CHAP. X. 
SEED. 
359 
effect: it is, however, doubtful whether it ever happens in 
nature, that the act of germination takes place under con- 
ditions so simple as those ; it is usually a more complicated 
phenomenon. 
Water is the agent to which we are most in the habit of 
assigning the power of causing the growth of seeds; to air 
and heat they are generally exposed more or less, and it is by 
the addition of water that the two latter are popularly con- 
sidered to be brought into active operation. According to 
De Candolle, it is a general property of seeds to absorb, during 
this period of germination, more than their own weight of 
water ; but no regular proportions have been remarked, and 
it is probable that the respective power of different seeds de- 
pends upon the nature of the matter deposited in their tissue. 
The effect of water may be supposed to be that of softening 
the tissue, of enabling all the parts to distend, and of dis- 
solving the soluble parts so as to render them fit to be taken 
into the circulation, as the young plant becomes capable of 
absorbing them. 
Germination cannot take place in vacuo ; nor in an at- 
mosphere of nitrogen or hydrogen, and still less in carbonic 
acid ; or at least, if in this latter gas some traces of ger- 
mination manifest themselves, they rapidly disappear: it 
can only occur in free oxygen. Of this but a small pro- 
portion is really necessary; from ^ to according to 
different observers. But 1 part of oxygen and 3 of nitrogen 
are the proportions which seem to be the most favourable, 
and this is not very different from the proportions in at- 
mospheric air ; viz. 1 of oxygen and 4 of nitrogen. A too 
large dose of oxygen weakens the young plant, by abstracting 
its carbon too rapidly. 
Experiments show that oxygen is not absorbed by the 
seed, but combines with its carbon, forming carbonic acid, 
which is thrown off*. When a seed ripens, a considerable 
quantity of carbon is stored up in its tissue, apparently for 
the purpose of enabling it to ‘‘maintain the unalterability ” 
to which its preservation is owing. This superfluous carbon 
renders it scarcely soluble in water. To enable the parts 
to be sufficiently moistened, it is therefore necessary that the 
A A 4 
