Chap. I] 



WATER 



(sclcrophylly) (Figs. 1 1, 12), fleshy (chylophylly or leaf-succulence) (Figs. 9, 10), 



or rudimentary and caducous (aphylly). In the last cases, the axes are rich 



in chlorophyll and carry on the process of assimilation. The axes are 



sometimes slender like rods, dry and hard, as in Ephedra, Spartium, and 



other plants (sclcrocauly), or they 



become short and thick, often 



even spheroidal, and filled with 



mucilaginous sap, as in the Cac- 



taceae {cJiylocauly or stcm-succu- 



Icncc). 



Reduction in surface is often 

 associated with the production 

 of thorns, because shoots or 

 leaves become pointed structures 

 rich in sclerenchyma and tran- 

 spiring but slightly, if at all ; 

 their utility in this form, as pro- 

 tective organs against animals, 

 if a reality, is only of a secondary 

 nature. 



Xerophyteswith pinnate leaves 

 have the power of automatically 

 adjusting the transpiring leaf- 

 surface. The mobile leaflets open 

 out under the moderate illu- 

 mination of the early morning, 

 or of a dull day, but close up 

 under intense insolation and the 

 profuse transpiration which ac- 

 companies it. The fact that 

 plants possessing pinnate leaves 

 with a relatively large and thin 

 leaf-surface thrive alongside of 

 aphyllous plants in the driest 

 regions proves how perfectly this 

 arrangement works. 



Other leaf-bearing xerophytes 

 have their leaves, or leaf-like 

 cladodes, arranged parallel to 



the incident rays of sunlight, and are consequently less intensely heated 

 and illuminated. This peculiarity disappears in many species when there 

 is no longer any danger of excessive loss of water, for instance in the 

 mangrove-tree, Sonneratia acida, when it is grown on a soil poor in salts, 



Fig. jo. Xerophilous structure. Wet saline soil of 

 tropical shores. Batis maritima. Plant with suc- 

 culent leaves. Natural size. After Dammer. 



