MOTILE MECHANISMS IN HIGHER PLANTS 459 



tips of roots which are placed in new positions relatively to 

 the vertical is very possibly the stimulus which sets the chemical 

 processes in the root awry, as Czapek has shown. But why 

 these should act in such a way as to produce the particular kind 

 of movement which follows on the stimulus — and the actual 

 direction of movement will depend on circumstances which are 

 bound up with the organism as a whole — we do not know. 



Much of what has here been said about these protoplasmic 

 mechanisms and of the difficulty of realising the actual condi- 

 tions under which they work is of course familiar to botanists who 

 are concerned with the behaviour of living plants. They are 

 also only too well acquainted with the experimental difficulties 

 that confront the investigator and are aware of the fact that the 

 ancillary sciences are as yet unable to afford much material 

 assistance. It may be, however, that an attempt such as this to 

 indicate even in the barest outline a few of the more obvious 

 problems of one branch of plant physiology may not be wholly 

 devoid of value. 



In contrast with the obscurity that enshrouds the operations 

 which are causative of movement in the living cell, the 

 mechanisms which are not so directly concerned with the con- 

 temporary action of living protoplasm have been materially 

 elucidated within the last fifteen years or so, mainly by the 

 efforts of Steinbrinck, Kammerling, Schwendener and others. 



The movements of various parts of the fruit by virtue of 

 which the seeds are shot out, often to a considerable distance, had 

 received full descriptive treatment at the hands of many of the 

 older observers and differential tensions were freely and some- 

 what loosely invoked to explain them. But it is only within the 

 last few years that we have gained an insight into the real 

 nature of the mechanism concerned and have learnt to under- 

 stand how it works. 



Taking the movements due to hygroscopic change first, namely 

 those depending on the unequal expansion or contraction which 

 occurs in the cell walls of the different parts of the motile organ 

 when it is wetted or dried, we find on the one hand that entire 

 tissue groups are involved, on the other that only single cell 

 membranes are concerned. All hygroscopic organs, however, 

 possess in common the faculty of absorbing water unequally on 

 opposite surfaces and so of curving inwards or outwards as the 

 case may be. Examples of such hygroscopic motile structures 



