WILLIAM KERR HIGLEY, Secretary of The Chicago Academy of Sciences.
Queen of the Marsh, imperial Drosera treads
Rush-fringed banks, and moss-embroidered beds.
--_Erasmus Darwin, in The Botanic Garden, 1789._
Some of the most interesting forms of nature are not the most
showy and are not easily observed by the untrained eye. Many of
their characteristics can only be known by carefully conducted
investigations, both in the field and in the laboratory.
The advance of science has shown us that it is as natural for some
plants to obtain much of their nourishment from the animal world, by
a true process of feeding, as it is for animal forms to obtain their
sustenance, either directly or indirectly, from the vegetable world.
There are many species among the lower orders of plants that are well
known animal parasites, but there are also, among our more highly
organized flowering species, forms that improvise a stomach and secrete
an acid fluid for the digestion of nitrogenous food which is afterwards
absorbed and used in tissue building. These are in no sense of the term
parasites.
Such a plant is our common round-leaved sundew (_Drosera rotundifolia_,
L.). The generic name Drosera is from the Greek, meaning dew.
This rather insignificant, but pretty little plant is distributed
nearly throughout the world, and is usually found in bogs, or in wet
sand near some body of water. The flower stalk is seldom more than six
or eight inches in height and bears very small white or pinkish-white
flowers.
The interesting feature of this species, however, lies in the rosette
of about five or six leaves growing from the base of the stem. These
leaves lie upon the ground and are usually about one-fourth to one-half
of an inch in length, and are generally nearly orbicular in form. The
upper side is covered with gland-bearing tentacles. The glands are
covered by a transparent and viscid secretion which glitters in the
sunlight, giving rise to the common name of the plant. There are
usually over two hundred tentacles on each leaf and, when they are
not irritated, they remain spread out. The viscid fluid of the glands
serves as an organ of detention when an insect lights upon the leaf.
The presence of an insect, or, in fact, any foreign matter, will cause
the tentacles, to which it is adhering, to bend inward toward the
center of the leaf and within a very short time all the tentacles will
be closed over the captured insect, which is soon killed by the copious
secretion filling its breathing apparatus.
Though these sensitive tentacles are not excited by either wind or rain
they are by the repeated touchings of a needle, or any hard substance.
It is said that a fragment of hair weighing but 1-78,740 of a grain
will cause a perceptible movement.
By experiment it has been shown that a bit of hard-boiled egg, or a
fragment of meat as well as an insect will cause not only an inflection
of the tentacles but also of the edges of the leaves, thus forming
an improvised stomach, the secretion of the glands then increasing
and becoming acid. At this stage the secretion is not only capable of
digesting but is also highly antiseptic.
This power of digesting and absorbing nitrogenous food is absolutely
necessary to the existence of the sundew, for it usually grows in a
poor soil and its few and not greatly elongated roots are of little
service except to absorb water, of which it needs a large amount for
the production of the copious secretion. Specimens may be developed by
planting in moist cotton and furnishing with plenty of water.
The length of time that the tentacles will remain inflected depends on
the vigor of the leaf and the solubility of the material causing the
excitement. The time varies from one to seven or eight days.
Easily dissolved and readily absorbed food in too large an amount seems
to cause overexcitement and overtaxation, and frequently results in the
death of the leaf.
The large number of insects, especially flies, captured by these plants
would lead one to believe that they are attracted by the odor of the
plant, or the purplish color of the tentacles, rather than by the
desire to use the leaves as a resting-place.
The sundew belongs to the natural order _Droseraceæ_. This contains
about one hundred and twenty-five species, of which one hundred and ten
belong to the genus Drosera, and are chiefly natives of Australia,
though the round-leaved species is common throughout the United States,
Europe, and Asia.
Closely related to the sundew is the Venus fly-trap (_Dionæa
muscipula_, Ellis). This is a native in the eastern part of North
Carolina only.
The leaf of this plant is provided with two lobes, which close quickly
when the sensitive hairs, which are situated on the upper surface of
the leaf, are irritated by an insect. The acid secretion flows out and
the leaves remain closed till digestion and absorption are completed.
Dr. Asa Gray has referred to this species as "that most expert of
fly-catchers."