185 



assume their natural position; apparently (hey are influenced only 

 by light, hut here again the expei'iinents of' I-'ficffek have shown 

 tliat gravitation plays an important part ; f. i. many leaves when 

 brought away from their normal position can return to it in the 

 dark, which evidently can be effected only -through the medium of 

 the gravitation-stimulus. 



Here again the lack of data regarding the position of the statocysts 

 in the leaves prevents us from prosecuting the I'esearch as to these 

 movements in connection with our theory. 



C. Stimulation by Light alone. 



Anxotonic curves are seldom caused by the light-stimulus alone; 

 the instance of this most fully investigated is that of the "trans- 

 versal heliotropism", whereby certain leaves place themselves per- 

 pendicular to the incident bundle of light. Hahkui.andt ') endeavours 

 to explain this movement by assuming that the middle-field of that 

 portion of the outer layer that adjoins the lowei' wall of the sen- 

 sitive epidermical cells is more sensitive to light than its surround- 

 ings. If now the leaf seeks to reach the desired position by the 

 shortest way, this must he accompanied by the quickest increase 

 in the intensity of the stimulus, exactly in the same way thus as 

 was assumed above with respect to the stimulus of gravitation. 



The stimulation of an sensitive organ causes everywhere a cer- 

 tain sensation or movement, whereby, however, the nature of the 

 sensation or of the movement, is determined solely by the special 

 properties of those parts of the organism which lie ouLtide the 

 perceiving sense-organ; consequently the nature of the stimulus 

 can never excercise any influence whatever u|)0u the effect that the 

 organism shows. 



If this conclusion should hold good for the plant too, as is very 

 probable from the nature of the case, and if we also bear in mind 

 that all anxotonic movements mentioned are executed in the same 

 way, it would follow that it is sufïicient for the plant to possess 

 only one single sensitive organ for all these movements, induced 

 by gravitation, by light or by both. 



Therefore not even for transversal-heliolropism an exception should 

 be made, for if we consider that a static apparatus without statoliths 

 (starch-grains) could not be stimulated by gravitation but can never- 

 theless remain sensitive to light, it might very well be possible 



1) Die Ijichtsinnesorgane der Laubbiattei-, f905, p. 127. 



