THE BIOLOGICAL CONDITIONS IN THE SOIL 241 



most acid plots there are only 7 species. Many instances 

 are found in England on the loams overlying the chalk 

 Downs in the southern counties, some of which are sour, while 

 others are not. Thus on the Wiltshire Downs, the so-called 

 " bake " land is sour, but it is surrounded by neutral or chalky 

 soils : on it is found the Sheep's sorrel {Rumex acetosella\ 

 Scarlet Pimpernel {Anagallis), and Knawel {Scleranthus 

 annuus), while the Toadflax {Linaria vulgaris), white and 

 bladder campion {Lychnis vespertina and Silene inflata) are 

 conspicuously absent, although common on the chalk in the 

 district.^ The influence of sourness on the wild vegetation of 

 uncultivated land (Harpenden Common) has been studied by 

 Hutchinson and McLennan (i40<^ and on woodland vegeta- 

 tion by E. J. Salisbury (242), in both cases the " lime require- 

 ment " was used as the measure of sourness. Their results are 

 given in Table LXV. Salisbury further finds Quercus sessili- 

 flora on the more sour and Q. robur on the less sour soils. 

 Attempts have often been made to draw up lists of plants in- 

 dicating sourness, but the problem is not altogether simple. 

 A plant may be eliminated from the natural flora not because 

 it cannot tolerate the degree of sourness in the soil, but be- 

 cause it tolerates this sourness less well than its competitors. 

 Another plant may flourish in sour soil, not because condi- 

 tions of sourness are suitable to it, but because of the absence 

 of effective competition. Thus sorrel is often described as an 

 indicator of sourness, but this is not entirely correct : it is not 

 the presence of sorrel that is symptomatic, for sorrel will 

 grow quite well on chalk soils : it is rather the presence of 

 sorrel, sweet vernal grass, etc., combined with absence of 

 clover. The effect of sourness, like that of any other adverse 

 factor, is to alter the balance in competition somewhat 

 against a particular set of plants, which tend therefore to be 

 eliminated in time. We shall find other cases (p. 301) where 

 even a small factor can profoundly affect the natural flora if 

 its action continues long enough. 



^ W. E. Brenchley, Annals of Botany, 1912, 26, 95-109. 

 16 



