WEEDS— PECULIARITIES AND DISTRIBUTION 425 



The Red Bartsia {Bartsia Odontites) affords a curious instance 

 of change of habitat in different places. In the eastern county 

 it is a characteristic clay weed, being practically confined to 

 such soil ; it is also presumably a chalk-hater since it is never 

 recorded as growing on the soils of the chalk downs. In 

 Wiltshire, on the contrary, it is rare on clay but a frequent 

 and characteristic plant of the calcareous soils, so entirely 

 belying its character. This total inversion of habitat is 

 apparently rare, as very few instances have so far been met 

 with. Cases of partial inversion, in which plants of localised 

 occurrence in one district are more generally distributed in 

 another, are fairly common. A few of the weeds that are most 

 generally associated with clay soils are Charlock (Brassica 

 Si/iapis), Greater Plantain (Plantago major), Knotgrass(Polygonum 

 aviculare), Black Bindweed (Polygonum Convolvulus), Wild Mint 

 (Mentha arvensis) and the Corn Buttercup (Ranunculus arvensis) ; 

 all these weeds are to be found on other types of soil as well. 



B. Weeds of Sand and Light Loams. — Sandy soils provide 

 conditions exactly opposite to those appertaining to clay soils. 

 They are composed of a considerable proportion of quite large 

 particles with some admixture of silt ; owing to the size of 

 these particles, however, the silicate of alumina, which is 

 generally present, is unable to bind the soil together as in the 

 case of the clays. Instead of being adhesive, the sandy soils 

 are very loose and friable ; the interspaces between the particles 

 are fairly large and so the force of capillarity is relatively small ; 

 the soil is also generally very poor in humus or decayed organic 

 matter and as humus is exceedingly retentive of moisture, in 

 its comparative absence the soil is unable to hold much water 

 in its interspaces, so that if the subsoil is also fairly permeable 

 the plants are subject to a considerable amount of drought at 

 their roots. On true sands the species of plants that occur 

 are practically only the Xerophytes, plants which are fitted with 

 very special adaptations to enable them to withstand lack of 

 water at the roots. On ordinary agricultural lands, true sands 

 such as occur on the seashore are rarely in evidence, very sandy 

 loams being the type usually met with. The species are some- 

 what characteristic and exhibit certain xerophilous characteristics 

 in a greater or less degree in accordance with the degree 

 of sandiness of the soil, which determines its water-holding 

 capacity and consequently the condition of comparative drought. 



