334 
PHYSIOLOGY. 
BOOK II. 
known, in which he attempted to force a radicle upwards and 
a plumula downwards, by enclosing them in tubes, which 
prevented the turning back of these parts. It was found that, 
as the radicle and plumula could not take their natural direc- 
tion, they became twisted spirally. These experiments, while 
they prove that the opposite tendencies of the radicle and 
plumula cannot be altered, still leave us in ignorance of the 
cause of such tendencies. We are equally ignorant of the 
cause of the directions of the leaves. Bonnet believed that he 
could explain that phenomenon upon the hypothesis of Do- 
dart just referred to, with respect to the radicle and plumula. 
According to him, the lower surface of the leaves is, like the 
radicle, composed of fibres which contract by humidity ; and 
the upper, like the plumula, of fibres that contract by dryness. 
As a proof of these assertions. Bonnet manufactured some 
artificial leaves ; the upper surface of which was parchment, 
which contracts by dryness, and the lower of linen, which 
relaxes by moisture. These leaves were submitted to the 
action of dryness and humidity ; and Bonnet found they were 
affected much in the same way as true leaves, — so easy is it 
to find proofs to support a favourite hypothesis." 
In consequence of the unsatisfactory nature of these and 
other theories, more modern physiologists have been satisfied 
with inscribing the particular directions taken by plants 
among the vital phenomena of vegetation. And this is, perhaps, 
as much as we are likely to ascertain relating to it, and 
all similar manifestations of the overruling power of nature. 
Dutrochet, however, being of opinion that some more direct 
explanation of this phenomenon is to be found, instituted a 
variety of experiments of a novel kind. " Seeing," he remarks, 
" that the stem is always directed towards heaven, and the 
roots towards the earth, we cannot but believe that there is 
some relation between the cause of gravitation and that of 
the life of vegetables : the constant direction of the stem 
towards the light leads us also to suppose that this agent per- 
forms some important part in determining the directions of 
the parts of plants. The stem must be placed in the midst 
of the atmosphere in order to develope itself ; the roots, 
on the contrary, require to lie within the earth. Hence, it 
