o 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.
The other modes of dispersal of seeds are so clearly adapted to their
special wants, that we feel sure they must have been acquired by the
process of variation and natural selection. The hooked and sticky seeds
are always those of such herbaceous plants as are likely, from their
size, to come in contact with the wool of sheep or the hair of cattle;
while seeds of this kind never occur on forest trees, on aquatic plants,
or even on very dwarf creepers or trailers. The winged seed-vessels or
seeds, on the other hand, mostly belong to trees and to tall shrubs or
climbers. We have, therefore, a very exact adaptation to conditions in
these different modes of dispersal; while, when we come to consider
individual cases, we find innumerable other adaptations, some of which
the reader will find described in the little work by Sir John Lubbock
already referred to.
_Edible or Attractive Fruits._
It is, however, when we come to true fruits (in a popular sense) that we
find varied colours evidently intended to attract animals, in order that
the fruits may be eaten, while the seeds pass through the body
undigested and are then in the fittest state for germination. This end
has been gained in a great variety of ways, and with so many
corresponding adaptations as to leave no doubt as to the value of the
result. Fruits are pulpy or juicy, and usually sweet, and form the
favourite food of innumerable birds and some mammals. They are always
coloured so as to contrast with the foliage or surroundings, red being
the most common as it is certainly the most conspicuous colour, but
yellow, purple, black, or white being not uncommon. The edible portion
of fruits is developed from different parts of the floral envelopes, or
of the ovary, in the various orders and genera. Sometimes the calyx
becomes enlarged and fleshy, as in the apple and pear tribe; more often
the integuments of the ovary itself are enlarged, as in the plum, peach,
grape, etc.; the receptacle is enlarged and forms the fruit of the
strawberry
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