312 



ASSIMILATION. 



iophyllum to the action of light colored by passing through 

 glass, ma}' be stated as follows : The activity of the plant in 

 decomposing carbonic acid diminishes with glasses used in the 

 order given : (1) uncolored " ground" glass, (2) yellow, (3) un- 

 colored transparent glass, (4) red, (5) green, (6) blue. By all 

 the experimenters now referred to, the evolved gas was collected 

 and examined. 



828. Measurement of the amount of assimilation. Sachs, in 

 1864, appears to have been the first to employ the now well- 

 known method of measuring the activity of the assimilative 

 process by counting the bubbles of gas which are given off 

 by a submerged water plant (see 814). Since the gas given 

 off by the plant is not pure oxygen, but is variable in compo- 

 sition, 1 the method cannot be regarded as sufficiently precise 

 for very accurate experiment ; but as it admits of such rapid 

 change in all external conditions, it answers for all practical 

 purposes. 



829. The effect of colored light upon the assimilative activity 

 of plants not submerged, as in the above experiments, but in 

 the air, was first examined by Cailletet, 2 in 18G7. He placed 

 the plant under bell-jars containing air with eighteen, twenty- 



1 For remarks upon the possible errors which may attend the use of this 

 method, consult Miiller (Pringsheim's Jahrb. vi., 1868, p. 478). 



2 L. Cailletet placed leaves in jars filled with air containing from 18 to 30 

 per cent of carbonic acid, and then exposed these to light which had passed 

 through colored glass. In one case the light was transmitted through a solu- 

 tion of iodine in carbon bisulphide. After an exposure of from eight to ten 

 hours, the amount of carbonic acid remaining undeeomposed by the action of 

 the leaves was found to be as follows : 



Two points must be specially noticed : (1) the striking effect of the large 

 amount of carbonic acid in the third series ; (2) the anomaly presented by the 

 green glass, which is quite unexplained. It is to be regretted that no fuller 

 account of the character of the glasses used is given (Comptes Remlus, Ixv., 

 1867, p. 322). 



