REPRODUCTION 273 



quency, is absolutely necessary. In other words, the stim- 

 ulus is but transient. Eepeated stimulation is needed to 

 continue in operation the processes by which the formation, 

 growth, and perfect development of flowers, and their ac- 

 cessory parts, are accomplished. In some few plants, flowers 

 which have been duly induced to form will develop per- 

 fectly in the dark, but in most plants abnormal characters 

 will appear, varying in importance from an insufficiency of 

 color to deformity or suppression of parts, or the failure of 

 the flowers to open. 



Sachs's hypothesis, * that certain specific substances, pre- 

 sumably made in the leaves under the influence of light, are 

 necessary for the formation of flowers, breaks down under 

 Vochting's experiments, for we cannot reasonably admit the 

 presence in the plant of corolla-forming material as distinct 

 from calyx-forming, etc. 



When a plant is so well nourished, has attained such a 

 size, and is under such favorable conditions generally, that 

 it is able to form the organs and substances of sexual re- 

 production, it must be that it will do so whenever the proc- 

 esses concerned in sexual reproduction are set and main- 

 tained in operation by a stimulus from outside. This 

 stimulus is the. light. Without it, the higher plant cannot 

 reproduce,, or even prepare to reproduce, itself sexually. 

 The plant is sensitive to that force which, in cases where the 

 plant is dependent upon insects for cross-pollination, will in 

 so far as possible ensure the visits of the necessary insects. 

 It would be interesting to determine whether other than 

 entomophilous flowers are so sensitive to light and so 

 dependent upon it. 



The influence of light is to be distinguished from the in- 

 fluences of food, water, and of the other substances, of heat 

 and of the other forces, essential to active life. With in- 

 suflicient warmth, food, or water, the plant will be imper- 

 fectly able, or quite unable, to reproduce itself by sexual 

 means ; but this inability is due to the effect of unfavorable 

 conditions upon all of its vital functions. Warmth, food, 



* Sachs, J. von. Lectures on the physiology of plants. English Ed., 

 Oxford 1887, and his special papers therein referred to (pp. 530, 534. etc.) 



18 



