GERMINATION.

107

therefore, moisture, a certain degree of heat, and thepresence of air.

327. Light appears to be prejudicial, and, therefore,darkness may be said to be also necessary to the perfectgermination of seeds; and these conditions are all securedby a good soil. The office performed by moisture is,in the first place, principally mechanical, serving tosoften and swell up the dry matters composing the seed,and, by assisting in the introduction of air, to facilitatethe changes requisite to germination: it likewise actschemically, its presence being essential to the conversionof starch into gum and sugar. It is also possible thatwater may be decomposed during germination, its oxy-gen combining with carbon to form carbonic acid.

328. When seeds germinate in a confined portion ofair, we find that the air does not increase in bulk at all;the nitrogen is not at all affected, but the oxygen isfound to have combined with a quantity of carbon, andto be converted into carbonic acid, and, in consequence,the insipid and comparatively insoluble starch containedin the seeds has changed into more easily soluble, sweet,and mucilaginous or gummy substances, fit for the nou-rishment of the embryo plant, before its roots and leavesare sufficiently developed to enable it to derive supportfrom the different sources of food presented to it.

329. When the access of air is prevented, whilst atthe same time seeds are exposed to moisture, they areunable to undergo this change; germination, or thegrowth of the embryo, cannot take place, and the seeds