those returning vessels that deposit
the new alburnum.

_Why is it inferred that these juices must be prepared in the plant itself,
by various secretions, and changes of the fluids which it absorbs?_

Because we find, that in the same climate, nay, even in the same spot of
ground, rue has its bitter--sorrel its acid--and the lettuce its cooling
juices; and that the juices of the various parts of one plant, or even of
one fruit, are extremely different. Sir James Smith mentions the
peach-tree as a familiar example. "The gum of this tree is mild and
mucilaginous. The bark, leaves, and flowers, abound with a bitter
secretion, of a purgative and rather dangerous quality, than which nothing
can be more distinct from the gum. The fruit is replete, not only with
acid, mucilage, and sugar, but with its own peculiar aromatic and highly
volatile secretion, elaborated within itself, on which its fine flavour
depends."--_Introduction to Botany, 6th edit_.

_Why are these juices readily found in the bark?_

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.

Common resin is ob