Sir Jno. Lubbock, F.R.S., lately delivered a most interesting lecture on
the above subject, in Leeds, England, some portions of which have a
special interest to the hybridist; and though it is not possible in
these pages to reproduce the whole discourse, I have thought a selection
from the report in the English _Agricultural Gazette_ might give
profitable food for thought to readers of the CANADIAN HORTICULTURIST.
The lecturer alluded to the difference existing in plants, not only
differences in form, size and, color, but also in other respects, some
being hairy, glutinous, sticky, &c. These may be accounted for in a
great measure by the relations borne by plants to insects, the visits of
which are generally necessary to ensure the fertilization of one flower
by the pollen of the other. In some cases, however, such as that of
_Drosera_, the object of the flower is to attract insects for the
purpose of devouring them, and therefore the leaves are covered with
sticky hairs, which bend gradually forward when an insect alights on the
leaf, and squeezes it slowly to death, its juices going to the
nourishment of its alluring and deceitful foe.
It is easy to see the advantage which flowers gain from secreting honey,
inasmuch as they are dependent for fertilization on the visits of bees
and other insects, which while feeding necessarily dust themselves with
pollen, and thus carry it from flower to flower. But it is less easy to
understand why honey should be secreted on those parts of flowers where
no pollen exists, at the base of the leaf-stalks, for instance. For the
explanation of this we are indebted to Mr. Bell, and Mr. Delphine, who
observed that in some cases upon the stems, and living upon the honey,
exist colonies of small ants, constituting a most efficient body-guard
against the attacks of leaf-cutting ants. They also protect plants from
the attacks of many other enemies, and are in their turn made use of by
various small species of Aphides who, by secreting a sweet fluid, of
which they allow the ants to avail themselves, convert them from enemies
into friends and thereby secure a cordial, instead of an angry
reception.
Harmless, however, and even useful as are ants which confining
themselves only to the stalks of flowering plants, they would generally
be positively injurious to the flowers themselves, as these would merely
rob them of their honey without repaying the debt by carrying the pollen
to others. Flowers therefore have been driven to protect themselves by
various devices, such as slippery surface, sticky glands or hedges of
hairs, which entirely prevent the ants from obtaining access to the
nectary. For the visits of ants to flowers would not only deprive them
of their honey, but would prevent the visits of those insects which are
so necessary to cross-fertilization. The instance of the mode in which
the common Fox-glove excludes the entrance of ants was given; the flower
is a close box, which contains the anthers, pistil and honey. It has the
specialties of a flower which is adapted for cross-fertilization by
insects, color, honey, and the arrangement of stamens and pistils, but
it is closed. The flower is adapted to cross-fertilization by
bumble-bees, and they alone can force open the box; to other insects it
is closed. Again, the beautiful rosy flowers of the _Polygonum
Amphibium_ are rich in nectar, and quite unprotected from the visits of
creeping insects so long as the plant is grown in water; the arrangement
of the stamens and pistils is such that it cannot fail of
cross-fertilization on the visit of any flying insect. When, however,
this plant is grown on land, and consequently liable to the visits of
creeping things, certain hairs terminating in sticky glands are thrown
out, effectively barring the entrance of these worse than useless
guests. The so called “sleep” of plants is another means of self-defence
adopted by those flowers whose fertilization is dependent upon the
visits of day insects; while on the other hand there are other species
of flowers adapted for moths and nocturnal insects, which expand towards
night, and scent the evening air with delicious perfume. The curious
life history of _Silene Nutans_ was then referred to, and the advantages
of early rising shown in those flowers which expand early in the morning
to receive bees, but close again before the later rising ants are astir.
Sir John proceeded to discuss the means by which insects provide
themselves with means of concealment, by imitating the appearance of
plants; the “walking stick” insect, and many larvæ are cases in point.
Some caterpillars, living on the under side of leaves, not only adapted
their color to those leaves, but actually, as their growth increased,
altered their markings so as to coincide with the fibres of the leaves.
Curiously enough, sometimes bright and striking colors were used as a
mode of protection. In these cases, however, they are accompanied by an
unpleasant smell and taste, so that the gorgeous dress which would seem
so dangerous is in fact a most effectual shield.
The lecturer pointed out that there were five principal types of
coloring among caterpillars. Those which live inside wood or leaves or
underground are generally of a pale uniform hue; the small leaf-eating
caterpillars are green, like the leaves on which they feed. The other
three types may “_si parva licet componere magnis_” be compared with the
three types of coloring among cats; there are the ground cats, as the
lion and puma, which are brownish or sand color, like the open places
they frequent; so also, caterpillars which conceal themselves by day at
the roots of their plant food, even if originally green, tend to assume
the color of earth. The spotted or eyed cats, such as the leopard, live
among trees, and their peculiar color renders them inconspicuous, by
mimicking the spots of light and shade among the foliage. Lastly, there
are the striped cats, as the tiger, which inhabit the jungle, whose
markings render them difficult to see among the brown grass they
frequent. The stripes of the tiger are transverse because he walks
horizontally on the ground; while the stripes on the caterpillar are
either longitudinal or oblique, the direction of the lines follows those
of the foliage; those caterpillars which cling to the grass in a
vertical position have longitudinal lines, while those which live on
mere leaves have oblique lines, corresponding with the oblique midribs
of the leaves.
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