ORGANIC RESPONSE 277 



able to exist and maintain a foothold. Such plants 

 usually absorb very rapidly such rain as falls, or 

 else have very long tap-roots that penetrate deeply 

 into the soil and take up the maximum amount 

 of water. Thus one of the morning glories (Con- 

 volvulus), which grows on the dry western plains, 

 instead of developing into a delicate, weak-rooted, 

 climbing vine, as do most of the Convolvuli, grows as 

 a sort of bush, a foot or so high, and sends a huge 

 root down twenty or thirty feet into the soil. 



Another advantageous structure found in plants 

 of such regions is one which brings about the storing 

 up of water. Succulents or fleshy plants have a 

 large amount of parenchyma tissue which holds 

 water as a sponge. This is true of many species of 

 cactus. In addition, such plants usually have the 

 exposed surface reduced to a minimum. This 

 retards and lessens the loss of water through evapora- 

 tion. In Cacti of various sorts the stem is succulent, 

 and the leaves, as such, are absent. Xerophytes 

 are also adapted to economize the water they have 

 in store and undergo long periods of drought. An 

 extreme example is the well-known " Resurrection 

 plant " (Selaginella), which grows on the sides of 

 rocky cliffs in Mexico and may be pulled up and 

 dried for years, only to uncurl and freshen up again 

 in a few hours when placed in a saucer of water. 



Midway between the two extremes just described, 

 the hydrophytes and the xerophytes, are the plants 

 we are most familiar with, the Mesophytes. These 

 are adapted to various degrees of temperature or 



