484 



PRACTICAL BOTANY 



(7) Reduction of leaf surface, as in the case of needle-shaped 

 (Fig. 248), scale-like, or other small, narrow kinds of leaf. 



(8) Partial or complete absence of useful leaves, as in the 



" switch plants," such 

 as Spartium, Casuarina 

 (Fig. 365), and so on ; 

 and in asparagus, the 

 cacti (Fig. 65), and 

 some euphorbias (Fig. 

 366). 



445. Discussion of 

 xerophytic characters. 

 The first four kinds of 

 characters operate, as 

 may be readily under- 

 stood, greatly to limit 

 the loss of water. Close- 

 ly packed parenchyma 

 cells m the leaf interior, 

 with few air spaces, 

 give little opportunity 

 for the water in the 

 interior cells to escape 

 into the internal atmos- 

 phere of the leaf and so 

 gradually pass off into 

 the air. When the epi- 

 dermis is thick, espe- 

 cially if it is covered 

 with a heavy, water- 

 proof cutinized layer 

 («<, Fig. 364), transpi- 

 ration when the stomata are closed is very scanty. Stomata 

 situated at the bottoms of microscopic pits or furrows are 

 much protected from drafts of air and therefore give oif vapor 

 slowly. And when the stomata-bearing leaf surface, as in the 



Fig. 365. Casuarina, an Australian switch 

 plant destitute of foliage leaves and depend- 

 ing on the chlorophyll-containing cells of the 

 bark for photosynthesis 

 Photograph hy Robert Cameron 



