CHAPTER X. 



ARABLE WEEDS. ASSOCIATION WITH CROPS. 



IT has already been pointed out that one of the chief factors 

 in determining the abundance or scarcity of a particular weed 

 is the degree of competition it is able to withstand success- 

 fully, and furthermore, that the above-ground struggle for 

 light is as important as the underground struggle for food 

 and water. This being the case, the habit of growth of the 

 various crops and the differing systems under which they are 

 cultivated have a great deal to do with determining the com- 

 position of the weed flora. A normal crop of clover or 

 temporary grass grows strongly and completely covers the 

 ground, so that it is impossible for the majority of weeds to 

 live at all except in bare patches where the crop has failed. 

 Consequently very few of the ordinary local farm weeds are 

 to be found among such a crop, unless the latter has come so 

 badly that the ground is not covered and the weeds have 

 an opportunity to live. On the other hand, there are a few 

 weeds, like geranium, campion, and field madder, which are 

 specially fitted to grow among a smothering crop, and these 

 plants are generally to be found in the clover. The seeds are 

 carried with the crop seeds and the weeds flourish among the 

 crops, but they are so impatient of cultivation, or are so 

 unable to withstand the open situation among cereals and 

 roots, that very few of them survive when once the clover or 

 grass has been removed from the ground. 



Amongst cereal crops matters are quite otherwise. The 

 crop plants grow in regular rows, well spaced out, and for a 

 long period during early growth the plants are small and offer 

 comparatively little competition to the weeds except in the 

 rows in which they are sown. This is most favourable for 

 the weeds, and if it were not for occasional cultivation the 

 crop would have a very poor chance as it would become 

 choked out or smothered by the weeds. Indeed, this is 

 occasionally seen to happen, especially on light sandy land on 



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