550 THE POPULAR SCIENCE MONTHLY 



weighing nearly a hundred pounds had a succulence of 3 on the scale 

 noted, in which condition it transpired water at the rate of 10 g. daily. 

 A year later the succulence had fallen but slightly, being 2.8; the rate of 

 transpiration, however, had decreased to one half, being now but 5 g. 

 daily. 



The theoretical explanation of the sudden drop in daily transpira- 

 tion given above will not suffice for this case which is a comparison of 

 successive seasons. The slowness of the rate of loss would allow ample 

 time for the diffusion from the great water-balance of the plant to take 

 up a deficiency at any given surface. Morphological alterations are not 

 found, and the theoretical explanation that presents itself would be that 

 the colloidal condition of the walls, or inner membranes had been 

 altered. The altered concentration of the cell-sap with its included 

 acids and other substances might well be responsible for a change similar 

 to that which takes place on the surface of a plate of jelly when acted 

 upon by various reagents. A second phase of interest in the acids of the 

 sap was found in their daily variations. Earlier the determinations 

 of the acidity of the sap were made rather at random, with the general 

 result that it w T as seen to be not affected by progressive desiccation. 

 Within the last few months, however, Professor H. M. Eichards has gone 

 into this matter more exactly, with the astonishing discovery that the 

 acidity of these plants is very great in the morning and decreases steadily 

 throughout the day until evening, when it begins to rise and continues 

 to increase until morning. So great is the amplitude of this change 

 that a cactus may contain four times as much acid in the morning as at 

 sunset. It is needless to say that the problem as to the making and fate 

 of this acid is a matter that excites the keenest interest in connection 

 with the respiration and food-construction processes in the plant. At 

 present the change seems to be directly dependent on the course of the 

 temperature. It is to be recalled that the water-holding power of the cell 

 colloids must be notably affected by this variation in the acids of the 

 cell. 



The probability of the absorption of water vapor from the air by 

 plants of the desert is one of perennial interest, especially to those who 

 take a sentimental view of desert life. The spines of cacti, especially 

 the large curved and hooked ones of the Echinocacti, will take up water 

 vapor, as has been demonstrated more than once in my work, but the 

 very small amount of moisture thus acquired is not available to the 

 living cells and is quickly lost when the plants are exposed to direct 

 sunlight. The bark of the ocotillo (Fouquiera) will absorb liquid water 

 and yield it to growing tissues, as has been found by Lloyd, the hairs of 

 some south African succulents have been found to absorb moisture, and 

 the fleshy beach plants will absorb either water vapor or liquid water 

 through the leaves, especially when in a desiccated condition. Doubtless 



