262 GERMINATION. 



fore, darkness may be said to he also necessary to 

 the perfect germination of seeds; and these condi- 

 tions are all secured by a good soil. The office per- 

 formed by moisture is, in the first place, principally 

 mechanical, serving to soften and swell up the dry 

 matters composing the seed, and by assisting in the 

 introduction of air to facilitate the changes requisite 

 to germination ; it likewise acts chemically, its pre- 

 sence being essential to the conversion of starch into 

 gum and sugar. It is also possible that water may 

 sometimes be decomposed during germination, its 

 oxygen combining with carbon to form carbonic 

 acid. 



684. When seeds germinate in a confined portion 

 of air, we find that the air does not increase in bulk 

 at all ; the nitrogen is not at all affected, but the 

 oxygen is found to have combined with a quantity of 

 carbon, and to be converted into carbonic acid, and, 

 in consequence, the insipid and comparatively inso- 

 luble starch contained in the seeds, has changed into 

 more easily soluble, sweet, and mucilaginous, or 

 gummy substances, fit for the nourishment of the 

 embryo plant, before its roots and leaves are suffi- 

 ciently developed to enable it to derive support from 

 the different sources of food presented to it. 



685. When the access of air is prevented, whilst 

 at the same time seeds are exposed to moisture, they 

 are unable to undergo this change ; germination, or 

 the growth of the embryo, cannot take place, and the 

 seeds decay; a portion of the water is decomposed, 



