CHAP. VIII. SEED. 299 



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 conditions so 

 simple as those ; it is usually a much more complicated phe- 

 nomenon. 



Water is the asjent 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 regidar 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. 



Boiled or distilled water, however, is not capable of bring- 

 ing about the germination of seeds, provided it entirely sur- 

 rounds them ; it is indispensable that oxygen should have 

 ready access to them. Germination in fact cannot take place 

 in vacuo ; nor in an atmosphere of nitrogen, or hych'ogen, and 

 still less in carbonic acid ; or at least, if in this latter gas 

 some traces of germination manifest themselves, they rapidly 

 disappear : it can only occur in free oxygen. Of this but a 

 small proportion is really necessary ; from J to -V, according 

 to different observers. But 1 part of oxygen and 3 of 

 nitrogen are the proportions which seem to be the most fa- 

 vourable, and this is not very different from the proportions 

 in atmospheric air ; viz. 1 of oxygen and 4 of nitrogen. A 

 too large dose of oxygen weakens the young plant by ab- 

 stracting its carbon too rapidly. 



Experiments show that the 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 jjurpose of 



