This section is from the book "The Botanical Magazine; Or, Flower-Garden Displayed", by William Curtis. Also available from Amazon: The Botanical Magazine; or, Flower-Garden Displayed, Volume I.
Apocynum Androsaemifolium. Tutsan-LeavED, Or Fly-Catching Dogsbane
Cor. campanulata. Glandulae 5 cum staminibus alternae.
APOCYNUM androsaemifolium caule rectiuseulo herbaceo, foliis ovatis utrinque glabris, cymis terminalibus. Linn. Syst. Vegetab. ed. 14. Murr. p. 258. Ait. Kew. v. 1. p. 303.
APOCYNUM canadense; foliis androsaemi majoris. Bocc. sicc. 35. t. 16. f. 3. Moris. Hist. 3. p. 609. s. 15. t. 3. f. 16.
In addition to the powerful recommendations of beauty and fragrance, the Tutsan-leaved Dogsbane interests us on account of the curious structure of its flowers, and their singular property of catching flies.
This species is a native of different parts of North-America; Mr. W. Hale, of Alton, Hants, who resided at Halifax in Nova-Scotia several years, brought me some seeds of it gathered in that neighbourhood, which vegetated, and produced flowering plants: it is not new to this country, being known to Morison who figures it, and to Miller, who cultivated it in 1731.
It is a hardy perennial plant, growing to about the height of a foot and a half, or two feet, and flowering from the beginning of July, to September; it has a creeping root, thereby it increases greatly in light dry soils, and warm situations, so as even to be troublesome; it will not thrive in a wet soil; with us it produces seed-vessels but rarely; is propagated by parting its roots in Autumn or Spring; Miller recommends March as the most proper season, or it may be raised from seeds, which in certain situations and seasons ripen here.
The flowers of this Apocynum have a sweet honey-like fragrance, which perfumes the air to a considerable distance, and no doubt operates powerfully in attracting insects; when a plant of this sort is fully blown, one may always find flies caught in its blossoms, usually by the trunk, very rarely by the leg; sometimes four, or even five, which is the greatest possible number, are found in one flower, some dead, others endeavouring to disentangle themselves, in which they are now and then so fortunate as to succeed; these flies are of different species, the musca pipiens, a slender variegated fly with thick thighs, is a very common victim, the musca domestica, or house fly, we have never observed among the captives.
Previous to our explaining the manner in which it appears to us that these insects are caught, it will be necessary that we should describe, in as plain a manner as possible, those parts of the flower which more particularly constitute this fatal fly trap.
On looking into the flower we perceive five Stamina, the Antherae of which are large, of a yellow colour, and converge into a kind of cone; each of these Antherae is arrow-shaped, towards the top of the cone their sides touch but do not adhere, below they separate a little, so as to leave a very narrow opening or slit between each, they are placed on very short filaments, which stand so far apart that a considerable opening is left between them, which openings, however, are closed up by processes of the corolla, nicely adapted to, and projecting into them; at the bottom of, and in the very centre of the flower, we perceive two germina, or seed-buds, the rudiments of future seed-vessels, surrounded by glandular substances, secreting a sweet liquid; on the summit of these germina, and betwixt the two, stands the stigma, in the form of a little urn, the middle of which is encircled by a glandular ring, which secretes a viscid honey-like substance, to this part of the stigma the Antherae interiorly adhere most tenaciously, so as to prevent their separation unless considerable force be applied; it is, as we apprehend, the sweet viscid substance thus secreted by the stigma, within the Antherae, which the fly endeavours to obtain, and to this end insinuates its trunk first into the lowermost and widest part of the slit, betwixt each of the Antherae above described, pushing it of necessity upwards: when gratified, not having the sense to place itself in the same position as that in which it stood when it inserted its trunk, and to draw it out in the same direction downwards, unfortunately for it, it varies its position, and pulling its trunk upwards, draws it into the narrow part of the slit, where it becomes closely wedged in, and the more it pulls the more securely it is caught, and thus this heedless insect, as Thomson calls it, terminates its existence in captivity most miserable.
In the incomparable poem of Dr. Darwin, entitled the Botanic Garden, there is a figure given of this plant; and in the Supplement we have the following account written by Mr. Darwin, of Elston.
"In the Apocynum Androsaemifolium the Anthers converge over the nectaries, which consist of five glandular oval corpuscles, surrounding the germ, and at the same time admit air to the nectaries at the interstice between each anther; but when a fly inserts its proboscis between these anthers to plunder the honey, they converge closer, and with such violence as to detain the fly, which thus generally perishes."
This explanation of a phaenomenon entitled to much attention, is widely different from ours; which of the two is most consonant to truth and nature, we shall leave to the determination of future observers.
In explaining the preceding appearances, to prevent confusion we called those parts which form the cone in the middle of the flower Antherae, but strictly speaking they are not such, the true Antherae being situated on the inside of their summits, where they will be found to be ten in number, making in fact the Apocynum a decandrous plant.