110

CHEMICAL CHANGES

337. The roots of a plant require little more thanmoisture. The water which they obtain from the soilcontains those saline and gaseous matters which theplants want, whilst the leaves require carbonic acid,ammonia, and the influence of light; and these theyobtain by growing up into the air.

338. When plants are shaded from the light, orcovered up, either artificially or by the leaves of sur-rounding plants, the action of light is impeded, andthey are unable to effect the proper decomposition ofcarbonic acid. Every one knows that under these cir-cumstances plants become unhealthy and send up longweak shoots, endeavouring to reach the light, and toplace their leaves in a situation where they will beexposed to its influence.

839. The germination of seeds may be readily effectedin water, although they are for the most part unable togrow and vegetate under that fluid. The change whichunder ordinary conditions is effected in seeds by theoxygen of the atmosphere is, under these circumstances,caused by the oxygen of the small quantity of commonair always dissolved or held in solution by water. In nocase can a seed germinate unless free oxygen is present,or some other means exist by which a portion of thecarbon in the seed can be removed, so as to cause thechange in the constituents of the seed before described.

340. In the process of malting, or converting rawgrain into malt, the object to be attained is to change alarge portion of the starch and gluten which the barley