CHAPTER III. SPECIAL PHYSIOLOGY OF MOVEMENT. 757 



The means by which, stimuli are transmitted is a matter which 

 is still under discussion ; but it appears that the means of trans- 

 mission is not the same in all cases. Whilst in some, such as 

 tendrils and the leaves of Drosera, the stimulus is probably trans- 

 mitted by means of the delicate protoplasmic filaments which con- 

 nect the protoplasm of adjacent cells (see p. 91) ; in others, for 

 instance Mimosa pudica, the stimulus is transmitted as a dis- 

 turbance of the hydrostatic equilibrium of the cells : it would, in 

 fact, appear that whilst the former means of transmission suffices 

 for a short distance, the latter is necessary when the distance to 

 be traversed is considerable. In Mimosa pudica there appears to 

 be a special tissue along which the stimulus is conducted : it be- 

 longs to the bast, and consists of large elongated cells with pitted 

 cellulose walls. 



The Latent Period. It has been observed, more especially with 

 regard to the heliotropic and other curvatures of radial growing 

 organs, that the curvature does not become apparent for some con- 

 siderable time after the stimulus has begun to act. This interval 

 is the Latent Period. For instance, it has been ascertained that 

 the primary shoots of Bean-seedlings do not begin to curve helio- 

 tropically for two hours after an exposure of one hour to unilateral 

 illumination. 



When, however, the stimulus to curvature has acted for a 

 sufficient time, the curvature will eventually take place even 

 though the stimulus has ceased to act : the conditions of curvature 

 have been induced, that is, even before the curvature can be 

 observed. 



In explanation, it may be suggested that, in growing members, 

 the stimulus is conveyed but slowly from the seat of irritability 

 to the seat of curvature ; and it is obvious that the curvature of a 

 growing cellular member is a process which, on mechanical grounds 

 in relation with the rate of growth, cannot be effected rapidly. 



15. Combined Effects of different Stimuli. Inasmuch 

 as it is commonly the case that the motile members, whether 

 growing or mature, are irritable to stimuli of various kinds, it is 

 clear that the assumption by them of any particular position is 

 the resultant effect of the stimuli which may be acting simul- 

 taneously. The phenomena in question are strikingly manifested 

 by growing members, and it is to these that the following account 

 especially refers. 



According to the position assumed in the course of their growth 



