270 PRINCIPLES OF BOTANY, ETC. 
han a mile with a forest, consisting of numberless 
stems, which at the top have the appearance of a 
close clipped bower. 
§ 264. : 
But there are varieties of stems, which at first 
sight seareely would be counted as such ; and which 
indeed, with regard to the structure of their vessels, 
are different. Che whole genus Cactus with its va- 
ricties is an instance of this kind: fig, 233, repre- 
sents astem of it. ‘The different links which com- 
monly are taken for leaves, are parts of the stem. 
The leaves themselves are subulate, fleshy points, 
which on their base are covered with small prickles. ) 
They fall off, as soon as a bark 1s properly formed, 
and their former place is marked by the remaining 
bundles of prickles. Uhe stem of some species of 
the genus Euphorbia, Cacalia and Stapelia, is of the 
same. nature. The links of the stem consist of a 
double net-work of air and adducent vessels; the 
whole is surrounded by a dense, cellular texture, or 
a fleshy substance, and the cutis itself, has such net- 
works of lymphatic vessels with apertures. 
| § 265. 
‘The thorn, (§ 47), is, with regard to its anato- 
mical structure, to be considered as a ligneous stem, 
and does in no respect differ from it. It arises most 
generally from an incompletely evolved bud which 
has begun to form itself, but wanting a proper sup- 
ply of nourishment, remains only in form of a very 
short, sharp, and bare twig. It is like the woody 
stem 
