ARAiu.i; \\KKI >s. ASSOCIATION WITH CROPS 161 



growth, and never have a chance to grow big enough to do any 

 damage. Thus it happens that good strong crops of spring- 

 sown wheat, oats, and barley are remarkably free from weeds 

 during the growing season. In July and August the ground 

 below the plant is clean, with only an occasional sickly weed 

 struggling for existence here and there. It is sometimes said 

 that the best way to get rid of weeds is to keep the crops 

 growing well and strongly, enabling them to carry out the 

 work of eradication themselves without much artificial aid. 

 If the season be unfavourable and a long drought prevents the 

 corn from growing away strongly the weeds have a much 

 better chance. Some of them can stand drought well, and as 

 at such times they often grow more rapidly than the crop they 

 are able to come into strong competition. 



Root crops are kept so well cultivated that there is no 

 definite evidence to show how they would behave towards 

 weeds if they were left to themselves. The ground is very 

 well prepared beforehand and the seeds are sown comparatively 

 late in the season, with the result that large numbers of the 

 weed se^ds have started into growth and been destroyed before 

 the crop is sown. The constant use of the hoe during growth 

 does not give the weeds any chance, and it is for this reason 

 that roots are considered to be such valuable " cleaning crops ". 

 This general association of weeds with crops is obvious, 

 but detailed investigation shows that a much closer connection 

 exists between certain crops and particular species of weeds. 

 Whilst most weeds of any importance are to be found associated 

 to some extent with every crop, yet many of them are specially 

 encouraged or discouraged by one crop or another. 



Comparatively few weeds are equally prevalent among all 

 crops and these few comprise some of the most abundant and 

 widely distributed plants. The others all have some special 

 relation to the crops, either of aversion or sympathy. In con- 

 sidering this question it is generally of little use to take into 

 account those weeds which occur very seldom, as the data are 

 not often sufficiently reliable to base conclusions upon. In a 

 few cases, however, this rqle has been departed from, where 

 the indications of association are particularly marked. 



An examination of the lists of the percentage of frequency 

 of the chief weeds among the different classes of crops shows 

 that roots, seeds, peas, and beans have the most direct influence 

 upon the weed flora. The weeds that are encouraged or 

 discouraged by these crops divide up into distinct classes with 



ii 



