the Leaf of Welwitschia mirabilis . 353 
The Spicular Cell. There is no need of describing in great detail the 
well-known spicular cell. One can distinguish two layers of wall : an 
outer comparatively thin and lignified wall, and an inner thick and un- 
lignified one ; crystals of calcium oxalate are embedded in the former. 
Miss Sykes, however, in her description reversed the actual order. Her 
error seems to be due to a misunderstanding of Bower’s correct state- 
ment : ‘ As the cell increases in size the cell-wall becomes differentiated 
into an outer cellulose wall and an inner lignified wall ’ ( 3 , p. 590). What 
he means by ‘outer cellulose wall ’ here is evidently nothing else but the 
middle lamella of the wall, from the inner layers of which the ‘ spicular 
cell ’ becomes differentiated (cf. 3 , PL XXXIII, Fig. 17). 
I have not observed any spicular cell in the cotyledons of my material. 
The Vascular Bundle. The vascular bundle of the adult leaf has 
already been fairly well described by Hooker (8), de Bary ( 4 ), Bertrand ( 1 ), 
and Sykes ( 19 ). The xylem elements are radially arranged, and are inter- 
spersed with parenchymatous cells (Fig. 7). The metaxylem consists of 
dense spiral or reticulate tracheides, with bordered pits at intervals on their 
radial and tangential walls. The secondary xylem consists chiefly of 
tracheides, generally with uniseriate bordered pits, and of a few tracheae 
with multiseriate bordered pits with transversely elongated orifice (I use 
the terms ‘ trachea ’, ‘ tracheide and ‘ vessel ’ as proposed by Rothert (cf. 
12 , p. 16), simply because it is more intelligible than de Bary’s usage). 
The tracheae are of the Gnetaceous character, viz. the perforation is incom- 
plete, and are perforated with one or occasionally two simple large round, 
or more or less irregular pits. 
The xylem-parenchyma is unlignified and never appears as a continuous 
ray in a transverse section. The cells are of the same height, and are 
never straight, but their walls are always more or less irregularly curved, 
and they give off short projections in the radial direction which come into 
contact with those of the neighbouring cells (Fig. 10). 
The sieve-tube is much elongated and possesses an oblique end-wall. 
The sieve-plates are of the Gymnospermic type, and are present not only 
on the sloping wall, but also on the vertical wall of both the radial and 
tangential sides, as in Cycas, Dammar and Ginkgo . They occur between 
two sieve-tubes as well as on the wall which abuts on the phloem- 
parenchyma (Fig. 11). 
The phloem-parenchyma is disposed radially, and the cells are vertically 
elongated and are nearly uniform in height. 
The transverse branch of the bundle has a similar structure to that of 
the main bundle on a much smaller scale. The xylem is composed chiefly 
of tracheides with reticulate thickening, with bordered pits at intervals. 
These connect with the metaxylem of the main bundle. Neither spiral 
