244 LITHOPHYTES SECT, vm 



Europe for instance, chasmophytes are merely species that also occur in other 

 situations ; yet these plants must be regarded as constituting a formation 

 of their own, because their flora is affected by the character of the locality, 

 and they form a definite society that develops in a particular habitat ; 

 moreover, they possess certain biological peculiarities enabling them to 

 occupy this type of habitat. But the formation has received only scant 

 attention. 1 



Thallophyta, Musci, Pteridophyta, and Spermophyta are all to be found 

 occupying such clefts. 



ADAPTATION. 



Rhizoids and roots are often constricted within narrow fissures and 

 flattened out as thin as paper. Roots of grasses, for instance, may form 

 a very flattened tuft filling the cleft. The tap-roots of other plants are 

 likewise flattened, when the clefts are narrow. The roots, guided by water 

 in the cleft, seem to penetrate very deeply ; ' their twine-like roots 

 penetrate incredibly deep into the moist interior of rock ' writes Christ 2 

 in reference to chasmophytes. Contractility of the main root seems to 

 be common, and to be of great importance to the species concerned. 



The constituent species are usually tufted in growth, and do not spread 

 far from the general root-system ; although means of vegetative migration 

 are not absolutely excluded, yet migratory plants with elongated horizontal 

 rhizomes are unusual. There rather occur creeping perennial herbs, 

 Fragaria for instance, that produce long thin epigeous runners, which 

 give rise to new shoots, either in another cleft or in the one in which the 

 parent plant is established. 



Rosette-plants are common ; the rosulous habit is, indeed, character- 

 istic of plants freely exposed to light. 



Xerophytic types are common, and succulent plants particularly so ; 

 in northern lands Crassulaceae, including Rhodiola and Sedum, and 

 Saxifragaceae, including Saxifraga Aizoon, occur ; in more southern 

 Europe Sempervivum appears. Moreover, there are plants, such as 

 Saxifraga oppositifolia and Silene acaulis, with small, thick, imbricate 

 leaves, or, like Diapensia, with small, dry, leathery leaves. Among peren- 

 nial herbs with rosettes of leaves may be mentioned Papaver nudicaule, 

 species of Draba, and other species that occur in great numbers on fell-fields. 



The warmer and drier the climate is, the more do mosses and lichens 

 retreat into the background, and the more abundant do Spermophyta 

 become. Many of these are xerophytes. On rocks of the calcareous 

 Alps, among stones in the stony tracts on the mountains of Herzegovina, 

 and in similar places, one often finds white, woolly species of Cerastium, 

 rigid tufts of species of Arenaria, species of Veronica, Alchemilla, Saxi- 

 fraga, and others, which form low, dense tufts, and possess small, stiff 

 leaves, a strong epidermis, abundant hairs, and many other signs of their 

 xerophytic nature. 



Even in lower parts of the Alps the succulent types, Sempervivum 

 and Sedum, become more common, and gradually lead us to true tropical 

 rocks. Here we can still see crustaceous lichens flourishing, but succulent 

 and other xerophytic Spermophyta have become more abundant. We 

 find not only rosette-plants, such as Bromeliaceae, Agave, Velloziaceae, 



1 Except from Ottli in Switzerland. - Christ, 1885. 



