strands
make connections later with some of the strands of the pinnae that differentiate in
the lamina. When the haut is shed, those parts of the branches of the primary strands
situated in the haut are lost, leaving the primary strands connected to the laminar
strands alone. In Borassus, during dissection of the palmate lamina, those parts of the
primary strands situated in the apical halves of the adaxial ridges are constricted, along
with the surrounding ridge, and shed. In the basal half of the adaxial ridge, the primary
strand makes connections with other neighbouring strands of the lamina. In Caryota,
the primary strands are comparatively few, since the primary plications are few.
The strands formed adaxial and abaxial to the tangential row of primary strands
are irregularly disposed, and are termed the adaxiai and abaxial complexes respectively.
These strands vascularize the rest of the lamina, and also the adaxial ridges. The
strands of the adaxial complex of Cocos are inversely oriented.
The primary strands extend to the thin ventral part of the sheathing base in
Cocos, Borassus, and Caryota, but are confined to the thick dorsal part in Phoenix.
The oblique courses of the strands on the two sides of the median ventral line of the
sheath as mirror images of one another, and their spatial and temporal sequence of
differentiation along two different transverse depths, account for their remarkable interlocking
as "warps and wefts" along the median ventral line.
The primary strands differentiate acropetally. The adaxial and the abaxial
strands show acropetal, basipetal, or discontinuous differentiation in different parts
of the leaf.
Although the basic pattern of vasculature seen in the younger stages does not
change, the vasculature of the mature leaf becomes very complex by the formation of
numerous additional bundles and branches, and their anastomoses, especially in the
sheath and rachis.
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