CHAP. LXIII CHASMOPHYTES 245 



and species of Yucca in America ; Aloe, Dracaena, Mesembryanthemum, 

 Aizoon, Sempervivum, Cotyledon and other Crassulaceae, or Senecio 

 (Kleinia) in Africa, including the Canary Isles x ; but also succulent- 

 stemmed euphorbias in the Old World, and Cactaceae in the 'New. 

 Besides these plants one finds grey-haired, small shrubs, such as species 

 of Croton and fragrant species of the verbenaceous Lippia, also small 

 fleshy-leaved plants, such as Peperomia, Pilea, Pedilanthus and, finally, 

 Orchidaceae with pseudo-bulbs. 



Many of these plants almost seem to live on air, yet they attain 

 a considerable size ; filled with sap they hang down in all their beauty 

 from jagged solid rock, at first sight seeming to be purely superficial, 

 but in reality sending their roots into crevices, and abstracting the water 

 retained there by capillarity. At certain seasons, and particularly in 

 the brief spring-time the brown or grey rocks are dappled with gay 

 flowers. 



Annual plants are rare on rocks, as they find but few spots suitable 

 for germination. In lands with a long, dry season they may, nevertheless, 

 play a prominent part. Very common on rocks in Madeira are Gnaphalium 

 luteo-album. Campanula Erinus, Gymnogramme leptophylla, two annual 

 species of Aichryson, and Sinapidendron rupestre ; even in the lowlands 

 annual herbs are richer in individuals than are perennial herbs, and only 

 exceeded in this respect by undershrubs. 2 



It is by no means universal for the vegetation on rocks to be exclu- 

 sively composed of xerophytic types. Mesophytes often form no incon- 

 siderable part of the plants present. This may be due to difference 

 in the crevices. Some of these receive water percolating from higher 

 parts of the mountain, and may remain moist throughout prolonged 

 periods of drought ; other crevices obtain their water exclusively from 

 the strictly local rain. Some crevices contain abundant detritus and 

 are therefore endowed with a greater power of storing water ; others 

 are poor in detritus and allow the water to pass away. The chemical 

 composition of the detritus also varies, as some crevices contain abundant 

 humus, in which numerous earthworms may lurk, whereas others are 

 poor in humus. Cracks in rocks supply an endless variety of habitats, 

 each of which forms a special kind of environment. 3 



Plants occupying crevices of the lower parts of rocks on the Danish 

 shores are halophytes which are mostly species belonging to the sandy 

 beach, whereas those higher up are perennial herbs and shrubs, the 

 majority of which are xerophytes, though some mesophytes occur. 4 



Ottli does not regard vertical rocks in the Alps as habitats poor in water. 

 Nevertheless the majority of plants that he terms lithophytes are xero- 

 phytes, and show that they find difficulty in obtaining water, especially 

 during winter, because the rocks retain no coating of snow. But many 

 mesophytes also occur. On the highlands of Madeira, according to 

 Vahl, 5 the vegetation on vertical rocks is a varied admixture of xerophytes 

 and mesophytes ; side by side with white woolly-haired undershrubs 

 grow ferns and liverworts. But in the hot dry lowlands of Madeira all 

 the springs dry up in summer, and the crevices of the rocks are not suffi- 

 ciently moist to permit mesophytes to replace the water lost in transpira- 



1 Christ, 1885. Vahl, 19046. 3 Ottli, 1903. 



4 Warming, 1906. 6 Vahl, 19046. 



