INTRODUCTION 3 



will agree that alien plants among arable crops are pernicious 

 and should be got rid of as far as possible. On the other 

 hand, the weeds of grass-land are seldom considered to be of 

 much significance, except when certain plants have specially 

 noxious qualities which compel the farmers to take measures 

 against them. For instance, buttercups are rarely eaten by 

 stock if plenty of other food is available, and they simply 

 encumber the ground when present in large quantities. 

 Nevertheless, nothing is done to get rid of them and most 

 people regard them with much tolerance, considering them 

 merely as signs that the ground is particularly fertile where 

 they grow luxuriantly. If, however, a little garlic, which 

 taints milk, is present on grazing land strong measures are 

 taken either to eradicate it or to ensure that milking cows do 

 not have access to the fields. If the same relative quantities 

 of buttercups and garlic were present on arable land, the 

 buttercups would be even more harmful than the garlic in 

 their action on the growth of the crop, and every effort would 

 be made to rid the land of them. Generally speaking, arable 

 weeds have the greater practical significance, and as a result 

 much more information is available with regard to them than 

 is the case with pasture weeds, so that it is possible to draw 

 more definite conclusions as to their occurrence. 



When land is well tilled the soil is constantly disturbed 

 by such operations as ploughing, harrowing, and cultivating. 

 After a crop is removed the soil is ploughed up, all vegetation 

 growing on it is buried and much of it is killed. If seeds are 

 buried in the soil they may germinate after the first ploughing, 

 but they in their turn are destroyed by later ploughing or by 

 the harrow and cultivator. If a cereal crop is grown the soil 

 is left more or less undisturbed when once the young plants are 

 well established, but if roots are wanted the land <f s kept culti- 

 vated the whole time the crop is on the ground. It is obvious 

 that these conditions are not at all favourable to the persis- 

 tence of weed plants unless they are so specially adapted to 

 hold their own that they can be more or less indifferent to 

 the rough treatment they get. For this reason the number of 

 plant species that occur as weeds is limited, and even among 

 this number there are very many which very seldom occur, 

 except on particular soils or under special circumstances. 

 There are few species that are really prevalent and widespread, 

 and every one of these is markedly well adapted to the con- 

 ditions of life. Furthermore, the weeds that are ubiquitous in 



