LEAVES WITH EEGAED TO LIGHT. 449 



result being determined by the light-stimulus. Therefore, when 

 the plant is on the klinostat, where the balance between epinasty 

 and apogeotropism is destroyed, the ligbt-stimulus is not strong 

 enough to keep the leaves in the diaheliotropic plane. 



The cases such as that of the bean seem to be intermediate 

 between those of It. Ficaria and of the cherry. There is evidence 

 that the leaves can remain for a considerable time approximately 

 at right angles to the light without the help of gravitation ; but 

 ultimately they may be forced by epinasty to relinquish the dia- 

 heliotropic plane. 



Thus the result of the experiments with the klinostat is, on 

 the whole, to confirm the view published in ' The Movements 

 of Plants '*, that the power which leaves have of placing them- 

 selves at right angles to the incident light is due to a specialized 

 sensitiveness to light — diaheliotropism, which is able to regulate 

 or govern the action of other external forces such as gravitation, 

 or of internal forces such as epinasty. 



Appendix on the Klinostat used in the above Experiments, made 

 after the design ©/"Horace Darwin. 



The klinostat f used in my experiments differs in construction 

 from that of Professor Sachs, which I had the advantage of seeing 

 in action in his laboratory. The principle on which my klinostats 

 are constructed is entirely due to my brother, Horace Darwin ; 

 and they have been found to answer so admirably that a detailed 

 description of the instrument may not be out of place X- 



The spindle of the klinostat (that is, the axis of rotation about 

 which the plant turns) does not form an integral part of the clock 

 which supplies the motive power, but is merely connected with 

 the clock by a loop of silk passing round two pulleys, one on the 



* Pp. 438-444. 



t The klinostat used by John Hunter {he. cit.) was made on the principle of a 

 water-clock. The beans which were used in his experiment were placed in a basket 

 filled with earth ; this was attached to a straight rod serving as a spindle, which 

 was supported at either end by a notch in the rim of a large tub filled with water, 

 which flowed slowly out through a minute hole close to the bottom of the 

 tub. The rotation was communicated by a piece of string wrapped round the 

 basket; to the free end of the string a floating weight was fastened; and as the 

 weight sank the basket rotated, at the rate of about one revolution in eight 

 hours. 



I The instruments were made by the Cambridge Scientific-Instrument Com- 

 pany, 18 Panton Street, Cambridge. 



