PLANT SOCIETIES 



245 



Notice how the spines are arranged on the surface, and 

 if there are any fruits, buds, or flowers, where they occur. 

 Peel off a little of the epidermis and observe its thick, 

 horny texture. Cut a cross section through a joint about 

 midway from base to apex and 

 examine with a lens. Notice the 

 thick layer of green tissue next 

 the epidermis, and within that, a 

 band of tough, woody fibers in- 

 closing the soft pulpy mass that 

 makes up the interior. (If the 

 woody layer is not easily made 

 out, allow your specimen to dry 

 for about twenty -four hours, and it 

 will become quite distinct.) Make 

 a longitudinal section through the 

 center of a joint and trace the 

 course of the woody fibers ; do 

 they get any more abundant 

 toward the base .'' Do any of them pass into the spine 

 clusters .-' What do the spines represent ? What is the 

 use of the green layer just under the epidermis ? Why 

 is it so much more abundant in the cactus than in ordinary 

 stems .'' Lay aside a section of a cactus plant, or a leaf 

 of yucca, agave, or other fleshy xerophyte to dry and see 

 how long it takes to lose its moisture. What would you 

 conclude from this as to its retentive power .-' 



470. — A plant of opuntia, 

 showing young branches and 

 flowers from the nodes. 



352. Other Xerophyte Adaptations. — Plants exposed to 

 periodic and occasional droughts frequently provide against 

 hard times by laying up stores of nourishment in bulbs and 

 rootstocks and retiring underground until the stress is over. 

 This is known to botanists as the geophilo7is, or earth-loving 

 habit. Others, as some of the lichens, and the little resur- 

 rection fern {P olypodium incaniini), so common on the 

 trunks of oaks and elms, make no resistance, but wither 

 away completely during dry weather, only to waken again 

 to vigorous life with the first shower. 



