Page:Popular Science Monthly Volume 79.djvu/550

546 plant the physical qualities of a huge roll of jelly. The comparison may be made more inclusive, however. As the spinose plants have a sap high in electrolytes, mineral salts, or of substances showing osmotic activity, so these plants are rich in suspension colloids, and simulate a mass of gelatine capable of taking in and holding great quantities of water. The most sketchy knowledge of the colloids prepares one to learn that the sap of these plants shows a very low osmotic pressure under ordinary conditions of growth. The melon cacti of Arizona have a drinkable sap which shows but 3 to 5 atmospheres of pressure, the

great tree cactus with its mucilaginous juice varies from 7 to 10 atmospheres and the opuntias (cylindrical) as high as 10 to 12 atmospheres (Fig. 2.) These values are to be contrasted with those given above for the spinose forms, which are seven to thirty times as great and with such ordinary broad-leaved shrubs as. the lilac, in which pressures from 20 to 30 atmospheres are the rule.

These purely physical features of the succulents are correlated with habits and modes of activity widely different from those of the spinose