Page:Popular Science Monthly Volume 79.djvu/554

550 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 transpiration 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 was seen to be not affected by progressive desiccation. Within the last few months, however, Professor H. M. Richards 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