162 
ARE SOILS ENRICHED, IMPOVERISHED, [Part III. 
growth of the tree. If this were so, the roots 
would soon be surrounded with such substances, 
and would be incapable of absorbing nutriment. 
In chalk districts eternal woods are found com¬ 
posed of nothing but beech ; in other soils, of 
nothing but oak. The oldest vineyards and the 
oldest hop-gardens are the best. And how 
many millions of acres are in this world covered 
with perpetual heath ! In all these cases, if the 
roots excreted substances unfit for nourishing 
the plants, the whole soil would have become 
saturated with them. Land plants grown in 
water are always unhealthy. Under these cir¬ 
cumstances, may not colouring matter, or other 
substances supposed by Macaire-Princep to be 
excretion, be the result of disease and decay or 
partial maceration of the roots. There is no 
discoloration of the water in which the seedlings 
of forest-trees are made to grow, while these are 
in health. 
I imagine that trees, in absorbing by their 
roots the moisture with which they come in 
contact, give off the unnecessary parts of this 
by transpiration in the air. I do not perceive 
what should cause roots to transpire when sur¬ 
rounded by moisture; or if they do, they must 
return, like the dog to his vomit, and again 
absorb their own transpirations. 
