458 SCIENCE PROGRESS 



Again, there seems to be no escape from the conclusion that 

 the character of the protoplasmic membranes of certain cells 

 may undergo rhythmical change in such a way as to allow the 

 water, together with dissolved salts, to be forced out from them 

 in certain determinate directions under the considerable pressure 

 exerted by the elastic cell wall. No satisfactory explanation, 

 however, has been given of the mechanism by which this 

 excretory process is effected, though the fact itself can hardly be 

 denied. Thus watery solutions are regularly and forcibly in- 

 jected from the cells of the roots into the dead wood elements 

 and this at a pressure of several atmospheres in some instances. 

 Such a circumstance is the more remarkable when it is remem- 

 bered that the cells which are so acting are in direct com- 

 munication with those external cells which are wrenching the 

 water away from the film which coats the minute particles of 

 which soil is composed. 



To return, however, to the mechanism of movement, a 

 further survey shows that a differential turgor in adjacent layers 

 of tissue does not afford an explanation of all the movements 

 which are associated with living protoplasm. 



For example, the tubular and often unicellular threads of 

 certain moulds, such as Phycomyces, exhibit fairly considerable 

 movements by virtue of which they bend in the direction of 

 incident rays of light. In this respect, they resemble the stems 

 of most of the higher plants ; like them, they are specially 

 sensitive to the more refrangible rays of the spectrum. But 

 inasmuch as each thread consists of a single tube, it is manifest 

 that no explanation based on unequal turgor in coherent and 

 adjacent cell rows can be invoked here ; to attribute the directive 

 movement simply to the specific irritability of the protoplasm is 

 mere circumlocution — and in such an explanation the question of 

 mechanism is altogether ignored. Moreover, no satisfactory 

 proof has been given of unilateral modification of the longi- 

 tudinal extensibility of the wall, which might meet the case. 



There are many other slow movements, e.g. the various 

 curvatures of roots in response to the stimulus given by gravita- 

 tion ; concerning the causes which produce these, we know 

 almost nothing. Much has been discovered as to the conditions 

 which may affect the movements and something even of the 

 subsidiary machinery in certain instances. The disturbance due 

 to the action of gravitv of the starch grains in the cells at the 



