188 



PLANT SOCIOLOGY 



decisive. Among the lichen associations of the Dolomites in the Tatra 

 are the Thelidium association of wet dolomite rocks and the Opegrapha 

 saxicola association. The latter may possibly occur also upon lime- 

 stone, but the former is dolomite constant. 



The epilithic and endoUthic algal communities of the south Tirolean 

 dolomite cliffs, first described by Diels (1914), are notable dolomite 

 associations. On the steep walls of the Schlern these communities of 

 cryptogams are distinctly stratified according to their light require- 

 ments. The rock surface is covered with an association of Scytonema 

 crassum and other species of Scytonema in tufts up to 0.5 mm. in length. 

 Below the coating of Scytonema comes a Cyanocapsetum, which is 

 really a stage preliminary and preparatory to the Scytonemetum. 

 Aside from these epilithic or surface communities, which can bear 

 intense light and drought, there are also very peculiar endolithic 

 algal groups penetrating as much as 0.8 mm. into the finest capillary 

 crevices invisible to the naked eye. Those immediately under the 

 surface of the rock consist of an orange-red Trentepohlia. Deeper in 

 the rock, with very weak light, occur extremely minute transparent 

 Gloeocapsa forms, which appear as green bands when the rock is broken 

 with a hammer. The question of the dependence of these algal asso- 

 ciations upon MgCOs needs further examination. 



Serpentine. — Raw serpentine soil supports plants which are 

 indifferent to lime as well as many lime-favoring and even lime- 

 constant species, such as Trisetum distichophyllum, Ranunculus par- 

 nussifolius, Oxytropis montana, Rhododendron hirsutum, Doronicum 

 grandiflorum, and Leontopodium alpinum. The occurrence of lime 

 plants may be connected with the presence of calcium in the serpen- 

 tine. A substitution of Ca by Mg ions may also be possible. Accord- 

 ing to Angel, an analysis of serpentine from Steiermark gave these 

 percentages : 



The rather high CaO content in the serpentine of Kraubat is 

 reflected in the floristic composition of the vegetation. Serpentines 

 of the region of Bruck, on the other hand, contain no CaO. 



Serpentine has a limited number of characteristic plant forms, 

 which are confined exclusively to magnesium sihcates or carbonates 

 (serpentine, magnesite) and may be considered strictly serpentine 



