430 SCIENCE PROGRESS 



being weeds of cultivated ground, are those most frequently 

 found by the wayside and in hedges. Shepherd's Purse, 

 Mouse-ear Chickweed and the ordinary Chickweed, the Wood- 

 bine or Wild Convolvulus, Cleavers, Groundsel, Sow Thistle, 

 Creeping Thistle and the common Dandelion are all universally 

 distributed in fields, no matter what the soil. 



A consideration of the results set forth justifies the con- 

 clusion that the chief factor in determining the diversity of the 

 weed flora is the texture of the soil rather than the difference 

 in the amounts of plant food available. In other words, the 

 degrees of coarseness or fineness of the soil and the correlated 

 differences in water-holding capacity have far more effect in 

 influencing the species of plants that occur than have the 

 variations in the amounts of available phosphoric acid, potash, 

 etc., present in the soil. The one chemical factor which has 

 a profound influence is chalk or lime, which is far-reaching in 

 its effects. This is most noticeable at the two extremes : 



i. When lime is conspicuous by its absence, the soil being 

 acid in consequence, a state of affairs that is definitely indicated 

 by the flora ; 



2. When the soil is practically pure chalk, providing a very 

 definite type of substratum and bearing a vegetation which 

 again is quite characteristic. 



Since lime is present in a greater or less proportion in 

 nearly all soils, between these two extremes the texture of the 

 soil is the predominant factor which determines the character 

 of the flora. 



So far we have been considering the weed flora solely in 

 its relation to the nature of the soil but another factor to be 

 considered is the character of the crop. Broadly speaking the 

 field crops fall into three main divisions : 



i. Cereals — Wheat, Barley, Oats. 



2. Roots — Swedes, Turnips, Mangels and by courtesy 

 Potatoes. 



3. Leguminous crops or " seeds " — Clover, Sainfoin, Lucerne, 

 Vetches, also Peas and Beans. 



The habits of these three sets of plants are quite distinct 

 and inevitably the difference influences the weed flora. The 

 cereal crops at first give the weeds a very fair chance; later 

 on, as they grow up and begin to fill out into ear, much of the 



