Weeds of Arable Land 83 



Root-crops, Mangel, Turnips (and Potatoes), not grown for their seeds, but 

 utilized at the end of a vegetative period, take comparatively little valuable 

 matter from the soil, but store photosynthetic carbohydrates as starch and 

 sugar. They usually add to the ground the material of their stems and 

 leaves ; and being fed to cattle, much of the material is also returned, 

 and tended in the form of farmyard-manure to the Wheat-crop. The 

 Mangel as a Chenopod, and the Turnip as a Crucifer, are eminently salt- 

 storing and halophytic by descent ; hence they will take any quantity of 

 manure, and what they do not use is also left for the next crop. 



The Weeds of agricultural land may be classified from several stand- 

 points, more particularly as : 



(1) Grasses and herbaceous plants of pasture-land. 



(2) Intrusives from woodland, hedges, and even from boundary-ditches. 



(3) Aliens, largely annuals, imported with the seed, or introduced in 



agricultural operations. 



The special ecological factors involved as characteristic of the special 

 associations, may be distinguished as : 



(1) The provision of new and unoccupied ground in optimum condition 

 for the germination of seeds, and over wide areas ; hence inviting coloniza- 

 tion and open competition, to an extent far beyond anything under natural 

 conditions of the environment. 



(2) The elimination of all arboreal forms and larger perennating stocks. 

 Ordinary herbaceous perennials are eliminated by ploughing ; only a few 

 residual types with particularly deep rhizomes or roots in the subsoil, or 

 with great capacity for regeneration from cut pieces of rhizome or root, can 

 remain effective. Owing to the great advantage gained by such plants in 

 open competition with mere seedlings, these may in time give rise to distinct 

 races of plant with special aptitude for this mode of existence, and thus 

 isolated by unconscious artificial selection (Dandelion, Carduus arvensis> 

 Convolvulus arvensiS) Triticum repens, Equisetum arvense). 



(3) With total elimination of arboreal forms and the majority of 

 perennating stocks, the field is left open more particularly for the activity of 

 annuals in greatly increased ratio. The annual weed, in fact, becomes the 

 normal associate of the annual crop. 



All field-crops, again, tend to be of annual duration, as worked in 

 association with marked annual rhythm of seasons, allowing the ground to 

 be wholly cleared of waste material at the end of each cropping, weeded, 

 manured, and re-cultivated to give full aeration of the soil. Perennial types 

 (Cotton, Castor Oil, Sugar Cane) are thus treated as annuals for purposes of 

 cropping in the tropics. Annuals are preferred, and the case of the biennial 

 turnip and mangel, utilized for their food-storage in the first year, only emphasizes 

 the point. 



The essential factor of such association implies that the periodicity of 

 the weed should run parallel with that of the crop, and different crops will be 

 characterized by their special weeds ; though a wide range of plants may 

 attempt to adapt themselves to all. Thus Poppies may colour crops of 

 Wheat and Barley in July, so vividly as to be a feature of the landscape ; 

 but are kept down in root-crops by weeding. Rapidly developing Charlock 

 may colour wide ranges of young cereals bright yellow in June ; but may 

 similarly colour turnip-fields as aftermath crop in September. 



From these general considerations it follows that : 



(i) New ground under favourable tillage may favour plants incapable 

 of holding their own in competition with other forms, in occupied land or 

 the small exposed areas permitted in a closed formation ; and such types 

 may persist under the special conditions without ever succeeding in estab- 



