ADAPTATION TO WATER 



157 



following t^-pes may be distinguished: (1) the normal form; (2) 

 the succulent form; (3) the dissected form; (4) the grass form; 

 (5) the needle form; (6) the roll form; (7) the scale form. In 

 many of these, subgroups based upon the structural protection, 

 viz., cuticle, hairs, and water cells, may be recognized. 



177. Normal leaf xerophytes. The leaf is normal in size and 

 shape, and of the usual dorsi ventral character. The necessary 

 decrease in transpiration is brought about by structural modi- 

 fications, rather than by a reduction in size. Three well-defined 

 subtypes may be recognized with respect to the structure used to 



Fig. 49. A normal leaf xerophyte, Arabis fendleri, in which water loss is 

 decreased by a cuticle, and the conversion of the sponge tissue into 

 a loose palisade tissue. 



secure protection. These are the cutinized, the lanate, and the 

 storage leaf. The cutinized leaf compensates for a low water 

 content by thickening the outer wall of the epidermis and render- 

 ing it impervious by the addition of cutin. The cuticle thus 

 formed sometimes becomes very thick, filling half or more of the 

 cell cavity. It is usually thicker upon the upper surface of hori- 

 zontal leaves, but is more uniformly developed upon upright or 

 oblique ones. The cuticle is often reinforced by a marked develop- 

 ment of palisade tissue. Cutinized leaves are usually leathery 

 in texture; and in addition, they are often evergreen. Practically 

 all xerophytes with smooth leaves of the normal form belong here, 

 though many of them have storage cells as well. Good examples 

 of this type are found in the bearberry, Ardostaphylus uva-ursi, 

 in species of Allionia, Pentstemon, etc. 



