242 The Living Plant 



fertilized by a male antherozoid which swims through the water 

 (figure 104). Now when this egg-cell is ready for fertilization, a 

 weak solution of malic acid pours out of the archegonium into the 

 water, and diffuses steadily outwards. As soon as some wandering 

 antherozoid perceives the presence of the acid, it turns and swims 

 directly towards the source of supply, and hence to the egg-cell, 

 which otherwise it would have no means to discover. And there 

 is reason to think that such a secretion of special chemicals at the 

 time when the egg-cells are ripe is very wide spread through the 

 plant and animal kingdoms, providing the method whereby the 

 swimming or growing male cells are enabled to find the female 

 cells. This function is obviously not simply advantageous but 

 indispensable. 



There are many important phases of chemotropism, but I 

 have the space to mention only one more. Water-plants, which 

 have floating leaves, alter the lengths of the petioles in accord- 

 ance with the depth of the water, a matter which can be shown 

 very beautifully by experiment. Now it is found that this regula- 

 tion is chemotropic, or, more exactly, aerotropic, for, as ex- 

 periment proves, petioles continue to grow until the leaves 

 reach a supply of free oxygen, when they stop. This case illus- 

 trates an additional fact about stimuli, viz. that they can serve 

 as signals to stop a process as well as to guide it ; and other cases 

 are known in which they act to start a process. Such stimuli 

 are probably very important in controlling the various processes 

 of growth, as our later chapter on that subject will demonstrate. 



Thigmotropism. This name is applied to those turnings and 

 movements made in response to a touch as a stimulus. The 

 most typical case is exhibited by tendrils, which, as the reader 

 will recall, are those long slender structures sent reaching out for 

 a support by a good many kinds of climbing plants. These 

 tendrils sweep in long slow courses through the air until they 

 touch some hard object, such as a stem, or a wire, around which 

 they then curl in three or four turns (figure 85), thus obtaining 



