178 THE BEAUTIES OF NATURE CHAP. 
for the association of species is less evident. 
The Larch and the Arolla (Pinus Cembra) 
are close companions. They grow together 
in Siberia; they do not occur in Scandinavia 
or Russia, but. both reappear in certain Swiss 
valleys, especially in the cantons of Lucerne 
and Valais and the Engadine. 
Another very remarkable case which has 
recently been observed is the relation existing 
between some of our forest trees and certain 
Fungi, the species of which have not yet 
been clearly ascertained. The root tips of the 
trees are as it were enclosed in a thin sheet 
of closely woven mycelium. It was at first 
supposed that the fungus was attacking the 
roots of the tree, but it is now considered 
that the tree and the fungus mutually benefit 
one another. The fungus collects nutriment 
from the soil, which passes into the tree and 
up to the leaves, where it is elaborated into 
sap, the greater part being utilized by the 
tree, but a portion reabsorbed by the fungus. 
There is reason to think that, in some cases 
at any rate, the mycelium is that of the 
Truffle. 
