MOTILE MECHANISMS IN HIGHER PLANTS 455 



by the unequal expansion which occurs in the different cell 

 membranes or parts of a membrane when wetted. The inequality 

 of swelling (and correspondingly of contraction during drying) 

 is related to a difference in the structure of the respective 

 regions ; it can often be detected in other ways than by the test 

 of water absorption. 



3. Cohesion of Water. — Mechanisms coming under this head 

 depend upon the resistance to rupture which is exhibited by a 

 mass of water when enclosed in a chamber the walls of which it 

 can wet. In this class are to be found some of the most remark- 

 able examples of delicately adjusted arrangements whereby the 

 production of movements suitable for particular purposes is 

 ensured. In all cases the cells concerned possess walls thickened 

 locally. The thickened part is more or less distorted by the pull 

 of the thinner portions as these are drawn inwards when water 

 is withdrawn from the interior of the cell by evaporation or other 

 means. The efficiency of this type of mechanism depends on the 

 condition that the walls are impermeable to air ; otherwise the 

 continuity of the contained water would be impossible when 

 the pressure within the cell is negative. 



Although the mechanisms in the two last-mentioned classes 

 may and often do work after the protoplasm has disappeared 

 from the cells directly concerned, the actual structure whereby 

 they are effective has been laid down previously by the living 

 substance. These instances rather gain than lose in interest by 

 this circumstance ; in this respect they are paralleled by the 

 water-conducting elements of the wood in which the conducting 

 tissues are very complex and the complexity is definitely related 

 to the functions they have to discharge. Nevertheless, the 

 functions themselves are not assumed until after the disappear- 

 ance of the protoplasm, which was the architect and builder of 

 these same cells and tissues. 



Botanists have long been familiar with the movements which 

 are due to the response on the part of living protoplasm to 

 external stimuli but in view of the numerous investigations of 

 which they have formed the subject, it is surprising how very 

 little we really know about them. This is even more true of the 

 rhythmic movements frequently met with in growing plants 

 which are not referable to any stimulating agent that can be 

 detected readily. 



