LIFE FORMS AND SYNECOLOGWAL UNITS 



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dead leaves and drifted dust, often traversed by tiny roots, holds water 

 very well and thus reduces the danger of drying out. Cushion plants house 

 a rich aggregate of minute animals and also serve as a rooting place for 

 many hemicryptophytes. Cushion plants are xerophytes with numerous 

 xeric adaptations (c/. especially Schroter, 1926; Hauri and Schroter, 1914). 

 They can also withstand low temperatures and violent winds and are there- 

 fore adapted for the occupation of climatically unfavorable hal:)itats in 

 high mountains, in the arctic and antarctic regions, as well as in deserts. 

 Typical examples are species of Anabasis, Minuartia, Saxifraga, and Andro- 

 sace and flat cushions such as Draba aizoides, M. sedoides, and Silene acaulis 

 (Fig. 149). The round, woody cushions (LufLkugelkissen) , such as those of 



Fig. 149. — Desert-like steppe with globular cushions of Anabasis arctioidcs near Ain 

 Chair, southern Algeria. {Photo by Daguin.) 



Alyssum spinosum and Astragalus tragacantha, are not to be classed with 

 the cushion forms, because of the lignified shoots, but rather among the 

 semishrubs and nanophanerophytes. 



A special group is presented by the mosses {Grinimia form), which appear 

 as rock pioneers; and l^y the turf mosses, with their flat cushions {Poly- 

 trichum form). Numerous bog mosses must be classed with the Polytrichum 

 form {Philonotis, Pohlia, Bryum species, and others). They are found in 

 the cold and temperate zones. 

 6. Chamaephyta sphagnoidea (bog mosses): mostly oligotrophic species of 

 Sphagnum with unlimited growth. They are dependent upon high humidity 

 and precipitation and are therefore entirely lacking in arid regions. On the 

 other hand, they advance far into the subarctic. 



