5 $2 LECTURE XXI. 



This question cannot at present be definitely answered, but it 

 may be pointed out that the excessive elongation of the cells 

 of the convex side of a curving multicellular organ, and 

 certain other phenomena to which attention was drawn when 

 we were discussing the mechanics of growth (p. 335, small 

 print), suggest that at least an active expansion of the proto- 

 plasm may take place. 



The recent observations of Kohl on the heliotropic, geotropic, and 

 hydrotropic curvatures of various organs (sporangiferous hyphae of 

 Phycomyces, root-hairs, roots, etc.) lend some support to these views as 

 to the intimate mechanism of curvature. He finds that the protoplasm 

 aggregates on the concave side of the curving cell, but it is not clear at 

 present what the exact significance of this fact may be. 



We will conclude our study of the mechanism of the 

 movements of growing organs with a brief consideration of the 

 spontaneous movements. The movement of nutation of mul- 

 ticellular organs is due, like the curvature induced by a stimu- 

 lus, to an inequality in the turgidity of the cells on two opposite 

 sides of the organ, an inequality which in this case also is to 

 be attributed to molecular changes in the protoplasm of the 

 cells, but occurring spontaneously. 



With regard to the intimate mechanism of the movement 

 of nutation of unicellular organs, we can only repeat what has 

 been said with regard to their induced movements, that the 

 movement is due to a difference in the condition of the proto- 

 plasm of the two opposite halves of the organ, of such a nature 

 that the protoplasm of the one side yields less readily to the 

 pressure of the cell-sap than does that of the other. 



Transmission of Stimuli. 



In speaking of the action of stimuli in inducing move- 

 ments of plant-organs it has been mentioned incidentally 

 that there is considerable evidence to shew that the stimulus 

 is transmitted from one part of the organ to another. Thus, 

 a transmission of the heliotropic stimulus appears to have 

 been made out in the case of certain cotyledons (p. 438), and 

 of geotropic (p. 468), hydrotropic (p. 479), and electrical 



