ON THE GROWTH OP PLANTS.

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have an affinity for each other, cannot combine in thedark. In the same way, that peculiar chemical changewhich goes on when plants grow, cannot proceed in thedark ; the decomposition of carbonic acid and water,and the combination of their elements in order to formlignin or starch, &c., cannot go on without light.

334. When seeds are sown in the earth, they areplaced in a situation where they are protected from theaction of light, and exposed to the influence of air andmoisture; and provided they are sufficiently warm,that is, above the freezing point of water, they willgerminate and produce plants. Below that temperatureseeds cannot germinate, because, as has been alreadysaid, the action of water is in the first instance to softenthe seed, and this it cannot effect when so cold as toassume the solid form itself. If buried too deep in thesoil, the free access of air to the seeds will be retarded,or even perhaps wholly prevented, and under these cir-cumstances of course they cannot grow.

335 . The nature of the soil too is of considerableimportance: very clayey soils allow a less perfect actionof air than those which are of a more porous and openstructure, and in the former, therefore, the process ofgermination proceeds more slowly.

336. When the young plants appear above the sur-face of the ground, their mode of growth is changed :they then require air and light, and being no longer pre-served by the soil from the action of light, are imme-diately able to effect the decomposition of carbonic acid.