P achy podium namaquanum , Welw. 935 
storing cells surrounded by a definite thick-walled epidermis. In transverse 
sections of spines slightly older cork cambium is seen forming in one of the 
layers of water-storing cells (Fig. 10). It has been seen arising near the 
periphery in the second layer of the water-storing belt, and has also been 
seen arising as deep as the seventh layer of this belt. When the cork 
cambium has arisen the lateral walls of the cells immediately outside it 
disintegrate, causing the tissue external to it to fall off. 
A transverse section of an old spine shows the small central group of 
cells now embedded in a mass of sclerenchyma. The parenchyma present 
in this central group of tissue in the young spine has been lignified, so that 
in transverse section the original xylem tracheides are no longer distinguish- 
able. The phloem becomes crushed and eventually disappears, leaving 
small spaces in the sclerenchyma (Fig. 11). The belt of thick-walled tissue, 
which gives to the spine its tough character, is formed by the lignification 
of the cell-walls of the parenchymatous ground tissue of the young spine ; 
the cells of which it is composed do not elongate. The surface of the spine 
is covered by a thick layer of tissue formed by the cork cambium, whose 
origin has been described above. This tissue resembles cork in origin and 
appearance, but stains with haematoxylin, and can therefore only be tissue 
destined to form cork, whose walls have not yet been suberized in the oldest 
spines examined. The lignification of the parenchyma progresses centri- 
petally, finally obliterating the vascular tissue to the extent described above. 
Owing to this the mature spine loses the function of water-storing and all the 
phloem elements possessed by the young spine. 
3. The Leaf. 
The leaves which crown the stem, borne singly on each protuberance, 
are i-§-2 in. long and about 1 in. broad, with an obovate-oblong lamina 
narrowing at the base into a very short hairy petiole. They are densely 
velvety on both sides, with a wavy margin and obtuse or retuse apex. The 
blade is bent up on either side of the midrib (involute), forming two grooves 
in which stomata are situated in small groups (Text-fig. 6). The stomata 
are also evenly distributed on both surfaces. They are very small, have no 
subsidiary cells, and are slightly above the level of the epidermis (Text- 
fig. 7 and Fig. 12); material has not been available for the study of their 
development. The epidermis is composed of oblong cells (e, and E 2 , 
Fig. 1 3 )> which have on their outer walls a thin cuticle which becomes very 
apparent when stained with a saturated solution of chlorophyll. It bears 
numerous trichomes and emergences whose complex structure is noteworthy 
when compared with the simple uniseriate or unicellular hairs found in all 
other members of the order 1 (except in the genus Oncinotis , where antler- 
Solereder ( 5 ), p. 529. 
