Others have an external bitter rind, as in the walnut; while in
the chestnuts and beech-nuts two or three fruits are enclosed in a
prickly involucre.
Notwithstanding all these precautions, nuts are largely devoured by
mammalia and birds; but as they are chiefly the product of trees or
shrubs of considerable longevity, and are generally produced in great
profusion, the perpetuation of the species is not endangered. In some
cases the devourers of nuts may aid in their dispersal, as they probably
now and then swallow the seed whole, or not sufficiently crushed to
prevent germination; while squirrels have been observed to bury nuts,
many of which are forgotten and afterwards grow in places they could not
have otherwise reached.[140] Nuts, especially the larger kinds which are
so well protected by their hard, nearly globular cases, have their
dispersal facilitated by rolling down hill, and more especially by
floating in rivers and lakes, and thus reaching other localities. During
the elevation of land areas this method would be very effective, as the
new land would always be at a lower level than that already covered with
vegetation, and therefore in the best position for being stocked with
plants from it.
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