124 WEEDS OF FARM LAND 



C. Weeds Characteristic of Sand and very Light Soil Deficient 



in Lime. 



D. Weeds Associated with Chalk, 

 (a) Plants characteristic of chalk. 



(//) Plants with a definite preference for chalk, but also 



occurring on all soils. 

 (c) Plants with a definite preference for light and sandy 



soils as well as for chalk. 



E. Weeds Associated with Peat Soils. 



A. Weeds of General Occurrence. 



By far the larger proportion of arable weeds are to be 

 found distributed over all types of soil instead of being 

 definitely associated with one or two particular soils. At first 

 sight it looks as though these weeds were absolutely indifferent 

 to the nature of their habitat, but a close numerical analysis 

 of observations made on their occurrence shows that there is a 

 considerable amount of differentiation, and that even though a 

 weed be of universal distribution it often has definite preferences 

 or mild dislikes for certain soils. This is shown both by the 

 frequency with which a weed occurs on a soil and by its 

 relative abundance, and the weed communities which are 

 characteristic of the different habitats owe a great deal to 

 those plants that are very tolerant and that yet have their 

 preferences as to soil. The presence of a quantity of the 

 universally distributed scabious (S. arvensis) on chalky land is 

 quite as characteristic as that of wild mignonette (Reseda luted), 

 which is practically confined to such situations. Indeed, there 

 are so few plants which are really soil indicators that if it 

 were not for the grouping into characteristic communities of 

 these weeds of general distribution it would be difficult or im- 

 possible to determine the weed communities that are associated 

 with the different soils. 



It is gradually becoming evident, however, that much of the 

 " universal occurrence" of certain weeds is more apparent than 

 real. Some of the commonest weeds, as knotgrass (P. aviculare), 

 show very great difference in character differences so great that 

 it seems impossible that they are due to mere local variations 

 brought about by particular conditions of soil and climate. 

 The knotgrass will vary from a type consisting of very thin, 

 wiry stems bearing tiny narrow leaves hardly inch long to 

 another of a much more succulent nature, in some cases with 



