124 



THE PLANT 



water for the time of extreme drouth. Xerophytic species of Helianthus 

 furnish examples of transverse bundles of storage cells, while those of 

 Mertensia illustrate the more frequent arrangement in which the water tissue 

 forms horizontal layers. 



2. The succulent form. Many succulent leaves are normal in shape and 

 size, though always thicker than ordinary leaves. Usually, however, they 

 are reduced in size and are more or less cylindrical in form. The necessary 

 decrease in transpiration is effected by the reduction in surface, the general 

 storage of water, a waxy coating, and, often also, by a very thick cuticle. 

 Agave, Mesembryanthemum, Sedmn, and Senecio furnish excellent ex- 

 amples of this type. 



3. The dissected form. The reduction in surface is brought about by 

 the division of the leaf blade into narrow linear or thread-like lobes which 



are widely separated. The 

 latter are themselves protected 

 by a hairy covering or a thick 

 cuticle, which is often sup- 

 plemented by many rows of 

 palisade, or by storage tissue. 

 Artemisia, Senecio, and Gilia 

 contain species which serve as 

 good examples of this type. 



4. The grass form. Xero- 

 phytic grasses and sedges have 

 narrow filamentous leaves 

 with longitudinal furrows 

 which serve to protect the 

 stomata. The furrows are 

 sometimes filled with hairs which are an additional protection, and the 

 leaves often protect themselves further by rolling up into a thread-like 

 shape. The elongated subulate leaves of /uncus and certain Cyperaceae 

 are essentially of this type, although they are usually not furrowed. 



5. The needle form. This is the typical leaf of conifers, in 'which a 

 sweeping reduction of the leaf surface is an absolute necessity. The rela- 

 tively small water loss of the needle leaf is still further decreased by a 

 thick cuticle, and usually also by hypodermal layers of sclerenchyma. 



6. The roll form. Roll leaves are frequently small and linear. Their 

 characteristic feature is produced by the rolling in of the margin on the 

 under side,, by which an almost completely closed chamber is formed for 

 the protection of the stomata which are regularly confined to the lower 

 surface of the leaf. The upper epidermis is heavily cutinized and the lower 



Fig. 34. Diplophyll of Mertensia linearis, 

 showing water cells (chresard, 3-% light, 1). 

 X130. 



