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 roundleaved 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 over-excitement and over-taxation, 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 Droseraceoe. 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 roundleaved species is common throughout the United States, Europe, and Asia.
Closely related to the sundew is the Venus fly-trap (Dionoea 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 flycatchers."