GROWTH 195 



many of them will droop. Cress grown in the light will 

 have short but healthy, thick and turgid stems. This 

 means that light is not without an influence upon the 

 growth or rather the elongation of stems ; but this 

 action is not an accelerating or favouring of growth 

 in length, but on the contrary an inhibiting of it. 

 The influence of light is not limited to this retarda- 

 tion of growth. If we have a plant in a room so that 

 it receives light always from one side, we shall see 

 that its young growing stem will bend over and turn 

 to the light, as we generally say. Evidently we have 

 no right to attribute any attractive power to the rays of 

 the sun, and there is no need to have recourse to such a 

 futile hypothesis. By comparing the two experiments 

 just described we can infer what is the explanation of this 

 phenomenon of the turning of stems towards the light. 

 Light retards the growth of stems, but a one-sided 

 illumination will not act with the same effect on both 

 sides of the stem the front will receive all the light and 

 the back will always remain in the shade. The front will 

 consequently grow a little more slowly than the back, 

 and the result will be the turning towards the light. In 

 a word we have here a case contrary to the action of 

 gravity. The force of gravity accelerates growth on 

 the side turned towards the centre of the earth the 

 stem withdraws from it. Light retards growth on the 

 side turned towards its source the stem turns to it. 

 The name heliotropism has been given to this 

 phenomenon. 



But if light retards the growth of stems, does it not 

 follow that plants must grow chiefly during the night ? 

 This question has been raised many a time, and has been 

 settled in various ways. These contradictions must not 

 puzzle us, because the point itself is a very complicated 

 one, and the observation of growth at such short 

 intervals requires rather delicate methods of investiga- 

 tion, which science has come to possess only recently. 



