METABOLIC PROCESSES IN THE PLANT. 329 



light also participate directly in the action. We shall make 

 experiments which go to show that access of light favours the 

 oxidation of organic acids outside the organism. A direct influ- 

 ence of the rays of light on the process of acid decomposition 

 in the cells of plants therefore appears to be quite possible. 



We know that if parts of succulent plants are left for some 

 time (e.g. for a night) in a limited quantity of air, they cause a 

 diminution in its volume. Oxygen, in fact, enters into combina- 

 tion with formation of organic acids. The same structures, on the 

 other hand, increase the volume of the air surrounding them in the 

 daytime, since they expire Oxygen. This Oxygen is, however, 

 not directly split off from the molecules of the decomposing 

 organic acids, but is a product of the actual assimilatory activity 

 of the chlorophyll corpuscles. The necessary Carbon dioxide is, 

 however, afforded in the manner above discussed by the decom- 

 posing organic acids. 



We now proceed to give directions for experiments which will 

 give information concerning the behaviour of organic acids in the 

 Crassulacege. 



For our observations we employ vigorous pot-plants, grown 

 under the most favourable external conditions, of Bryophyllum 

 calycinum, Echeveria metallica, or Rochea falcata (the last plant 

 is particularly suitable). In comparative work on the accumu- 

 lation or disappearance of acids in the tissue of the leaves, we 

 employ either the two opposite leaves of a pair (e.g. Rochea), or 

 we experiment with a single leaf only (e.g. Echeveria metallica), 

 and divide it longitudinally into two portions as nearly as possible 

 similar. The objects are weighed immediately after removal 

 from the plant. If it is not required to determine the quantity of 

 free acid which they contain until some time has elapsed, they are 

 placed, e.g., on moistened blotting-paper, and covered with bell- 

 glasses. We determine the amount of acid in the leaves by 

 titration. The quantity of potash used in titrating gives a direct 

 measure of the acidity of the cell-sap (for method see 130). 



We first carry out the following investigation : A plant of 

 Rochea is exposed during the day to direct sunlight, and towards 

 evening, about five or six o'clock, a pair of leaves are cut off. If 

 we experiment with Bryophyllum, we take a few pairs of leaves ; 

 in experiments with Echeveria we use only one of the large leaves. 

 Half of the material is at once tested acidimetrically, the other 

 half next morning, the objects having been kept till that time in 



