54 PLANT GEOGRAPHY 



or most swampy situation if lime is present. Here it 

 would seem as if competition plays an important part. 

 Closely related species often differ markedly in their 

 requirements as to the chemical composition of the soil; 

 but such differences may not be apparent unless the 

 species occur together so as to compete. Thus the 

 Milfoil (Achillea Millefolium L.) seems always indifferent 

 as regards much or little lime; A. moschata Jacq. will 

 grow, either alone or with A . Millefolium, on calcareous 

 soil, but not if A. atrata L. is present; whilst, on the 

 other hand, A. atrata, indifferent when alone or with 

 A. Millefolium, cannot grow on siliceous soil if A. 

 moschata be present. In other words, A. moschata is 

 relatively less calciphile than A. atrata. 



Undoubtedly the importance of lime as determining 

 the composition of the flora of any habitat may be due 

 to its neutralising the acidity of the soil, or to its in- 

 fluencing in this way the growth of certain soil-bacteria 

 or mycorhizae. 



The action upon plant-form of a substance in the 

 soil -water usually inert is strikingly exemplified in the 

 case of Viola calaminaria Lej., a markedly modified 

 form of V. lutea Huds., confined to the neighbourhood 

 of the zinc mines of Aix-la-Chapelle, and yielding a 

 notable proportion of zinc in its ash. 



SLOW CHANGES IN EDAPHIC CONDITIONS. The edaphic 

 conditions of a locality may undergo a cycle of extreme 

 change without any"^ change of climate or geological 

 change of level. Thus a pond containing submerged 

 aquatics, such as Char a and Myriophyllum, may become 

 overgrown by what is termed the " Limnaea " formation, 

 rooting in the muddy bottom with leaves floating on 

 the surface, such as 1he Batrachian Ranunculi, Nuphar, 

 and Potamogeton. These may in turn be ousted by 

 reeds, such as Scirpus lacustris L. and Phragmites, or 

 by such semi -aquatic marsh -plants as the Bog -bean 

 (Menyanthes trifoliata L.), the Flowering Rush (Butomus 

 umbellatus L.), the CEnanthes, Acorus Calamus L., and 

 Iris Pseudacorus L. These may in turn give place 

 either to Sedges followed by meadow-grasses, or to 

 Sphagnum, followed in time by Ling, Birch, and Pine. 

 Ground which has thus become pine-forest from being 

 covered by water, in which, of course, the water-level 

 has become greatly lower, may conversely pass once 



