6 WEEDS OF FARM LAND 



food supply, have shown how actively aerial competition comes 

 into play quite independently of the root competition. 1 When 

 the plants were grown at some distance from one another so 

 that no shading occurred growth was strong and even, a large 

 amount of dry matter being produced. When a number of 

 plants were closely crowded together they were far less healthy, 

 and the amount of dry matter formed by individual plants de- 

 creased as the amount of shading by neighbouring leaves in- 

 creased. The crowded plants were unable to carry on adequate 

 carbon assimilation owing to deficiency of light caused by the 

 overlapping of leaves. As this decrease of growth occurred even 

 when there was no lack of food for the individual plants it is 

 obvious that aerial competition must play a still more import- 

 ant part in the field where the plants are competing for food 

 as well as light. Many of the methods and processes of 

 cultivation are directed towards the elimination of this com- 

 petition of weeds with crops, the same end being attained by 

 various systems of farming. 



Under ordinary farm management the various crops are 

 grown in rotation, one of the simplest being the four-course 

 rotation, barley, seeds, wheat, roots. The seeds are sown 

 down with barley in spring, and if germination is good the 

 heavy growth of the barley tends to keep down weeds, while 

 the clover manages to mark time until the barley is cleared oft 

 in the autumn. The habit of growth of the clover prevents 

 the ordinary weeds from making much headway, although a 

 number of special weeds are often introduced by this crop. 

 Consequently, when the clover makes way for the wheat the 

 land is not weed-ridden if the barley and clover have been 

 good. The weeds have not had much chance to develop and 

 to seed, and those introduced by the clover are not usually 

 permanent. With the autumn-sown wheat the weeds get 

 their chance. The seeds buried in the soil rush into growth, 

 and the wheat crop is full of a great variety of weeds unless 

 some proportion of them is removed by spring cultivation. 

 Even so the wheat stubble is a veritable flower garden, and 

 is full of many varieties of weeds ripening their seeds in 

 readiness for the future. After such a fouling crop as wheat, 

 therefore, roots follow on well, as they offer a good oppor- 

 tunity to clean the land and to improve the condition of the 

 soil by cultivation, ready for the next cycle of crops. 



1 Brenchley, W. E. (1919), "Some Factors in Competition," Joum. 

 Applied Biology, VI, Nos. 2 and 3, pp. 142-170. 



