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The cause productive of such movement has been attempted to be 
explained by several philosophic writers. Darwin thus endeavours to unfold 
this mystery. 
When the seed falls naturally upon the earth, or is buried artificially in 
shallow trenches beneath the soil, the first three things necessary to its 
growth are heat, water, and air. Heat is the general cause of fluidity, 
without which no motion can exist; water is the menstruum, in which the 
nutriment of vegetable and animal bodies is conveyed to their various organs ; 
and the oxygen of the atmosphere is believed to afford the principle of 
excitability so perpetually necessary to all organic life; and which renders 
the living fibres both of the vegetable and animal world obedient to the 
stimuli which are naturally applied to them. 
Whence we may in some measure comprehend a difficult question; 
why the plume of a seed sowed upon, or in the earth, should ascend, and 
the root descend, which has been ascribed to a mysterious instinct; the 
plumula is stimulated by the air into action, and elongates itself, where it is 
thus most excited; and the radicle is stimulated by moisture, and elongates 
itself thus, where it is most excited, whence one of them grows upwards in 
quest of its adapted object, and the other downward. 
Hunter, when inquiring into this subject, made the following experi¬ 
ments. 
I took, says this great experimentalist, a tub about eighteen inches 
deep, and about two wide, and filled it with fine mould, in which I planted 
some beans and peas; their eyes were placed in various directions, and over 
the surface was spread a close meshed net. The mouth of this tub was 
turned down, being raised about three feet from the ground, and was sus¬ 
pended between two posts. Round the tub, and over its bottom, which 
was uppermost, were placed wet straw, mats, &c. to take off any influence 
the sun or air might have upon its contents, and a small hole was bored in 
its bottom, to which was fixed a small long tube that came through the 
straw. This was intended for pouring some water, if I found the earth get 
dry, into the tub. Under the mouth of the tub I placed looking-glasses, in 
such a way that the light was thrown upon the mouth of the tub, or surface 
of the earth. The weather w~as fine, so that through the whole day there 
was the reflection of the looking glasses upon the surface of the mould, 
which was much more powerful than day-light without the direct rays of 
the sun. This I continued till I conceived that the beans and peas had 
