84 Plant-life of the Oxford District 



lishing themselves in any other. In this way, arable land may come to bear 

 a special flora peculiar to these fields, either predominant, or mingling with 

 residual and intrusive forms of the vicinity, and largely ' alien '. 



(a) The greatest success in such association will ensue as the parallel- 

 ism of periodicity in crop and weeds becomes the more exact ; and this 

 will be increased by natural selection, as one type is favoured at the expense 

 of others in coincidence of germination, seed-maturation, and seed-harvesting. 

 The periodicity of each particular crop will be associated with a special 

 selection of weed-plants ; particular forms becoming characteristic, and 

 others less so, or residual, as the conditions of the main crop become more 

 precise or more narrowly exacting. 



Thus Wheat (Beans and Oats), autumn-sown and flowering in late June, 

 as a tall shading crop, will be generally associated with a class of weeds 

 entirely different from those of Mangels (Turnips) sown in spring, with open 

 distribution, no special canopy, and pulled in late autumn, with little dis- 

 turbance of the weeds. Spring-sown Barley (Oats), as a tall crop with 

 dense canopy, rapidly maturing (July), may represent a third class of crop, 

 with its characteristic weeds, though combining features of the two pre- 

 ceding types. 



But the subject is not so straightforward as this implies ; since one is 

 not usually dealing with plants growing continuously in successive seasons on 

 the same ground. If this were the case, the constitution of the association 

 would probably soon become obvious. Owing to general adoption of some 

 form of rotation of crops, the same field does not usually carry the same 

 crop for 2, years in succession ; and thus the weed debris of one crop becomes 

 the initial equipment of the next. 1 New intrusives may be added with the 

 new seed, and a general mingling of weeds result. In such case, shading 2 

 and weeding in early stages are the determining factors. The greatest floral 

 display of the arable fields is given by non-weeded corn-fields in the early 

 stages of their growth (June, Midsummer), and again in the autumn fields of 

 root-crops. Casual weeds, flowering and fruiting late in the season, have no 

 chance in a corn-crop, but may grow freely with mangel (Chenopods, Poly- 

 gonum). On the whole, neglected root-crops give the greater variety of 

 species, though the corn-fields afford the greater display of individual forms 

 (Charlock, Radish, Poppy). The general list of corn-field weeds comprises 

 about 60 species. 



A further source of complication is introduced by the manner in which 

 ploughing operations tend to bury seeds at some depth these remaining 

 dormant through one or more seasons, to be again brought to the surface the 

 next time the ground is turned over. The consideration of the weeds of an 

 arable field then implies some knowledge of the condition of the crops for 

 several years previously. Each field requires to be taken on its own merits, 

 and over a period of several seasons, the appearance, disappearance, and 

 reappearance of special types being carefully noted. Data require to be 

 collected for (i) The weeds of the main crop ; (2) The special case of plants on 

 any road -track or way across the crop ; (3) The plants of the hedges adjacent 

 to the crop. These may be sorted out according to their prevalence in 

 successive seasons, and each field becomes a study in itself. The more 

 agricultural outlook further includes the collection of data showing the rela- 



1 Seed-wheat is particularly well cleaned, and common weeds as Poppies, Lychnis Githago, 

 Charlock (Sinapis arvensis\ Sonchus arvensis, etc. , are largely residual from a previous Bean-crop. 



2 Wheat sown in 6-inch drills gives a uniform mantle over the soil in early stages, and 3 bushels 

 of seed may be used on heavy soils. The same crop might be produced with half a bushel of seed 

 and more scattered plants. A close growth in early stages gives protection, as well as shading of 

 weeds. Broadcasting still gives a more effective smothering mantle : wide drills only encourage 

 weeds. 



