1 64 WEEDS OF FARM LAND 



brought by the wind, throughout the year in the case of 

 groundsel, and in late summer and early autumn in the case 

 of corn sow-thistle. 



A comparison of Table I with Table X shows that very few 

 weeds are indifferent both in regard to the soil they occupy and 

 the crops amongst which they grow, shepherd's purse, creeping 

 thistle, curled dock, groundsel, and field speedwell being the 

 only ones in this category. The other weeds that are found as 

 often in one crop as in another are more particular as to soil, so 

 that in their case the nature of the soil, and not that of the 

 crop, determines the distribution. On the other hand, most of 

 the weeds that have no preference as to soil are greatly in- 

 fluenced by the nature of the crop, and therefore in this case 

 the crop, and not the soil, is the determining factor in dis- 

 tribution. This means that the nature of the soil and the 

 type of crop act independently on the weed flora and that the 

 weeds themselves respond in various ways to the action of 

 the two factors which are necessarily at work at one and the 

 same time. 



2. (a) Weeds Specially Associated with Root Crops. A 

 careful consideration of all the results does not reveal a single 

 weed that has a particular association with root crops. Roots 

 are sown quite late in the season, which gives the opportunity 

 of cleaning the land well beforehand. During the growing 

 season the cultivator and hoe are kept at work, so that only 

 the most persistent and ubiquitous weeds are able to maintain 

 a footing. 



The constant cultivation among root crops tends to make 

 a great clearance of the weeds, as under conditions of good 

 farming comparatively few plants are allowed to attain any 

 size. In this way the abundance of the weeds is greatly re- 

 duced, but most of the more common species are to be found 

 among the roots throughout the season. This is because the 

 stores of seeds or underground parts in the soil are so great 

 that the very cultivation which clears away one crop of seed- 

 lings prepares the soil for another crop to germinate or start 

 into growth. Some weeds, however, seem to resent the inter- 

 ference and are often absent from roots in fields that they 

 frequent when other crops occupy the ground. 



