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EFFECTS OF LIGHT

decay ; a portion of the water is decomposed, and theoxygen and hydrogen which it contained unite with theelements of the vegetable matter, and form carbonicacid, carburetted hydrogen, and other substances.

330. The chemical changes attendant on the processof germination are very different from those which goon during the growth of a complete plant. Before theformation of leaves, and in the very earliest stages of itsgrowth, a plant requires only warmth, moisture, andthe presence of air ; but subsequently, it requires, inaddition to these, carbonic acid and light.

331. It is the oxygen of the air alone which is essen-tial to the germination of seeds, for the nitrogen beingunable to combine with carbon, under ordinary circum-stances, is quite useless in diminishing the quantity ofthat substance contained in the seeds: its presence inthe air is, however, very useful, serving to dilute theoxygen and to prevent its acting too rapidly. Seeds arefound to germinate very quickly in pure oxygen gas,but the plants produced are weak and unhealthy.

332. In the next stage of the growth of plants, orwhen they have leaves, they begin to absorb carbonfrom the air instead of parting with it; and this theydo by decomposing the carbonic acid always present inthe air, a power which they possess when exposed tothe influence of light.

333. The effects of light in increasing chemical affinityare highly curious, and but very imperfectly understood :there are a good many substances which, although they