THEIR INJURIOUS EFFECTS 573 



white clover ley, or a well-developed plantain on a lawn, to 

 appreciate the extent of damage done by plants of this 

 character. 



Weeds such as small bindweed {Convolvulus arvensis, L.) and 

 black bindweed (^Polygonum Convolvulus, L.) wind round and 

 climb up the stems of any plants in their neighbourhood in order 

 to place their leaves in a favourable position in regard to light 

 and air. In doing so they often press the leaves of the plants 

 supporting them closely to the stem, and thus prevent the 

 proper development of the crop. Corn crops are often seriously 

 damaged in this manner, and not infrequently osiers and rasp- 

 berry canes are injured in the same way. 



Many weeds use various crops as supports, but do not wind 

 round them as the bindweeds do. Some of them, such as tares, 

 climb by means of tendrils, while others, cleavers for example, 

 are covered with stiff-hooked hairs, which enable them to climb 

 and expose their heads to the light. The weaker-stemmed crops, 

 such as cereals, are often pulled to the ground by the weight of 

 those climbing and winding weeds. 



b. Weeds not only screen off light and air, but they utilise and 

 consequently deprive the crop of various important manurial 

 constituents which would find their way into it if the weeds were 

 not present. The analyses of weeds of various kinds often show 

 a specially high percentage of potash and phosphates, and 

 there is little doubt that with their extensive root-system they 

 collect a considerable proportion of the manures which are ap- 

 plied to the soil but not intended for their benefit. Perhaps this 

 robbery of fertilising constituents is of little practical moment, 

 but plants of all kinds take up very considerable amounts of 

 water from the soil and transpire it into the air; the ground 

 thus becomes comparatively dry. Where weeds are present they 

 compete for the water-supply of the soil, and reduce the amount 

 available for the crop. In this manner they are responsible for 

 the stunted character and reduced yield of crops overrun by them. 



