HYDROHARMOSE 



123 



intensely xerophytic habitats, and, consequently, it is difficult, if not unde- 

 sirable, to place such xerophytes under a particular group. The best that 

 can be done is to recognize the types arising from extreme or characteristic 

 modification, and to connect the less marked forms as closely as possible 

 with these. Halophytes differ from xerophytes only in the fact that the 

 chresard is determined by the salt-content of the habitat, and not by the tex- 

 ture of the soil. In consequence, they should not be treated as a distinct 

 group. 



168. Types of leaf xerophytes. 

 In these, adaptation has acted 

 primarily upon the leaf, while 

 the stem has remained normal for 

 the most part. Even when the 

 leaves have become scale-like, 

 they persist throughout the grow- 

 ing season, and continue to play 

 the primary part in photosyn- 

 thesis. The following types may 

 be distinguished : 



I. The normal form. The 

 leaf is of the usual dorsiventral 

 character. In place of a reduc- 

 tion in size, structural modifica- 

 tions are used to decrease 

 transpiration. With respect to 

 the protective feature that is 

 predominant, three subtypes may 

 be recognized. The ciitinized leaf 

 compensates for a low water-con- 

 tent by means of a thick cuticle, 

 often reinforced by a high de- 

 velopment of palisade tissue. 

 Such leaves are more or less 

 leathery, and they are often evergreen also. Arctostaphylus and many 

 species of Pentstemon are good examples. Lanate leaves, i. e., those with 

 dense hairy coverings on one or both surfaces, as Artemisia, Antennaria, 

 etc., regularly lack both cuticle and -palisade tissue. The protection against 

 water loss, however, is so perfect that the chlorenchym often assumes the 

 loose structure of a shade leaf. Storage leaves usually have a well- 

 developed cuticle and several rows of palisade cells, but their characteristic 

 feature is the water-storage tissue, which maintains a reserve supply of 



Fig. 33. Staurophyll of Bahia disseda, 

 showing extreme development of palisade 

 (chresard, 3-9^; light, 1). X 130. 



