320 DR. E. DE FRAINE ON THE 
between the two (in transverse section), throughout the internode, until the 
nodal region of the next segment is reached. 
2. The Anatomy, and the Course of the Vascular Bundles. 
The structure of the free part of a leaf segment is, in its essential features, 
similar to that of any other succulent dicotyledonous leaf. 
Since the connate leaf-bases and their two free segment-tips form a kind 
of socket into which the internode above seems to fit, the dorsal side of the 
leaf necessarily develops an assimilating area, usually consisting of two or 
three layers of palisade parenchyma. The rest of the leaf is made up of 
large parenchymatous cells devoid of chloroplasts, the intercellular spaces are 
very small, and the tissue evidently has its function in water storage. In the 
reddish forms this aqueous layer is filled with red cell-sap. A single-layered 
epidermis covers the palisade tissue; in it the stomata, which vary slightly in 
size and shape in the different species, are abundantly scattered, they show 
no peculiarity of structure and occur flush with the surface. 
The development of the cuticle is usually very slight, except in S. glauca 
and S. fruticosa, and is in marked contrast with //aloenemum strobilaceum, 
which plant is well protected by a great thickness of cuticle. Between the 
palisade laver and the aqueous tissue run the numerous vascular bundles 
which constitute the network of the veins found in the free segment-tips, 
and the “ cortical” series of bundles of the segment in its fused region. 
The structure which has been deseribed is characteristic not only of the free 
segment-tips and of the tubular leaf-sheath, but also of the internodal foliar 
sheath throughout the greater part of its length. Towards the base of the 
internode, however, the cells of the palisade layer of the sheath begin to 
diminish in length and finally disappear, the cells of the aqueous tissue 
shrink, and ultimately the sheath is reduced to a zone five or six rows deep 
composed of small parenchymatous elements (Pl. 15. fig. 1). The stem in 
this region consists of a ring of vascular bundles, surrounded by two or 
three layers of. pericyclic cells, and the endodermis abutting on to the much 
reduced foliar sheath ; no true stem cortex exists (text-fig. 2, 1) ; the endo- 
dermis however persists. The free leaf-sheath (ls.B) surrounding the node 
fuses with the structure as above described, but by the time the nodal region 
is passed the zone of small celled tissue, representing the foliar sheath of 
the internode above (ls. А), has entirely disappeared and the endodermis now 
abuts on to the new foliar sheath ; the endodermis is the “limiting layer” 
so evident throughout the internodes (text-fig. 2, 2 & 3). 
The structure of the foliar sheath is, in its essential features, similar in all 
the Salicorniex examined. 
