SOIL FOR GREENHOUSES 179 



weakened condition of the tissues (making the plant sub- 

 ject to the attack of enemies, especially fungi), and so on 

 through a list of other possibilities. To illustrate more 

 fully, we may say that in our experience, the Lady Hume 

 Campbell violet seems to thrive best on relatively light soil ; 

 that is, a soil which contains a comparatively small amount 

 of clay. The Marie Louise, on the other hand, does best 

 with more clay, or in other words, a heavier soil. The 

 violet soils of the Poughkeepsie region contain from eight 

 to fifteen per cent of clay, and it is here that the highest 

 success is attained with this particular variety. In parts 

 of Maryland and Virginia, where the Campbell violet is 

 grown extensively, the soils frequently contain six to eight 

 per cent of clay, so that it will be seen that there is con- 

 siderable difference as regards the structure of the two 

 classes of soils. It follows of course that where soils 

 showing so much difference in structure are used, the 

 plants in each case have different sets of factors to which 

 they must adapt themselves, and in doing this they may 

 be so modified as to materially affect the development of 

 the flowers." 



145. Mixing soils. " Soils of both the heavy and mod- 

 erately heavy types are found in many places, and if they 

 do not occur naturally, the desired effects may be approxi- 

 mated by combination of light and heavy soils. We 

 have never found it satisfactory to lighten heavy soil by 

 mixing it with sand alone. This takes away the life of the 

 soil, and plants never succeed so w r ell in it as when the 

 desired conditions are produced by mixing a heavy and a 

 light soil. For example, we may have in one part of a 

 field a soil containing fifteen to twenty per cent of clay, 

 and in another, one containing four or five per cent. By 

 mixing these two soils in equal proportions, a combination 



