A plant is a wonderful organism, yet how few of us realize it as we
casually glance at the flowers growing by the wayside. We see a beautiful
flower; we know that in the course of time it withers and fades away; and
we know that the next year the plant grows up again, sends forth its
buds, which at the proper time unfold their petals, and so the cycle
continues year after year, while we give little thought to the change
that occurs, the cause and its effect. Volumes might be written, and a
great many have been since the time of Darwin, upon the many interesting
processes by which various flowers are propagated. As this book is
confined chiefly to the identification of flowers, we will give but a few
illustrations between flowering seasons. We see in most flowers a thing
of beauty; their real and, to them, most important function is to produce
seeds to perpetuate the species.
The parts of a flower that are necessary to produce seeds are the pistil,
with its stigma at the top and ovule at the base, and the stamens with
their pollen-laden anthers. A flower that has these organs is known as a
perfect flower; if, in addition, it has a corolla and calyx, or petals
and sepals, it is known as a complete flower. On the opposite page are
shown a number of flowers with their parts named.
[Illustration: PARTS OF FLOWERS]
In order that seed may be set, it is necessary that ripened pollen from
the anthers should come in contact with the usually sticky stigma, whence
it will be transmitted down the style and germinate the ovule. As is well
known to be the case with the higher animals, cross-fertilization is
necessary in order to insure a good, vigorous species. It is also evident
that should the pollen continue to fertilize the ovule in the same
flower, the plants in successive generations might become weakened and
finally die out and the species be lost. To avoid such a calamity,
flowers are constructed so as to facilitate cross-fertilization and the
means that some of them adopt towards insuring that end are remarkable.
The stamens on most of the simple flowers usually curve outwards, so that
the pollen-laden anthers are far enough removed so that there is little
danger of the pollen falling on the stigma, at least until after
cross-fertilization has already taken place. Others have either the
anthers or stigma ripen first, so that it must necessarily be pollen from
another blossom that quickens the seed.
We all marvel at the industry of the honey bee; how tirelessly it buzzes
from flower to flower, from each gathering a drop of the nectar, with
which it fills its cells; but we do not always realize the double duty it
is doing, for it is a most reliable and active agent for the propagations
of a great many plants. Many butterflies, bees, and even beetles
unconsciously accomplish the same result, and it is now conceded that
each has special colors that are attractive to them. For instance, the
bumblebee has a strong preference for blues and purples. The observer
will also notice that a bee makes the rounds from flower to flower,
taking all of one kind and passing by other species. While this habit
undoubtedly avoids some complications, even should he mix his drinks and
visit in succession flowers of widely different species, confusion would
not be apt to result, for the stigma of one species is usually not
responsive to pollen brought from blossoms of another family.
Botanists go a step further; not content with the discovery that certain
insects like certain colors, they claim (and apparently with good reason)
that the bright and showy petals are for the sole purpose of attracting
insects; they are, in fact, bill boards advertising the fact that there
is a store of honey there ready for the asking. On the other hand, those
flowers that are self-fertilized or wind-fertilized have inconspicuous
blossoms.
The stigma always partially obstructs the entrance to the food supply, so
that the visiting bee must brush against it, and in doing so will leave
some of the pollen that he has brought from the last flower visited on
its sticky surface. The pollen-dust is attached to the insect in various
ways, usually simply by his brushing against the anthers with his hairy
body, for it is found that nearly all the useful insects have downy or
hairy bodies; other flowers set a sort of spring gun and when the insect
steps on the trigger he is showered with the germs (Laurel for example);
still others have clefts to catch the legs of visitors, releasing them
only if they are strong enough to tear away the pollen masses (such a
flower is well illustrated in the Milkweed). Besides having bright
colored petals, many of the flowers also have a pleasing odor, this also
serving to attract certain kinds of insects; others have very unpleasant
odors, like the skunk-cabbage or even like that of putrid meat, as in the
carrion flower and the purple trillium, these odors being apparently for
the purpose of attracting certain scavenger insects. There are also some
flowers, like the evening primrose, that are seen at their best after
dusk, when the light-colored petals are widespread and a delicate perfume
given off to attract the moths and sphinges that visit them.
It is evident that a flower secreting honey may be visited by unwelcome
guests, ones that will accept of the nectar, but will make no useful
return. Any insect with a shiny, smooth body whether winged or not, is of
little use in fertilizing a plant, for even should it receive pollen, it
will in all probability have fallen off before the next flower is
visited. Ants being particularly fond of sweet things and so small that
they can enter a flower without disturbing the anthers, frequently drain
the nectar cups so no useful insect will visit them, and they fail to
reproduce their kind. Nature has a number of quite effective ways of
preventing thefts of this kind, one of the most common ways being to
provide the plant stem with bristly hairs, forming a very difficult
barrier for any crawling insect to overcome; others have a tuft of hair
at the very entrance to the honey cells which bar the way for unwelcome
guests, but readily allow the bee to insert its tongue; still others are
protected by recurved leaves or by sticky stems, or as in the toadflax by
a two-lipped flower, which will open under the weight of a bumblebee, but
is effectively closed to any lighter insect.
The seeds, having been set, are enclosed in a capsule composed of the
closed and dried sepals, the petals having fallen off; in a pod as in the
peas and beans; or in fruit, as inside an apple, which is formed by the
base of the flower enlarging about the seeds with the calyx remaining at
the top of the ripened fruit, or in the strawberry, where the seeds are
on the outside of the berry and the calyx at the bottom of the fruit. It
is plain that should these seeds simply fall to the ground, plants of a
single species would soon become so crowded in a small area that the
earth could not support them. Consequently various means are furnished
different plants for the dispersion of their seeds. A great number, like
the thistles, milkweeds and dandelions, have plume-like parachutes
provided for each seed, so they can float away on the breeze to new
fields; those that have their seeds embedded in fruit are entrusted to
birds to be carried where fate wills it; others, like beggar-ticks,
burdock, etc., have spines to attach themselves to the clothing of people
or to the coats of animals that brush against them.
[Illustration: KINDS OF LEAVES]
As certain insects prey upon plants or rob them of their nectar, so
certain plants prey upon insects, literally eating them or absorbing them
into their system. Best known among these are the pitcher plants, a swamp
species, whose leaves are pitcher-like and with a hood or awning over the
top to keep out the rain; these leaves are half filled with a sweet fluid
that attracts insects, makes them tipsy and causes their death in the
watery grave, the plant feeding largely upon the resulting broth. Of
another type is the round-leaved sundew, also a common plant; its leaves
are covered with short bristly hairs, a drop of gum glistening at the end
of each. A fly investigating these is soon caught in the sticky gum and
the leaf slowly folds together, enveloping the victim in what might be
termed the stomach of the plant. Perhaps the most interesting and surely
the most peculiar plant is the Venus fly-trap, which is found only in
eastern North Carolina. At the end of each leaf is apparently a smaller
one, perhaps an inch in diameter; this is fringed around the edge and
rather bristly in the centre. These central bristles are very sensitive
and if touched or an insect lights upon the leaf, the two parts of the
leaf instantly clasp together on the central stem as a hinge. If nothing
is caught, in a short time the trap opens again; if, however, the attempt
has been successful it will remain closed for several days or a week,
until the victim is entirely absorbed by the glands on the inner surface
of the leaf.
As in the animal world, so in the plant world; always a struggle for
existence, the strong surviving and the weak falling by the wayside. The
old adage that “In union there is strength” is amply proved by many of
the composite flowers, such as the asters and goldenrods, whose stalks
are not only capped with numerous flower-heads, but each flower-head is
composed of hundreds of little perfect florets, so closely set together
that even should an insect but crawl across the flower-head he will
fertilize a number of them. That their plan is a good one is seen by the
steady increase in the numbers of these flowers and the rapid strides
with which they occupy new territory. On the other hand, compare such
flowers as the lady’s slippers, fringed gentian and numbers of others
that are yearly becoming less common.