WEEDS-PECULIARITIES AND DISTRIBUTION 427 



chalk. Spurrey, Sheep's Sorrel and Annual Knawel {Scleranthus 

 annuus) are all in evidence, whilst far more Scarlet Pimpernel 

 is present than ever occurs on the calcareous soils. The tran- 

 sition from the chalk flora to the acid soil flora is remarkably 

 abrupt and is accounted for by the thinness of the la}^er of 

 " bake." The local theory is that this soil is derived from 

 chalk but that all the lime has been removed by leaching, only 

 an acid, sandy residue being left. 



C. Weeds of Chalk Soils. — The calcareous soils are those 

 derived from the Chalk rocks, both from the chalk proper and 

 from the Chalk Marl underlying it. The soils vary from those 

 which are essentially pure white chalk to those of the red 

 lands and the chalk loams, which are rather heavier in texture 

 but still truly calcareous. 



The type of vegetation found upon chalky soils, especially 

 on chalk downs, is very distinctive. Chalk, besides being of 

 such a decidedly alkaline nature, provides a substratum of a 

 peculiar type and naturally one would expect to see this 

 reflected in its flora. There are certain species of plants to 

 which chalk acts as a veritable poison, killing them outright. 

 Such plants, in consequence, are absent from all such soils 

 and receive the collective name of " calcifuges " or plants which 

 flee from chalk. The Sweet Chestnut and some species of 

 Pines (Pinus Pinaster) cannot tolerate chalky soils ; consequently 

 their presence in the vegetation is always an indication of the 

 non-calcareous nature of the substratum. Plants of the same 

 genus but of different species frequently behave quite differently 

 in this respect. For instance, whilst Pinus Pinaster cannot 

 tolerate chalk soil, Pinus Laricio prefers a lime-containing 

 substratum and reaches a much better development in such 

 a situation. It is not easy to determine the reason why lime 

 acts as a poison to certain species of plants. Lime-hating plants 

 grown on calcareous soils do not contain so much chlorophyll 

 in their leaves as those grown on non-calcareous soils. The 

 deficiency in the green colouring matter implies necessarily a 

 decrease in the assimilating power of the plant and hence the 

 process of nutrition is adversely affected, causing a condition 

 of partial starvation. This may partly explain the toxic effect 

 of chalk upon such plants. 



The only species which are really absent from calcareous 

 soils are those to which lime is absolutely poisonous. In the 



