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THE POPULAR SCIENCE MONTHLY 



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 



Ficj. 3. An Invested Echinocactus absorbing Wateb xhbough a Clay Cup 



IMBEDDED IN THE BASAL PORTION. 



great tree cactus with its mucilaginous juice varies from 7 to 10 atmos- 

 pheres 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 



