Dry Land Plants 225 



Bracken and the Carex together, and to call this district 

 where they are dominant the Bracken-Carex association. 

 Not that they occupy all the ground; some portions of 

 what is essentially the same country, especially round the 

 Cambridgeshire border, are v under cultivation and other por- 

 tions have been ploughed up but have relapsed into waste. 



In this northern portion Ling (Calluna Erica] only occurs 

 here and there, and always in a secondary position, neither is 

 Gorse very common. Both these plants seem to suffer con- 

 siderably from rabbits. The comparative rarity of the Cal- 

 luna seems due to the large amount of calcareous matter that 

 the sandy soil contains, as the chalk is nowhere very far 

 below the surface. If the surface soil is thick this calcareous 

 matter, being less in amount, will not affect the flora to the 

 same extent as on a shallow soil, where the plants are con- 

 tinually receiving fresh supplies of chalk. Hence Ling, being 

 a calcifuge plant, should be expected upon deep surface soils. 

 This it is not possible to determine without digging. As a 

 rule however it is found in the depressions between low hills, 

 and since the sandy covering would by denudation tend to- 

 wards the valleys it is in these slight hollows that the greatest 

 depth should be found. 



South by the river Lark, where there are deposits of post- 

 glacial gravels overlying chalk, Ling is the dominant plant. 

 Here upon Tuddenham heath, further to the south upon Kent- 

 ford heath, and probably in one or two other scattered localities 

 it is abundant. Between Tuddenham and Kentford heaths 

 chalk covered with blown sand occurs, and here it is exceed- 

 ingly rare. Carex and Bracken take its place, as they do in a 

 narrow strip between the Calluna area and the rough meadows 

 bordering the river. The river has worn away the greater 

 portion of the gravel, the chalk is but a little distance below 

 the surface and the Ling cannot grow. 



This eastern district, presenting as it does in the main but 

 two types of vegetation, Carex-Bracken and Heather, can yet 

 when examined in more detail be broken up into several plant 

 M. & s. 15 



