ae PRINCIPLES OF BOTANY, &c. 
with the vascular cutis. The most careful  cultivas 
tion cannot convert aprickle into a shoot, as its air. 
vessels beeome-very rapidly lhgneous,. and separate 
from the inner bark, and it is therefore only kept 
from. dropping off, by the covering cutis. Prickles 
have sometimes a peculiar shape; they are almost of 
the shape of contorted tendrils in Nauclea acwleata 
and other plants. Even the stipulae of some plants 
are’ converted into prickles, for instance, Robinia 
pseudacacia, Berberis vulgaris, &c. 
§ 267. 
Tendrils ‘have the same structure with regard to 
their vessels, which herbaceous stems have. ‘They 
are in fact petioh without the leafy expansion, but 
which, having not wasted their sap in the formation. 
of leaves, have grown longer, and on this account 
have become too thin and feeble to keep their 
straight direction. Hence arises their twisted shape. 
It appears, as if the diminished force of the current 
of air hassome influence upon the tendril. For each 
plant ‘that’ supports itself by tendrils, when distant 
from a wall, tree or shrub, sends out all its tendrils 
towards that side’ on which the plant is.to attach 
itself. At least this phenomenon can scarcely be ex- 
plained in any other way. 
§ 268. | 
The pith which is found in the centre of stems, 
(§ 278), is a soft and spongy cellular texture, which 
commonly is of a remarkably splendid white colour. 
ft is not the least different from cellular texture, 
4 and 
