349 
the Leaf of Welwitschia mirabilis. 
The constant addition of new bundles at the base of the adult leaf is the 
peculiarity of Welwitschia, which is not known anywhere else. The phylo- 
genetically primary design of the nervation is somewhat blurred in the 
young leaf, and is completely obliterated in the adult leaf, but it is well 
retained in the cotyledon. 
The Epidermis. The epidermal cell is practically prismatic, with 
a thin inner wall and very much thickened outer wall (Figs. 3, 6). The 
cell-lumen is very much narrowed towards the outer side of the cell, owing 
to the enormous thickening of the lateral walls. Two layers can be dis- 
tinguished in the outer wall : a cuticularized layer and a non-cutinized layer. 
The cuticularized layer contains a considerable amount of minute crystals 
and granules of calcium oxalate. The non-cutinized layer chiefly consists 
of cellulose and does not react to phloroglucin. The middle lamella of the 
partition-wall between two epidermal cells is very conspicuous. The cuticle 
is not very thick and measures about 2*5 jut in the thickest part, and is far 
thinner on the guard cell. 
The description and figure of the epidermis given by Sykes ( 19 , p. 180) 
differ surprisingly from my own. One of the most striking things is that 
the outermost layer, which should be nothing else but cuticle, is said to 
come down at the corner of the cells through the middle (cuticularized) 
layer and touches the cellulose wall. This should be the middle lamella of 
the partition-wall between two contiguous epidermal cells. It might appear 
as if it were a projection of the cuticle, but it is not a continuation of the 
latter at all. I was also at a loss to understand the misty covering of the 
epidermal cells delineated in her Fig. 3. It seems to me that it may represent 
a portion of the outer wall of other epidermal cells lying in the lower level 
of focus, since there is no superficial layer of wax or anything of that sort. 
The epidermal cells of the cotyledon more or less differ from those of 
the leaf. In surface view they are rather irregular, and are elongated 
longitudinally. The size of each cell is larger than in the adult foliage 
leaf. In transverse section (Fig. 5) each cell appears broader than high. 
In my material there is no evidence of a cuticularized layer ; the outer wall 
consists chiefly of cellulose, and is very much thicker than the other, and 
shows the middle lamella above referred to clearly. The thin cuticle covers 
the whole surface. 
The epidermal cells of the young leaf are of irregular shape and larger 
size than those of the adult leaf, just as in the cotyledon. But when the 
development of the bundles of secondary origin begins, then the epidermal 
cells show a tendency to assume a more regular shape, and their size 
becomes smaller than that of those near the apex ; this probably shows the 
beginning of the secondary structure of the leaf. 
Two layers, cellulose and cuticularized, are already differentiated in 
B b 
