260 CAUSES OF GERMINATION. [BOOK n. 



If any one of them is abstracted, the other two are of no 

 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 their 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 depends upon the nature of the matter 

 deposited in their tissue. One effect of water may be sup- 

 posed to be that of softening the tissue, of enabling all the 

 parts to distend, and of dissolving the soluble parts so as to 

 render them fit to be taken into the circulation, as the young 

 plant becomes capable of absorbing them. Another effect is 

 to yield hydrogen by its decomposition. 



Germination cannot take place in vacuo ; nor in an atmo- 

 sphere of nitrogen or hydrogen, and still less in carbonic 

 acid ; or at least, if in this latter gas some traces of germi- 

 nation manifest themselves, they rapidly disappear : it can 

 only occur in free oxygen. Of this but a small proportion 

 is really necessary; from ^ to -, according to different 

 observers. But one part of oxygen and three of nitrogen 

 are the proportions which seem to be the most favourable, 

 and this is not very different from the proportions in atmo- 

 spheric air ; viz., one of oxygen and four 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 



