Because they appear to be matured, or brought to greater perfection, in
layers of wood or bark that have no longer any principal share in the
circulation of the sap. Thus, the vessels containing them are often very
large, as the turpentine cells of the fir tribe, in all the species of
which these secretions abound. The substance from which spruce-beer is
made, is an extract of the branches of the _Abies Canadensis_, or Hemlock
Spruce; a similar preparation is obtained from the branches of _Dacrydium_,
in the South Seas.

_Why, in the spring, is the herbage under trees generally more luxuriant
than it is beyond the spread of their branches?_

Because the driving mists and fogs becoming condensed on the branches,
cause a frequent drip beneath the tree not experienced in other places;
and thus keep up a perpetual irrigation and refreshment of the soil.

_Why are certain plants useful or injurious to others that grow in their
vicinity?_

Because of certain fluids which the roots excrete from their slender
extremities; and in this manner the likings and antipathies of certain
plants may be accounted for. Thus, it is well known that the creeping
thistle is hurtful to oats, _erigeron acre_ to wheat, _scabiosa arvensis_
to flax, &c.

_Why are some resins odorous?_

Because they contain essential oil; some afford benzoic acid when heated,
and these have been termed balsams; such as tolu balsam and benzoin.