DISTRIBUTION OF WEEDS 19 



many being formed that even if a very large percentage is 

 lost or destroyed the surviving minority will be amply suffici- 

 ent to provide an abundance of descendants. This method is 

 very common and effective among arable weeds, especially 

 with those which grow close to the ground. The various pro- 

 cesses of cultivation are all inimical to the survival of weeds ; 

 when the seeds germinate the seedlings are liable to be ruth- 

 lessly cut down at various stages of growth. Under these 

 circumstances the presence of large quantities of seeds in the 



FIG. 5. KNOTGRASS (Polygonum aviculare). 



soil is most favourable for the weeds as the seeds do not all 

 germinate at once, so that if and when the first batch of seed- 

 lings is destroyed, a fresh batch which may have more chance 

 of reaching maturity springs up at short notice. This may 

 be continued time after time, as seeds from one crop of weeds 

 usually vary greatly in the time that elapses before germina- 

 tion, the period sometimes ranging from a few days to several 

 years. Scarlet pimpernel, knotgrass (Fig. 5), mouse-ear 

 chickweed, spurry, chickweed, and speedwells of various 

 species are all low growing arable weeds which form such an 

 abundance of seed that when once they are established it is 



2" 



