HYDROHARMOSE 125 



one often protected by hairs. This type is found especially among the 

 genera of the Ericales, but it also occurs in a large number of related 

 families. 



7. The scale form. Reduction of leaf surface for preventing excessive 

 water loss reaches its logical culmination in the scale leaf characteristic of 

 many trees and shrubs, e. g., Cupressus, Tamarix, etc. Scale leaves are 

 leathery in texture, short and broad, and closely appressed to the stem, as 

 vvi'^11 as often overlapping. 



169. Types of stem xerophytes. In these types the leaves are deciduous 

 early in the growing period, reduced to functionless scales, or entirely absent. 

 The functions of the leaf have been assumed by the stem, which exhibits 

 many of the structural adaptations of the former. Warming^ has distin- 

 guished the following groups : 



1. The phyllode form. -The petiole is broadened and takes the place of 

 the leaf blade which is lacking. In other cases, the stem is flattened or 

 winged, and it replaces the entire leaf. This type occurs in Acacia, 

 Baccliaris, Genista, etc. 



2. The virgate form. The leaves either fall ofl: early or they are reduced 

 to functionless scales. The stems are thin, erect, and rod-like, and are often 

 greatly branched. They are heavily cutinized and palisaded, and the stomata 

 are frequently in longitudinal furrows. This type is characteristic of the 

 Genisteae; it is also found in Ephedra, many species of Polygonum, 

 Lygodcsmia, etc. 



3. The rush form. In Hcleocharis, many species of Juncus, Scirpns, and 

 other Cyperaceae, the stem, which is nearly or completely leafless, is cylin- 

 drical and unbranched. It usually possesses also a thick cuticle, and several 

 rows of dense palisade tissue. 



4. The cladophyll form. In Asparagus the leaves are reduced to" mere 

 functionless scales, and their function is assumed by the small needle-shaped 

 branches. 



5. The flattened form. As in the preceding type, the place of the scale- 

 like leaves is taken by cladophylls, which are more or less -flattened and leaf- 

 like. Ruscus is a familiar illustration of this form. 



6. The thorn form. This is typical of many spiny desert shrubs, in which 

 the leaves are lost very early, or, when present, are mere functionless scales. 

 The stems have an extremely thick cuticle, and the stomata are deeply 

 sunken, as a rule. Collet ia and Holacantha are good examples of the type. 



^Lehrbuch der Oekologischen Pflanzengeographie. 2d ed., 196. 1902. 



