CHAPTER V. 



* Some exception may be taken to this statement from the fact that the periodically motile and 

 irritable parts of plants are also sometimes heliotropic and geotropic, as is the case with the leaves 

 of Phaseolus and the filaments of Cynaracere. But this only proves that mature parts, when in 

 abnormal conditions, commence growing afresh. These conditions here consist in the light falling 

 unequally on the two sides, or on the upper side of contractile organs turning downwards (as in 

 Phaseolus). In the same manner mature petioles of ivy begin growing afresh in the dark, or 

 when the light is unequal on the two sides, and mature nodes of Grasses when placed in a horizontal 

 position, the former on the shaded, the latter on the under side. 



I 



PERIODIC MOVEMENTS OF THE MATURE PARTS 

 OF PLANTS AND MOVEMENTS DEPENDENT 

 ON IRRITATION. i 



Sect. 26. Definition. The logical arrangement of our subject requires us to 

 devote a separate chapter to the little that we yet know on the mechanism of the 

 periodic movements of leaves and foliar structures and those due to irritation, in 

 order to call the attention of the reader to the fact that although these movements 

 present many external resemblances to those described in the last chapter, they are 

 nevertheless the result of altogether different causes, and have nothing to do with 

 the phenomena of growth, a distinction to which sufficient attention has not at 

 present been paid. 



The movements now under consideration are disdnguished most conspicuously 

 from those described in the last chapter by the fact that they do not arise during 

 growth and are not caused by it, but on the contrary are only manifested when 

 the organs in quesdon are perfectly mature \ and when the peculiarity of their in- 

 ternal structure, which renders the phenomenon possible, is fully developed ; while 

 the movements we have hitherto been considering cease with the completion of the 

 growth of the organ. The greater number of the movements which are brought 

 into play during growth — as the curvatures caused by heliotropism or geotropism or 

 by the pressure of supports on tendrils and climbing plants — produce new per- 

 manent conditions, since growth is modified. It is only when the action has been 

 a very transitory one that heliotropic or geotropic curvature or that of tendrils 

 due to irritability, can again be effaced by further growth. During these processes 

 the organ is advancing towards maturity ; the changes which have not been effaced 

 are therefore, as it were, stereotyped. 



The case is quite different with the changes now to be described. They take 

 place in organs whose growth is completed, but whose structure allows the tissues 



