232 



THE PLANT. 



X. 



These two diagrams do not agree as to the position of the 

 maximum assimilation, the former placing this in the orange 

 red, where the darkest absorption band of chorophyll actually 

 occurs ; the latter on the greenish side of the yellow, where the 

 rays brightest to the eye occur. The former diagram is 

 probably the more correct. 



In connection with the action of light upon plants, it may be 

 pointed out that though light of low refrangibility (i.e., near 

 the red end of the spectrum) is undoubtedly most active in 

 promoting carbon assimilation, yet light of short wave length 

 (i.e., from the violet end of the spectrum) is of most im- 

 portance in directing the motion of the leaves or of affecting 

 the growth in length of the shoots. 



With reference to our knowledge of the method by which the 

 carbon dioxide of the atmosphere is brought into contact with 

 the chlorophyll-containing cells, considerable advances have re- 

 cently been made. Blackman in 1895" described experiments 

 by which he proved that carbon dioxide found its way into (in 

 assimilation) and out of (in respiration) the leaves almost ex- 

 clusively by the stomata, and not as was generally believed- 

 by diffusion through the cuticle and epidermis. When the rate 

 at which carbon dioxide is absorbed by a vigorously growing 



* Phil. Trans., vol. 186 (1895), 485. 



