LIGHT AND VEGETATION 79 



enter into organic compounds which constitute, with water, 

 almost the whole of vegetable matter. By what mechanism 

 are these syntheses accomplished? Many investigations have 

 been made to try to find the answer to this question and 

 some of them will be described in the three following 

 chapters. 



2. Phototropism, or the action of light on the form of 

 growth. Plants, in general, grow towards the hght. Recent 

 research on this phenomenon has shown that the curvature 

 of a stem towards the light is due to unequal elongation 

 of the parts that receive more or less illumination. It has led 

 to the discovery of substances called hormones, or auxins, 

 which influence growth and which are dependent on 

 illumination. 



3. Photoperiodism, or the effect of the daily period of 

 illumination. A plant needs a certain quantity of light each 

 day, but the way in which it receives that hght is extremely 

 important. The effect of intense illumination for a short time 

 is quite difterent from that produced by weaker but more 

 prolonged illumination. The artificial lengthening of the day 

 by means of even very low-powered lamps, the action of 

 which must be neghgible from the point of view of energy, 

 can profoundly modify the development of the plant, for 

 example, by stimulating flowering or by preventing it. 



Many phenomena of this kind have been discovered and 

 studied only comparatively recently, but they are very 

 important when we come to consider the adaptation of 

 different species of plants to a particular latitude and climate. 

 Besides these three main questions, there are numerous other 

 actions known to be due to the visible part of Hght, and 

 these have been grouped together in the present chapter. The 

 small amount of space devoted to them does not necessarily 

 imply that they are of secondary importance, but often simply 

 that they have not yet been completely investigated. 



It is quite possible that in the future, among the phenomena 

 that we shall now describe in a few lines, a discovery of a 

 process of luminous action of great theoretical interest will 



