4 2 4 SCIENCE PROGRESS 



A good loam provides the type of soil which is the most 

 favourable to plant growth. The loams are intermediate in 

 texture between sand and clay and the disadvantages of both 

 the latter are eliminated by the admixture. A finely grained 

 soil containing plenty of humus (derived from decayed vege- 

 table matter) retains a sufficient supply of water to encourage 

 the growth of the majority of plants, provided the subsoil be 

 sufficiently permeable to admit of good drainage and that water- 

 logging be avoided. Such a soil, whilst it contains all the 

 moisture the plant needs, also has sufficient air in its interspaces 

 to enable the roots to breathe freely, so that, other things being 

 equal, all the vital processes can go on satisfactorily. Conse- 

 quently the great majority of species are to be found on soils 

 of this type, the relative dominance or comparative scarcity or 

 absence being determined by the different local conditions. 



The weed floras of the heavy soils are characterised at the 

 outset by the fact that there are very few species which can be 

 said to be symptomatic, i.e. there are practically no plants which 

 are confined to clay, appearing nowhere else. The number of 

 species occurring is less than on the lighter types of soil and 

 they are such as are more or less universally distributed, 

 though in different districts some plants show a greater or 

 less predilection for heavy land. Still, though the species are 

 restricted in number, the actual quantity of weed present is 

 equally great. It frequently happens that a weed which is 

 characteristic of a certain type of soil in one part of the country 

 may in another be conspicuous by its absence either from fields 

 of the particular soil or from the whole district. For instance, 

 the Scentless Mayweed {Matricaria inodora) is almost constantly 

 associated with the clay soils investigated in Bedfordshire, whilst 

 on the equally heavy land around Bath no trace of it is to be 

 found. The explanation in this case may be that the plant is 

 shy of lime, as the eastern clay is non-calcareous in composi- 

 tion. Again, in Bedfordshire the Hoary Plantain (Plantago 

 media) is practically absent as a cornfield weed, whereas in 

 the Somerset district it occurs frequently and is the one species 

 that is only found on clay. This again may be accounted for 

 by the fact that the plant is notably a denizen of limestone 

 districts, 1 as the Oolitic series of clays around Bath contain much 

 limestone thus providing a congenial soil for this plantain, 

 1 Bentham and Hooker, Handbook of the British Flora, p. 366. 



