16 PHILOSOPHY OF ZOOLOGY. 
displaces the soil, which is specifically heavier than itself,— 
while the latter bursts through its earthy covering, and 
rises to a determinate height in the atmosphere, not in con- 
sequence of its specific levity, for its density is greater 
than that of the cireumambient air. 
As the radicle is incapable of procuring nourishment for 
the growth of the future plant, in immediate contact with 
its surface, it sends out from its substance numerous fibres, 
which are multiplied in all directions to search after food, 
and convey it to the proper organs. The number and 
direction of these fibres, and the distance to which they 
extend, are regulated by the wants of the plant, and the 
supply in the soil. The stem in some plants rises unas- 
sisted in a vertical direction ; in others, it requires the sup- 
port of a foreign body, or remains prostrate on the ground ; 
and the leaves assume every variety of aspect and position. 
Various explanations, intended to account for the de- 
scent of the root and ascent of the stem, widely different 
from the one to which they are here referred, have been 
offered by different naturalists. One supposes the root to 
be attracted downwards by the earth, and the stem to be 
attracted upwards by the sun,—another, that the root is 
stimulated by the earth, while the stem is stimulated by 
the air, so that each lengthens in the direction of the 
greatest excitation,—and a third ascribes the descent of 
the root to the weight of its watery juices, and the ascent 
of the stem to the buoyancy*®& its gaseous secretions. 
But these hypotheses, while they fail in explaining the 
phenomena, many of which have not been contemplated, 
indicate in_ their authors a strong desire to employ the 
principles of chemistry or mechani¢s in the solution of the 
difficulties attending theexamination of organized bodies, 
and a decided and unphilosophical aversion to admit the 
existence of the vital prineiple. 
