ON THE RELATION OF VEGETATION TO SEASONS. 
203 
cells, their stomates, their vessels, and their intercellular passages are filled, and they 
drop off. At this time a plant is nearly exhausted of its fluid sap, the watery 
portion of which it had exhaled during- the summer and autumn, all the parts are 
dry and solidified, so as to suffer little from evaporation ; and the roots themselves, 
having for some time been but feebly in action, are firm and not liable to be easily 
broken ; every thing is in a state of languor, and prepared to renovate the enfeebled 
powers of the plant by the slow and gradual absorption of fluid during the winter. 
It is in the autumn, then, that both theory and practice direct us to transplant 
trees. At that season every circumstance concurs to render the operation practica- 
ble ; but if we wait till the spring, the spongelets, which form during the winter, 
are likely to be destroyed, and many causes may call the already turgid plant into 
growth before the roots have had time to form new spongelets. 
The seasons of growth and repose are so essential to vegetation, that, as is 
familiar to all gardeners, it is scarcely possible to prevent plants preparing themselves 
for their annual changes, whatever artificial means may be employed to maintain 
them in a uniform atmosphere, and to protect them from those causes which usually 
bring about repose : and this is certain, that if we succeed in preventing the cessa- 
tion of growth, the plants which are the subject of the experiment uniformly, in 
the end, fall victims to the forced and unnatural condition in which they are 
maintained. 
If annual changes in their condition be requisite to the well-being of plants, so 
in like manner are the diurnal changes of light and darkness. If plants were kept 
incessantly growing in light, they would be perpetually decomposing carbonic acid, 
and would, in consequence, become so stunted that there would be no such thing as 
a tree, as is actually the case in the polar regions. If, on the contrary, they grow in 
constant darkness, their tissue becomes excessively lengthened and weak, no decom- 
position of carbonic acid takes place, none of the parts acquire solidity and vigour, 
and, consequently perish. But under natural circumstances, plants, which in the 
day become exhausted by the decomposition of carbonic acid, and by the emptying 
of their tissue by evaporation, repair their forces at night by inhaling oxygen 
copiously, and so forming a new supply of carbonic acid, and by absorbing moisture 
from the earth and air, without the loss of any portion of it. Such being the case, 
we must conclude that plants grow chiefly by day, and this is conformable to the 
few observations that have been made on the subject. Oneyer found the stem of a 
Belladonna lily, and plants of wheat and barley, grow by day nearly twice as fast as 
at night ; and Mulder states that he has arrived at a similar result in watching the 
development of other plants. 
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