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4 ALPINE PLANTS. 
food within reach of the plants. If the 
drainage is good, no substances which are 
injurious to the roots of the plants can collect 
on the sub-soil. 
These noxious matters the rain washes 
away in the draining, and leaves room for 
the descent of the roots into wholesome 
soil. 
It may be argued that drainage in hot, dry 
soil would render drier a soil already parched 
up. So far as my own experience goes, this 
is not the case. 
A little judgment must be used in cases 
of this sort, and instead of the drainage 
being, say, 12in. from the surface, as in clayey 
sub-soil, in sandy dry places it must be 
deepened so that the roots can _ penetrate 
further. They will thus be rendered more in- 
dependent of the surface moisture. 
There is no doubt that the soil shrinks 
under the influence of the heat in proportion 
to the quantity of clay or peat it contains. 
Sandy soil diminishes very little in bulk by 
dryness. Thus, in heavy soils in dry weather 
the roots are very much compressed and the 
air excluded from them, and the plants are 
