482 



GENERAL CONSIDERATIONS 



[Ch. XIX 



Breadth. 



'Cells of convex side . . 

 Cells of concave side . . 

 Cells in normal condition 



0.125 mm. 



0.020 



0.099 



0.045 mm. 



0.025 



0.035 



0.042 mm. 



0.026 



0.032 



The importance of imbibition for geotropism is also shown by 

 the experiment of cutting a slice off from the upper part of the 

 horizontal root, which then curves only slowly ; whereas, if 

 the cut surface is placed down, the curving takes place with 

 abnormal rapidity. 



It being admitted that tropisms are, for the most part, 

 largely, if not chiefly, due to differential imbibition, the ques- 

 tion arises. How can this produce a turning ? It is easy to see 

 how it might be possible in the case of a multicellular plant, 

 but tropism also occurs in animals and unicellular organs, as 

 ■e.g. the hyphse of Mucor. It has been suggested, to meet this 

 difficulty, that the unequal growth affects primarily the cell- 

 walls ; but this explanation does not quite meet the case of 

 hydroids. A general statement, applicable to multicellular 

 plants and animals and to unicellular organs, may be made, if 

 the cell-wall be recognized as living, as follows : The imbibi- 

 tion distends the living substance on the convex side, the 

 water probably being in part drawn from the concave side. 



Going a step farther, the tropic agent produces such a 

 change in the molecules of the curving region as to cause them 

 to imbibe water with abnormal rapidity. This change may, 

 however, be produced by the agent either directly or indi- 

 rectly. In all cases in which the sensitive part of the organ 

 becomes the curving part, the action of the agent is direct. 

 Examples are found in the thigmotropism of tendrils, in the 

 phototropism and thigmotropism of stems, and, probably, in 

 "traumatropism. In cases in which the curving part is remote 

 from the sensitive part, as in geotropism, the action of the 

 agent is indirect. In these cases there must be a transmission 

 of a stimulus from the sensitive part (the tip) to the respond- 

 ing part, producing the required molecular change there. 



Concerning the nature of the changes induced by the tropi- 



