4 Alpine Plants. 
collect on the subsoil; for the rain in its passage 
washes away these noxious matters, leaving only 
wholesome material to receive the roots as they 
descend. 
It might be supposed that drainage in hot, dry soil 
would render drier a soil already parched up, but ex- 
perience seems to show that such is not the case. 
A little judgment must, however, be used in dealing 
with such exceptionally warm light soils. Hence, 
instead of the drainage being say one foot from the 
surface, as in clayey subsoil, in sandy arid places it 
must be deepened, so that the roots can penetrate 
further, for in this way they will be rendered more 
independent of the surface moisture. 
The importance of proper mixture of soil is there- 
fore evident, so far as it promotes satisfactory 
drainage. 
But the zature, as well as the condition, of the soil 
has also very direct physical bearing upon plant 
welfare. 
Under the influence of heat, soil shrinks in pro- 
portion to the quantity of clay of peat it contains, 
whereas sandy soil diminishes very little in bulk 
by dryness. 
Hence, not only is air excluded from the roots of 
plants by heavy soils in dry weather, but the plants 
themselves are also placed in a condition exceedingly 
unfavourable to vitality through constriction of the 
vessels in the “collar” of their stems, and even more 
immediately through the destruction of those delicate 
root-hairs by which they take in their nourishment, 
under the compression to which they are subjected. 
