CHAP. XIV. 
DIRECTIONS TAKEN BY ORGANS. 
407 
tempted to force a radicle upwards and a plumula down- 
wards, 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 direction, 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 Dodart just 
referred to, with respect to the radicle and plumula. Ac- 
cording 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 
contracts 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, per- 
haps, as much as we are likely to ascertain relating to them, and 
all similar manifestations of the overruling power of nature. 
Dutrochet, however, being of opinion that some more direct 
explanation of such phenomena is to be found, instituted a 
variety of experiments of a novel kind. Seeing,” he re- 
marks, “ that the stem is always directed towards heaven, and 
the root 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 may 
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