ARABLE WEEDS. ASSOCIATION WITH SOILS 147 



White mustard or charlock (Brassica alba} is often found 

 growing alongside the ordinary charlock (Brassica sinapis} on 

 chalky soils, though it is not so frequent in occurrence even 

 in such favoured situations, but whereas the Brassica sinapis 

 is abundant on all types of soil with a leaning towards the 

 heavy land, Brassica alba is rarely seen except on typical 

 chalk land and in a less degree on light soils. The two 

 species are very similar in habitat and colour of flower, and 

 farmers rarely distinguish between the two forms, the chief 

 difference being in the shape of the leaves and the character 

 of the fruits. When white mustard does occur it is very often 

 dominant, particularly on chalk, where it is the chief weed 

 every second time that it occurs. Curiously enough, though 

 the species is so uncommon on the heavy and medium land, 

 yet in such situations it is sometimes dominant instead of 

 being occasional or scarce, as is usually the case with plants 

 that show preference for special types of soil when they occur 

 outside the preferred habitat. 



Hardhead (Centaurea nigra} is as common on chalky soil 

 as the white mustard, but it is more or less insignificant in 

 quantity, and never reaches a position of dominance. The 

 plants are larger and therefore attract attention, but the 

 species is more associated with pasture than with arable land. 



Toadflax (Linaria vulgaris} and wild mignonette {Reseda 

 luted] are always connected in the popular mind with chalky 

 land, and indeed they seem curiously characteristic and con- 

 spicuous, although they are rarely present in any considerable 

 quantity. Corn gromwell (Lithospermum arvense} and lamb's 

 lettuce (Valerianetla olitorid) are most insignificant, occurring 

 but seldom and in very small quantities, but nevertheless they 

 are just as characteristic of chalky soils as are the more abun- 

 dant species like white mustard and toadflax. 



(b} Plants with a Definite Preference for Chalk, but also 

 occurring on all Soils. This class of weeds is much better re- 

 presented than that of plants characteristic of chalk. The 

 plants in this section do not exhibit violent dislike to any type 

 of soil, but they are certainly much more often associated with 

 chalky land than with any other. Some of the commoner 

 weeds come under this heading, such as pimpernel, fumitory, 

 field pansy, and campion, and only a very few are at all 

 infrequent in distribution on arable land, these being hog- 

 weed, greater knapweed, round leaved toadflax, and wild carrot. 

 This is only another instance of what usually occurs. Plant 



