740 



ECOLOGY 



arrangement of the parts. For example, in Raoidia (fig. 1060) and in 

 other alpine or arctic cushion plants, the short stems are so closely packed 

 together that only the tips are exposed. Cushions are formed by various 

 mosses (as Leucobryum, Bartramia, and Dicranuni) and by some lichens 

 (as Cladonia rangiferina, fig. 898), but among seed plants the habit is 

 rare outside of inclement climates. An additional advantage from 



FIG. 1063. A spherical cactus (Echinocactus Wislizenf), representing an extreme con- 

 trast to a thin-leaved tropical evergreen, since it has a minimum transpiring surface in pro- 

 portion to its volume; note the numerous recurved spines. Photograph by FULLER. 



dwarf ness is increased ground protection; as a consequence, transpira- 

 tion is still further reduced and temperature changes are less rapid. 

 Perhaps the advantages of proximity to the ground are best realized in 

 rosette herbs, whose leaves often are closely appressed to the soil (fig. 

 1036). A third and perhaps the greatest advantage in dwarf ness is 

 seen where the latter is most in evidence, namely, in alpine and arctic 

 regions. There it makes possible protection by snow, and con- 

 sequently there is a suspension of transpiration during the very 

 months when it would be most dangerous because of the cessation 

 of absorption; furthermore, snow covers protect greatly from cold. 



