568 LECTURE XXI. 



cannot at present be decided, any more than the question, 

 alluded to above, as to whether or not the protoplasm actively 

 contracts on stimulation. In any case the protoplasm is not 

 wholly passive in the process of recovery. The restoration of 

 its resistance to the filtration of the cell-sap under pressure is, 

 like the loss of it, an active process, and is doubtless de- 

 pendent upon molecular changes, of a nutritive nature, going 

 on within it. 



Taking all these facts and inferences into consideration, 

 we come to the conclusion that the variations of turgidity 

 upon which movement in response to a stimulus depends, are 

 to be attributed to molecular changes in the protoplasm of 

 the cells. We may accept Cohn's dictum that "the living 

 protoplasmic substance, the primordial utricle, is the es- 

 sentially contractile portion of the cell," but we are not 

 justified in following him when he compares the motile cells 

 of the filaments of the Cynareae to unstriated muscular fibres, 

 and says that "now we know plants which (so to speak) 

 actually possess muscles." We do not know whether or not 

 the protoplasm of these motile cells contracts actively like a 

 muscular fibre ; all that the facts before us tend to establish 

 is that the state of aggregation of the protoplasm varies so 

 as to lead to corresponding variations in its resistance to the 

 escape of the cell-sap by filtration under pressure. This being 

 so, it will be perhaps well, in speaking of these phenomena, to 

 use, not the term contractility, but the term motility, sug- 

 gested in a previous lecture (p. 372). 



We may now take up the consideration of the daily periodic 

 movements of motile organs, taking Mimosa pudica as our 

 illustrative case. With regard to the primary petiole, we 

 learned in the previous lecture (p. 543) that it has no proper 

 nyctitropic movement, that the fall at night is not the re- 

 sponse to the diminution in the intensity of the light, but is 

 the mechanical effect of the nyctitropic change in position of 

 the secondary petioles and leaflets. It might be thought that 

 the fall of the primary petiole in the evening is to be ascribed 

 to the stimulating influence of the diminution in the intensity 

 of the light, inasmuch as the position then assumed bears a 



