Chap, in.] the Movements of Plants. 557 



rays of light were more heliotropically active, and the fa 

 confirmed by Payer in 1842 ; but Dutrochet in 1S43 mainta 

 and incorrectly, that it is the brightness of the light, and not 

 its refrangibility, which is the determining factor. Zanto 

 found in 1843 that red, orange, and yellow light arc heliotro- 

 pically inactive. Gardner on the contrary in 1X44, and 

 Guillemain in 1857, came with the help of the spectrum 

 to the conclusion that all its rays are heliotropically active, and 

 the question long remained hampered by these contradictory 

 statements, till it was taken up again in 1864. This was 

 a similar case to that of the question of the effect of varie- 

 gated light on the elimination of oxygen and the formation of 

 chlorophyll. Daubeny had given attention to the subject in 

 1836 and inclined to the view, that it was the brightness of the 

 light rather than its refrangibility which was the important 

 point; and Draper's observation, made with the spectrum in 

 1844, that the elimination of oxygen reaches its maximum in 

 yellow light and decreases on each side of it, was generally 

 understood as though it was a question only of the brightness 

 of the light. It is only within recent times that this view has 

 been abandoned, and in the same way all the investigations 

 which have just been mentioned were not settled till after 

 i860, and were scarcely turned to any theoretical account. 



The bright point in the history of phytodynamics at this time 

 is Briicke's treatise in 1848 on the movements of the leaves in 

 Mimosa, not only on account of the very important results which 

 it records, but still more for the exactness of its method which 

 has made it a model of research in these subjects. lie first 

 established the essential difference between the periodical 

 nocturnal position of the leaves of Mimosa and the position 

 which they assume when irritated, and showed that the former 

 is connected with an increase in turgidity, the latter with 

 relaxation ; he showed further that if the upper half of the 

 organ is removed, the periodical movements and the irrita- 

 bility both continue. Of great importance to the theory was 



