PLANT FORMATIONS. S 1 9 



Each one of these limited areas or spots of soil furnishes a char- 

 acteristic ecological or vegetation type of plant community. 

 The climatic formations are thus infiltrated, as it were, with 

 plant communities determined by soil conditions, in some places 

 filling in cavities or interspaces, and in other cases actually 

 intermingling with the elements of the climatic types. These 

 plant communities are termed by Schimper edaphic formations. 

 Thus rocky places or shores, sandy areas or dunes, certain 

 swamps or moors, etc., represent interspaces in the woods type 

 which are occupied by edaphic, or soil, plant formations. 



In prairie regions where the ground is rolling, subordinate 

 herbs may give the formation a variegated appearance, i.e., 

 when Primula officinalis occupies the dry places and P. elatior 

 with differently colored flowers occupies the damp places. At 

 Simplon in dry alpine meadows, Senecio unifloris with large 

 flowers occurs on thin soil covering rocks, Senecio incanus occupies 

 deeper soil, but never were they mixed. The hybrid form oc- 

 cupied soil of an intermediate depth (Schimper). In culture 

 meadows or pastures where the surface of the ground is undu- 

 lating the field is often covered with patches of yellow and white 

 flowers. In the low ground there is a rich infiltration of butter- 

 cups (Ranunculus acris), and on the higher ground of ox-eye 

 daisies (Chrysanthemum leucanthemum). So in the forest 

 region conditions of the ground may be such as to modify the 

 appearance of the forest, encouraging certain kinds of trees in 

 one locality, others in another. The modifying influence, how- 

 ever, is not sufficient in these cases to change the vegetation 

 type. It still retains the climatic character. 



991. Open edaphic formations. These are controlled by phy- 

 sical or mechanical conditions of the soil of such a nature that 

 the struggle for existence is with the adverse conditions of the 

 ground and not with rival plants. The result is the same as 

 in the desert, where the struggle is with climate, not with soil, 

 and there may be a mixture of the elements of the woods or 

 prairie regions. There are three marked types of open edaphic 

 plant societies: ist, the vegetation of rocky places; 2d, the 



