Martynia and its Humble Servants.
BY JOSEPH CRAWFORD, PH.G.
From an Inaugural Essay.
This subject is chosen to show, not the presence of some powerful alkaloid or other valuable therapeutical, principle which I think is wanting, but rather some of the relations existing between plants and insects, and to awaken a deeper interest among students for observing the indigenous Materia Medica and the wonderful forms exhibited by plants.
The sciences of Botany and Entomology have been full of delightful interest to their respective students that the idea of connecting the two (in their earlier history) was almost disregarded, but now the one is known to be as dependent on the other for the perfection of its species, as the other is on it for the perpetuation of its species. The Martynia has been selected, not as showing these relations to their utmost, but as a common example and full of untiring interest.
It is a native of the Southwestern States, but is cultivated in the eastern section annually for its flowers and fruit, the latter for pickles and condiments of like nature. The species proboscidea is the one under consideration, and about the only one that receives any horticultural attention. It belongs to the natural order Bignoniaceae, and is commonly called Unicorn Plant, from the resemblance of the curved capsule and prolonged beak (with horny texture) to a horn, and the specific name is consequently easily-derived. The genus was named in honor of Professor Martyn, of Cambridge.
The plant is about two or two and one-half feet high, occasionally prostrate from weight of branches and fruit; leaves entire, large, round and heart-shaped, oblique at base, upper alternate, lower on very long petioles, and all nearly horizontally expanded. Inflorescence a large many-flowered raceme; calyx bell-shaped, with five unequal lobes, the upper lobe narrow, ⅜ to ½ inch long; the others are nearly equal. The lower portion of the calyx is split open to base, and subtended on either side near the top by a, large fleshy conical bract as long as the calyx, and of the same color, or a little darker. The corolla is gibbous, inflated, about an inch long, or longer, hangs obliquely on a pedicel twice its length; the lobes nearly equal, spreading, about half an inch broad. The lower lobe is somewhat longer and a trifle broader, and furnishes the most characteristic marking as a temptation or solicitation of insect aid that can be found in any order outside of the Orchidaceae, represented in this section of the United States and will be described later. The tube of the corolla is spotted with yellow and purple both on the interior and exterior; the lobes have their share also in the same colors, but not so much of the purple dotting. There are four perfect stamens, didynamous, the fifth only partly developed, club-shaped and woolly; the filaments are long, and one or two are twisted. The anthers are regularly two-celled, and rectangular when opened, when they expose the pollen on the surface in four miniature bricks, by the cohesion with the anthers. The filaments diverge in their recurvature, but always meet, to cast their pollen, in adjacent pairs. The pistil is also recurved, and exserted at all times beyond the stamens; stigma bifid, the upper portion rolled back; and the lower rolled inwards and somewhat longer than the upper.
On account of this arrangement, the anthers overcapped or covered by the stigma, it scarcely receives any pollen by the natural source, but most, if not all, through the agency of insects, in various ways.
One of the largest of its humble servants in this cause is the female of Bombus virginicus, which can be seen in early mornings flying from flower to flower seeking the nectariferous secretion. Its size will just admit its entrance into the tube of the corolla, and it must consequently brush its hirsute thorax against the opened anthers, and a quantity of the pollen adhering to it by contact is brushed off on the lower lobe of the stigma of the next flower visited; the stigma of the same flower is not fertilized by its own pollen by this bee, because of its sensitiveness; when the insect brushes against it on entering, it immediately closes in order to retain any pollen accumulated, and remains so for a lapse of time, or until the insect has left the flower.
This bee is one of its best benefactors, as on account of its size it cannot enter the tube of the corolla without touching either the stigma or stamens and deposit pollen obtained from the last visited flower and receive a new supply from the present one; and, aiding fertilization as described, its presence is an assurance of the complete fertilization.
Although this depends principally upon the bulk of the insect, yet a number of these winged friends are small and have each their peculiar mode of assistance.
"Nature abhors perpetual self-fertilization," is true, but she always supplies the deficiency by having the aid of the wind, birds or insects in the distribution of the pollen; the wind for dioecious plants generally, and for small flowers with an interruption or transposition of parts. Those dependent on insects have some special attraction or solicitation for them, generally in a peculiar form of the flower or beautifully bright coloring of the whole or a part of it.
This plant depends on this last entirely; from its dependent position and quiet coloring of the exterior surface of the corolla and calyx, and a great portion of the interior of the corolla, it would be passed unnoticed by the myriads of winged insects constantly passing near, were it not for the beautiful marking on the lower portion of the corolla. It begins at the insertion of the corolla on the receptacle, is prolonged through the tube in brilliant golden lines about the size of the filaments, and terminates on the lower lobe in bright golden splotches, in exact imitation of the stamens discharging their pollen. The quiet pearl hue of the background adds so greatly to the deception that even the instructed are too apt to consider them the essential organs without further investigation.
One of its numerous small friends is the Mellisodes prunosa, very frequent at all hours of the day and extremely busy; its size and general appearance is similar to that of the worker of Apis mellifica, and unobservant individuals are likely to confound them; but they can easily be distinguished by the little triangular white spot on the head, just above the mouth, and by their seeking separate flowers; the Apis scarcely visiting the Martynia, while the Melissodes crowd the corollas to the verge, so eager are they for the nectar secretion. They are as industrious and persevering as their relative, for frequently can they be seen crowding the whole length of the corolla, waiting patiently for their turn at the fountain of nectar, and one, undisturbed, will remain from 15 to 20 minutes at a single flower. It is only when congregated thus that they are of any service in aiding fertilization, as on account of their size they have easy access to all parts without distributing much pollen; but when a number of them are assembled the advent of a new arrival causes a flutter of excitement momentarily among the little congregation, pollen is detached, and, adhering to portions of their bodies, is transported to the stigmas of other flowers which are thus fertilized.
The Melissodes prefers the base of the flower for its exertions, but the genus Halictus, represented by a beautiful female, is content with the pollen, and works very industriously at it, letting no small amount fall on the backs of the "waiting congregation" of Melissodes, etc., and, these subsequently transporting it to the stigmas of other flowers, cross fertilization is again produced. This insect is much smaller than the preceding, much shyer and a great deal more active, but not as numerously represented.
In describing the plant nothing was said about the viscid glands covering its entire surface, giving it a somewhat glaucous hue, and one unaccustomed to the plant would in all probability, on hasty inspection, rank it as velvety to the touch, but on a more extended investigation the mistake would be clearly shown and the true nature reveal itself by its resinous secretion. Although so numerous, they can scarcely be described without the aid of the microscopical instrument, except that they are of various sizes, the largest about a line in length, rather rigid, composed of from four or six to ten or twelve transparent elongated cells, terminated in a flattened globe (mostly), but when a foreign substance touches the tip a threadlike glutinous mass is drawn out, exhausting the gland in part and causing it to assume another shape. They are more numerous and larger on the under side of leaves, main stem and parts of the inflorescence, while the upper side of the leaves is covered with a shorter kind, nearly free from resin (probably from attrition of elements), and the long curved pods are comparatively free from it.
The office of the glands is at present unknown, but undoubtedly they are of consequence in assimilation to the plant in this wise: from their glutinous nature, small insects meet their fate by passing too near, or, forced by the wind upon them, and struggling, they become still more entrapped, and death relieves them of their misery, and they become disintegrated by unseen forces. Whether or not they are absorbed in the liquid or gaseous form is yet to be ascertained; many writers have approached the subject, but accomplished nothing. Darwin and Mrs. Treat have proved the presence of gastric juice in Drosera and Dionaea; but then the adaptations are dissimilar, these being mobile and the Martynia immobile.
It is not altogether unlikely that the odor of the plant assists in the "slaughter of the innocents," by alluring them to it, as it is very offensive to many people having occasion to pass it. From the upper surface of a leaf taken from the middle of the stem were counted 16 hemipterous and 1 coleopterous, and on the lower surface 112 dipterous and 5 coleopterous insects, insects with a few living aphides, in different stages of development. This is the number from one ordinary leaf, and it is very easily seen that a few of these plants in a garden must necessarily rid it of countless numbers of these, apparently, creatures of detestation, and render man an unseen service of good. Some of these thus caught were too minute to specify, and others comparatively rare; such were a male and female Halictus. Phyllobreta dilaticornis was very frequent; Ascogaster basalis quite numerous, and Haltica fuscula was very well represented. It was noticed that the Halictus, in its flight to the flowers, frequently came to its death on the glands beneath.
From these few facts we are safe at least in pronouncing Martynia an entomophilous plant, an insecticide, and in all probability insectivorous, worthy of considerable attention, more than I have had time to bestow, and from two partial afternoon associations with it.
The investigation of species was assisted by Prof. Ezra T. Cresson and Mr. Aaron, of the Academy at Shanuonville, Pa.