INTRODUCTION. 879 



condition, of curving geotropically and heliotropically. I observed both these sets 

 of phenomena, for example, in the motile organs of Phaseolus, and in the irritable 

 filaments of the Cynarese. These heliotropic and geotropic curvatures are necessarily 

 accompanied by growth. To meet this it may be pointed out, (i) that the periodic 

 movements and the movements due to irritation which we are now considering are 

 not dependent upon growth but upon an alternate contraction and dilatation of the 

 cells; (2) that there are other organs which are also capable of renewed growth 

 under abnormal conditions after their growth under normal conditions has ceased. 

 We found this to occur in the nodes of Grasses. When these have ceased to grow 

 in the erect position, they exhibit sharp geotropic curvatures due to a vigorous 

 growth of the lower surface when they are placed horizontally. The periodically 

 motile and irritable organs evidently possess the same property. They are not only 

 irritable and periodically motile after their growth under normal conditions has 

 ceased, but they are capable of renewed growth under abnormal conditions. The 

 conditions are abnormal when light falls upon these organs from one side only, or 

 when they are placed in an unusual position which is more or less nearly horizontal ; 

 then heliotropism and geotropism are brought into play. 



Now that sufficient stress has been laid upon the distinction between organs 

 which are motile while growing and organs which are motile after they have ceased 

 to grow, the points of resemblance of the two may be considered. The first of 

 these is the fact that for each kind of movement which is exhibited by the special 

 motile organs, a corresponding kind may be observed in growing organs. Thus, 

 the spontaneous periodic movements of the former correspond to the spontaneous 

 nutations of growing stems, leaves, tendrils, and flowers. The variations of tempe- 

 rature and of the intensity of light which act as stimuli upon the former, by increas- 

 ing or diminishing the turgescence of the tissues, also affect many growing leaves 

 and cause flowers to open or shut, by accelerating the growth of one side or the 

 other. The motile organs of Mimoseae, Oxalidese, and Cynareae, which are sensitive 

 to contact, correspond to the growing tendrils and roots which are sensitive to 

 pressure. 



This comparison tends to show that the causes which induce contraction and 

 dilatation of the cells of motile organs by modifying their turgidity, may also retard 

 or accelerate the growth of growing organs. The deeply-seated connection be- 

 tween these phenomena will become evident if what was said in Sect. 14 as to the 

 causes of growth be compared with the following account of the causes of the 

 periodic movements and of those due to stimulation. According to my theory, the 

 hydrostatic pressure which the cell-sap exercises, in consequence of its increase in 

 quantity by endosmosis, upon the extensible cell-wall, is an essential condition of the 

 growth of the cell ; fresh solid matter is deposited between the micellae of the 

 stretched cell-wall, it therefore grows, and thus growth is a perpetual over-stepping 

 of the limit of elasticity of the stretched cell-wall. Everything which increases the 

 turgescence of the cell promotes its growth, everything which diminishes the tur- 

 gescence is prejudicial to its growth. If these effects are produced in different 

 degrees upon the two sides of a growing organ, corresponding curvatures will be 

 produced. If these effects are produced in a mass of tissue the cell-walls of which 

 have ceased to grow but are very extensible and very perfectly elastic, an increase of 



