WEEDS— PECULIARITIES AND DISTRIBUTION 415 



On all land used for agricultural purposes, the plants — 

 crops and weeds alike — are growing under somewhat artificial 

 conditions and are considerably influenced by human agency. 

 Tillage of the soil, the various manures used, the residual 

 effects of previous crops, the cultivation of a variety of crops 

 in rotation, are all factors that play their part in influencing 

 the growth and development of the plants growing on any 

 particular soil ; the aim of the agriculturist is so to utilise these 

 factors that the crop receives the greatest amount of aid and 

 encouragement whilst the weeds are kept as much as possible 

 in abeyance by unfavourable conditions of existence. 



In cultivated ground weeds are obnoxious for several reasons: 



1. They are plants of strong vital activity and if allowed to 

 grow freely they rob the crop of a great deal of raw food 

 material that would otherwise be left at its disposal. Pot 

 experiments have been made 1 with sand, field soil and clay to 

 test the relative demands made by certain plants upon the 

 available food in the soil. The seeds of certain crops — rye, 

 wheat, barley and oats — and of various common weeds — Char- 

 lock, Corn Cockle, Poppy, Sheep-sorrel, etc. — were germinated 

 in different soils and the drain upon the soil measured by 

 ascertaining the amount of carbon assimilated. It was found 

 that assimilation was considerable during the first thirty days 

 but that the weeds excelled the cereals in rate of growth, 

 especially on soils containing a limited supply of plant food. 

 The baneful effect of weeds is specially manifest on poor soils, 

 as these suffer a greater proportional loss from the growth of 

 weeds than better soils. 



2. Weeds absorb the soil moisture and so lessen the supply of 

 water available for the crop. Many weeds are luxuriantly leafy 

 and give off a great amount of water from their leaves by 

 transpiration, so that in a dry summer much water is wasted 

 by the alien plants and the crop suffers from the effect of 

 drought far more severely than would otherwise be the case. 



3. If unchecked, weeds tend to grow very luxuriantly and 

 to crowd the crop, robbing it of light and air, so that the 

 plants do not have room or opportunity to attain their maximum 

 development. Some of the most vital physiological processes — 

 assimilation and respiration — are dependent upon a free access 



1 E. Vitek, "Weed Studies," Monatsh. Landw. 3 (1910), No. 11, pp. 333-44. 

 Ref. in Exp. Stat. Rec. xxiv. p. 543. 



