330 PHYSIOLOGY OF NUTRITION. 



complete darkness in moist air under a bell-glass. I found, e.g., 

 that the two members of a pair of Rochea falcata leaves weighed 

 respectively 12'G gr. (a), and 13'6 gr. (6). a was examined in the 

 evening, immediately after being cut off, b not till the next morning. 

 The pulp from a required for neutralisation 2 - 6 c.c., that from 1 12'5 

 c.c., of dilute potash. For every 10 gr. of leaf substance employed 

 these numbers give for a 2*1 c.c., for b 9'2 c.c., of potash, a differ- 

 ence of 7'1 c.c. Another plan is to cut only one leaf from the 

 plant in the evening, and determine its acidity at once, while the 

 second leaf is not removed till the next morning, the plant having 

 meanwhile been kept in complete darkness. The experiments 

 always indicate a considerably higher acidity in the case of leaves 

 which have been kept some time in darkness. 



To prove still more certainly that the acidity of the sap of 

 Crassulaceous plants diminishes in presence of light, but in- 

 creases in darkness, it is necessary to make the following experi- 

 ment : Two opposite leaves of a plant of Rochea are cut off early 

 in the morning. One leaf is halved longitudinally, and in one- 

 half, after weighing it, we at once determine the quantity of acid 

 in the cell-sap by titration. The second half we hang under a bell- 

 glass, the air within which is saturated with moisture, and exclude 

 the light by covering over with a cardboard cylinder. The second 

 leaf is hung up under a bell-glass, the space within which is also- 

 saturated with moisture, and exposed to very bright diffuse day- 

 light, care being taken that the back of the leaf also receives light 

 reflected from a suitably arranged mirror. Proceeding in this 

 Avay the objects are exposed to approximately the same conditions 

 of temperature, and diminution of acidity due to rise of temperature 

 is, as far as possible, excluded. If in the evening we determine 

 the acidity of the half leaf and of the entire leaf, we shall find 

 that the former is richer in acid than the latter (comparing natu- 

 rally equal weights of fresh leaf substance). We easily satisfy 

 ourselves of the important influence of temperature on the process 

 of acid decomposition in the leaf of Crassulaceous plants, if in the 

 early morning we remove a few of the acid leaves from plants of 

 Bryophyllum, Rochea, or Echeveria, which have been exposed to 

 normal conditions of environment, and in some of them determine 

 the acidity at once, while in the remainder the acidity is deter- 

 mined after they have been kept in complete darkness for about 

 twelve hours, part at a lower temperature (say 12-16 C.), part 

 in a thermostat at a temperature of 30 C. It will appear that 



