406 
PHYSIOLOGY. 
BOOK II. 
CHAPTER XIV. 
OF THE DIRECTIONS TAKEN BY THE ORGANS OF PLANTS. 
The substance of all that is known upon this subject has 
been combined with some excellent observations of his own, 
by Dutrochet, in a memoir, of which 1 shall avail myself in 
the following remarks. 
“ The general phenomena of nature,” says this writer, 
“ which are daily before our eyes, are often those which 
mankind considers the least attentively. Those who are 
unaccustomed to reflect upon such subjects can scarely be- 
lieve that there is any very extraordinary mystery in the 
ascent of the stems of vegetables, or in the descent of their 
roots ; and yet this is one of the most curious circumstances 
connected with vegetable life. The downward direction of 
the roots may appear easy of explanation : it may be said 
that, like all other bodies, they have a tendency towards the 
centre of the earth, in consequence of the known laws of 
gravity (as is the opinion of Knight, in PhiL Trans, for 
1806) ; but on what principle, then, is to be explained the 
upward tendency of the stem, which is in direct opposition to 
those laws ? And here lies the difficulty. Dodart is the first 
who appears to have paid attention to this circumstance; he 
pretends to explain the turning backwards of seeds sown in an 
inverted position by the following hypothesis : — He assumed 
that the root is composed of parts that contract by humidity ; 
and that the stem, on the contrary, contracts by dryness. 
For this reason, according to him, it ought to happen that, 
when a seed is sown in an inverted position, the radicle will 
turn back towards the earth, which is the seat of humidity ; 
and that the plumula, on the contrary, turns to the sky, or 
rather atmosphere, a drier medium than the earth. The 
experiments of Du Hamel are well known, in which he at- 
