WEEDS— PECULIARITIES AND DISTRIBUTION 431 



light and air is cut off and consequently the weeds suffer, though 

 not before they have had time to reach a considerable develop- 

 ment. The leguminous crops, on the other hand, tend to spread 

 out laterally from the first and to cover the ground very closely, 

 so that they choke out of existence many weeds that would 

 otherwise be found. The difference in the method of cultivating 

 the crops also affects weeds most vitally. From the very 

 beginning, root crops are kept well cultivated, the weeds being 

 constantly cut down by the hoe, so that few weeds representing 

 few species occur among them, the presence of many weeds 

 indicating poor cultivation. The dense covering growth of the 

 Leguminosae accounts for the absence of many species which 

 are to be found associated with all other crops, such as Wild 

 Mint, Plantain and Annual Meadow grass {Poa annua). Several 

 species of plants, such as Fat Hen, Cleavers, Knot-grass, 

 Chickweed, etc., were absent or very rare in seed crops in 

 Bedfordshire whilst they were frequently met with in association 

 with them in the West. This may probably be accounted for, 

 as many of the seed crops of 191 1 had been unable to make 

 much headway after they were sown, owing to the prolonged 

 drought, so that the weeds had a fair chance to get ahead 

 before the crop was big enough to smother them out. A few 

 plants, on the other hand, are almost constantly associated with 

 seed crops and indeed are confined to them ; such are the 

 Thyme-leaved Sandwort (Arenaria serpyllifolia), Mouse-ear 

 Chickweed, Wild Madder {Sheradia arvensis) and species of 

 Geranium. It is a moot question whether these species are 

 always indigenous to the localities in which they occur, as when 

 they are found in seed crops and nowhere else in the neigh- 

 bourhood it seems probable that in many cases they have been 

 introduced from other places and sown with the crop seed. 



Parasitic Weeds 



There is one class of plants which is of peculiar interest 

 from an agricultural point of view, on account of the curious 

 mode in which they live and because when the plants occur 

 in quantity they are so very detrimental to the well-being 

 and life of the crop. These are the parasites or plants which 

 live at the expense of others instead of leading independent 

 lives. All normal plants obtain their raw food material 

 first hand from mineral substances dissolved in the soil 



