Plant Communities of the World 289 



arable ground. Treated every year in the same way, this ground 

 has developed rather stable communities in two orders. The 

 Segetalia, sown cultures, plowed once a year, have in middle 

 Europe a weed community, Agrostemmatetum githagonis. Gen- 

 erally distributed is Agrostemma githago; brilliantly blue and 

 red shine Centaur ea cy units and Papaver rhoeas; characteristic 

 also are Spergularia arvensis, Pisum arvense, Lunaria vulgaris. 

 Olitoria, hoed cultures, truck crops, are both plowed and worked 

 over, in the season, by hoeing. These crops, such as potatoes and 

 other vegetables, are not only manured and hoed, but often are 

 watered also, and vegetation is somewhat hydrophilous. We can 

 call the association Chenopodietum polyspermi, by reason of the 

 frequent Chenopodium polyspermum accompanied by Pamcum 

 cr us gain, Mercurialis annua, Euphorbia peplus, Amaranthus 

 ascendens, A. retroflexus. Polygonum persicaria, etc. 



24. Rupideserta: RocI^ deserts. — This is the chomophyte vege- 

 tation of rocks which is dependent on detritus on the rock or 

 in crevices, and thus is clearly influenced by the rock habitat. 

 (When crevices become large, or detritus plentiful, this latter 

 influence ceases and we have stony forests or grasslands.) In the 

 detritus on the rock the habitat of the exochomophytes is rather 

 xeric, and in the crevices with the chasmochomophytes, or chas- 

 mophytes, it is not. The prime condition is edaphic; climatic 

 factors noticeable are altitude and exposure, both of strong in- 

 fluence on the local climate. Rock deserts are found locally 

 almost everywhere in mountains, in erosion valleys, and on 

 slopes and cliffs in the lowlands. In the Swiss Alps are to be 

 distinguished the Potentillion caulescentis, on a calcareous habi- 

 tat and with a montane-subalpine association characterized by 

 Potentilla caidescens, Kernera saxatilis, Asplenium rutamuraria, 

 Lilium croceum, and Globularia cordifolia, among others, and 



