There are six native species of the cypripedium in this Eastern region, varying in shape and in color--shades of white, yellow, crimson, and pink. The mechanism of their cross-fertilization is the same in all, with only slight modifications.
The most common of the group, the _C. acaule_, most widely known as the moccasin-flower, whose large, nodding, pale crimson blooms we so irresistibly a.s.sociate with the cool hemlock woods, will afford a good ill.u.s.tration.
The lip in all the cypripediums is more or less sac-like and inflated.
In the present species, _C. acaule_, however, we see a unique variation, this portion of the flower being conspicuously bag-like, and cleft by a fissure down its entire anterior face. In Fig. 16 is shown a front view of the blossom, showing this fissure. The "column" (B) in the cypripedium is very distinctive, and from the front view is very non-committal. It is only as we see it in side section, or from beneath, that we fully comprehend the disposition of stigma and pollen. Upon the stalk of this column there appear from the front three lobes--two small ones at the sides, each of which hides an anther attached to its under face--the large terminal third lobe being in truth a barren rudiment of a former stamen, and which now overarches the stigma. The relative position of these parts may be seen in the under view.
[Ill.u.s.tration]
The anthers in this genus, then, are two, instead of the previous single anther with its two pollen-cells. The pollen is also quite different in its character, being here in the form of a pasty ma.s.s, whose entire exposed surface, as the anther opens, is coated with a very viscid gluten.
[Ill.u.s.tration: Fig. 16]
With the several figures ill.u.s.trating the cross-fertilization, the reader will readily antic.i.p.ate any description of the process, and only a brief commentary will be required in my text.
I have repeatedly examined the flowers of _C. acaule_ in their haunts, have observed groups wherein every flower still retained its pollen, others where one or both pollen ma.s.ses had been withdrawn, and in several instances a.s.sociated with them I have observed the inflated lip most outrageously bruised, torn, and battered, and occasionally perforated by a large hole. I had observed these facts in boyhood. The inference, of course, was that some insect had been guilty of the mutilation; but not until I read Darwin"s description of the cross-fertilization of this species did I realize the full significance of these telltale evidences of the escape of the imprisoned insect.
Since that time, many years ago, I have often sat long and patiently in the haunt of the cypripedium awaiting a natural demonstration of its cross-fertilization, but as yet no insect has rewarded my devotion.
[Ill.u.s.tration: Fig. 17]
At length, in hopelessness of reward by such means, I determined to see the process by more prosaic methods. Gathering a cl.u.s.ter of the freshly opened flowers, which still retained their pollen, I took them to my studio. I then captured a b.u.mblebee, and forcibly persuaded him to enact the demonstration which I had so long waited for him peaceably to fulfil. Taking him by the wings, I pushed him into the fissure by which he is naturally supposed to enter without persuasion. He was soon within the sac, and the inflexed wings of the margin had closed above him, as shown in section, Fig. 17. He is now enclosed in a luminous prison, and his buzzing protests are audible and his vehemence visible from the outside of the sac. Let us suppose that he at length has become reconciled to his condition, and has determined to rationally fulfil the ideal of his environment, as he may perhaps have already done voluntarily before. The buzzing ceases, and our bee is now finding sweet solace for his incarceration in the copious nectar which he finds secreted among the fringy hairs in the upper narrowed portion of the flower, as shown at Fig. 18 A. Having satiated his appet.i.te, he concludes to quit his close quarters. After a few moments of more vehement futile struggling and buzzing, he at length espies, through the pa.s.sage above the nectary fringe, a gleaming light, as from two windows (A). Towards these he now approaches. As he advances the pa.s.sage becomes narrower and narrower, until at length his back is brought against the overhanging stigma (Fig. 18 B). So narrow is the pa.s.s at this point that the efforts of the bee are distinctly manifest from the outside in the distension of the part and the consequent slight change in the droop of the lip. In another moment he has pa.s.sed this ordeal, and his head is seen protruding from the window-like opening (A) on one side of the column. But his struggles are not yet ended, for his egress is still slightly checked by the narrow dimensions of the opening, and also by the detention of the anther, which his thorax has now encountered. A strange etiquette this of the cypripedium, which speeds its parting guest with a sticky plaster smeared all over its back. As the insect works its way beneath the viscid contact, the anther is seen to be drawn outward upon its hinge, and its yellow contents are spread upon the insect"s back (Fig. 18 C), verily like a plaster. Catching our bee before he has a chance to escape with his generous floral compliments, we unceremoniously introduce him into another cypripedium blossom, to which, if he were more obliging, he would naturally fly. He loses no time in profiting by his past experience, and is quickly creeping the gantlet, as it were, or braving the needle"s eye of this narrow pa.s.sage.
His pollen-smeared thorax is soon crowding beneath the overhanging stigma again, whose forward-pointed papillae sc.r.a.pe off a portion of it (Fig. 18 B), thus insuring the cross-fertilizing of the flower, the bee receiving a fresh effusion of cypripedium compliments piled upon the first as he says "good-bye." It is doubtful whether in his natural life he ever fully effaces the telltale effects of this demonstrative _au revoir_.
[Ill.u.s.tration: Fig. 18 A]
Such, with slight modifications, is the plan evolved by the whole cypripedium tribe. Darwin mentions bees as the implied fertilizers, and doubtless many of the smaller bees do effect cross-fertilization in the smaller species. But the more ample pa.s.sage in acaule would suggest the medium-sized Bombus as better adapted--as the experiment herewith pictured from my own experience many times would seem to verify, while a honey-bee introduced into the flower failed to fulfil the demonstration, emerging at the little doorway above without a sign of the cordial parting token.
[Ill.u.s.tration: Fig. 18 B]
Occasionally I suppose a fool b.u.mblebee is entrapped within the petal bower and fails to find the proper exit, or it may be--much less a fool--having run the gantlet once too often, decides to escape the ordeal; hence the occasional mutilated blossom already described.
One of the most beautiful of our orchids, though its claims to admiration in this instance are chiefly confined to the foliage, is the common "Rattlesnake-Plantain," its prostrate rosettes of exquisitely white reticulated leaves carpeting many a nook in the shadows of the hemlocks, its dense spikes of yellowish-white blossoms signalling their welcome to the bees, and fully compensating in interest what they may lack in other attractive attributes.
[Ill.u.s.tration: Fig. 18 C]
The single flower is shown enlarged in Fig. 19--A, a young blossom, with a.n.a.lyses B and C, the latter indexed; D, an older blossom, with similar a.n.a.lyses (E and F). Both sorts are to be found upon every spike of bloom, as the inflorescence begins at the base and proceeds upward. As we look into the more open flower we observe a dark-colored speck, which, by a.n.a.lysis, proves to be the lid of the anther. This portion is further shown enlarged in Fig. 20, A. If we gently lift it with a pin, we disclose the pollen ma.s.ses in the cavity (B) thus opened (C, profile section), the two pairs united to a common viscid gland at the base, this gland again secreted behind a veil of moist membrane, as also shown at B. This membrane is, moreover, very sensitive to the touch. Below the flattened tip of the column, and at a sharp inward angle, is the stigma.
In the freshly opened flower (Fig. 19, A) the column inclines forward, bringing the anther low down, and its base directly opposite the V-shaped orifice in the lip, which also is quite firmly closed beneath the equally converging upper hood of the blossom. The entrance is thus much narrowed. If we insert a pin in this V-shaped entrance it comes in contact with the sensitive membrane below the anther, and it is immediately ruptured, as shown at Fig. 20, D. The sticky gland is brought into immediate contact, and clasps the pin, which, now being withdrawn, brings away the pollen, as in E and F. Thus it is naturally removed on the tongue of its sipping bee.
[Ill.u.s.tration: Fig. 19]
[Ill.u.s.tration: Fig. 21]
The further demonstration will be better shown by profile sections (Fig.
21). Nectar is secreted in the hollow of the lip indicated, somewhat as in the cypripedium. If we now imitate with a probe the habit of the insect and the action of its tongue, we may witness a beautiful contrivance for cross-fertilization. We will suppose the bee to be working at the top of the spike. He thrusts his tongue into the narrow opening (G). The membrane protecting the pollen-gland, thus surely touched, ruptures as described, and the exposed gland attaches itself to the tongue, being withdrawn as at H, and located on the insect"s tongue, as in F, Fig. 20. The bee leaves this flower cl.u.s.ter and flies to another, upon which it will usually begin operation at the bottom. The flower thus first encountered is an old bloom, as in Fig. 19, D. Its sepals are more spreading, the lip slightly lowered, and the column so changed as to present the plane of the stigma, before out of sight, in such a new position as to invariably receive the pollen. The tongue of a bee entering this flower conveys the pollen directly against the stigmatic surface (I), which retains its disentangled fecundating grains, as at J, and the flower"s functional adaptations are fulfilled.
[Ill.u.s.tration: Fig. 20]
[Ill.u.s.tration: A. Extended. B. Folded beneath the head.]
In the allied Spiranthes, or "Lady"s-Tresses," a somewhat similar mechanism prevails, by which fertilization is largely effected by the changed position or angle of the stigma plane.
And thus we might proceed through all the orchid genera, each new device, though based upon one of the foregoing plans, affording its new surprise in its special modification in adaptation to its insect sponsor--all these various shapes, folds of petals, positions, colors, the size, length, and thickness of nectary, the relative positions of pollen and stigma, embodying an expression of welcome to the insect with which its life is so marvellously linked. Occasionally this astounding affinity is faithful to a single species of insect, which thus becomes the sole sponsor of the blossom, without whose a.s.sociation the orchid would become extinct. A remarkable instance of this special adaptation is seen in the great Angraec.u.m orchid of Madagascar, described by Darwin; and inasmuch as this species glorifies Darwin"s faith in the truth of his theory, and marks a notable victory in the long battle for its supremacy, it affords an inspiring theme for my closing paragraphs.
Among the host of sceptics--and were they not legion?--who met this evolutionary and revolutionary theory with incredulity, not to say ridicule or worse, was one who thus challenged its author shortly after the appearance of his "Fertilization of Orchids," addressing Darwin from Madagascar substantially as follows: "Upon your theory of evolution through natural selection all the various contrasting structural features of the orchids have direct reference to some insect which shall best cross-fertilize them. If an orchid has a nectary one inch long, an insect"s tongue of equivalent length is implied; a nectary six inches in length likewise implies a tongue six inches long. What have you to say in regard to an orchid which flourishes here in Madagascar possessing a long nectary as slender as a knitting-needle and eleven inches in length? On your hypothesis there must be a moth with a tongue eleven inches long, or this nectary would never have been elaborated."
Darwin"s reply was magnificent in its proof of the sublime conviction of the truth of his belief: "The existence of an orchid with a slender nectary eleven inches in length, and with nectar secreted at its tip, is a conclusive demonstration of the existence of a moth with a tongue eleven inches in length, _even though no such moth is known_."
Many of us remember the ridicule which was heaped upon him for this apparently blind adherence to an untenable theory. But victory complete and demoralizing to his opponents awaited this oracular utterance when later a disciple of Darwin, led by the same spirit of faith and conviction, visited Madagascar, and was soon able to affirm that he had caught the moth, a huge sphinx-moth, and that its tongue measured eleven inches in length.
[Ill.u.s.tration]
Here we see the prophecy of the existence of an unknown moth, founded on the form of a blossom. At that time the moth had not been actually seen at work on the orchid, but who shall question for a moment that had the flower been visited in its twilight or moonlight haunt the murmur of humming wings about the blossom"s throat would have attested the presence of the flower"s affinity, for without the kiss of this identical moth the Angraec.u.m must become extinct. No other moth can fulfil the conditions necessary to its perpetuation. The floral adaptation is such that the moth must force its large head far into the opening of the blossom in order to reach the sweets in the long nectary.
In so doing the pollen becomes attached to the base of the tongue, and is withdrawn as the insect leaves the flower, and is thrust against the stigma in the next blossom visited. This was clearly demonstrated by Darwin in specimens sent to him, by means of a probe of the presumable length and diameter of the moth"s tongue. Shorter-tongued moths would fail to remove the pollen, and also to reach the nectar, and would thus soon learn to realize that they were not welcome.
The Angraec.u.m also affords in this long pendent nectary a most lucid ill.u.s.tration of the present workings of natural selection. The normal length of that nectary should be about eleven inches, but in fact this length varies considerably in the flowers of different plants, this tendency to variation in all organic life being an essential and amply demonstrated postulate of the entire theory of natural selection. Let us suppose a flower whose nectary chances to be only six inches in length.
The moth visits this flower, but the tip of its tongue reaches the nectar long before it can bring its head into the opening of the tube.
This being a vital condition, the moth fails to withdraw the pollen; and inasmuch as the pollen is usually deposited close to the head of the moth, this flower would _receive_ no pollen upon its stigma. This particular blossom would thus be both barren and sterile. None of its pollen would be carried to other stigmas, nor would it set a seed to perpetuate by inheritance its shorter nectary.
Again, let us suppose the variation of an extra long nectary, and the writer recently saw a number of these orchids with nectaries thirteen inches in length. The moth comes, and now must needs insert its head to the utmost into the opening of the flower. This would insure its fertilization by the pollen on the insect"s tongue; and even though the sipper _failed_ to reach the nectar, the pollen would be withdrawn upon the tongue, to be carried to other flowers, which might thus be expected to inherit from the paternal side the tendency to the _longer_ nectary.
The tendency towards the perpetuation of the short nectary is therefore stopped, while that of the longer nectary is insured.
_THE MILKWEED_
The singular hospitality of our milkweed blossom is nowhere matched among Flora"s minions, and would seem occasionally in need of supervision.
Just outside the door here at my country studio, almost in touch of its threshold, year after year there blooms a large clump of milkweed (_Asclepias cornuta_), and, what with the fragrance of its purple pompons and the murmurous music of its bees, its fortnight of bloom is not permitted to be forgotten for a moment. Only a moment ago a whiff of more than usual redolence from the open window at which I am sitting reminded me that the flowers were even now in the heyday of their prime, and the loud droning music betokened that the bees were making the most of their opportunities.
Yielding to the temptation, I was soon standing in the midst of the plants. The purple fragrant umbels of bloom hung close about me on all sides, each flower, with its five generous horns of plenty, drained over and over again by the eager sipping swarm.
But the July sun is one thing to a bee and quite another thing to me. I have lingered long enough, however, to witness again the beautiful reciprocity, and to realize anew, with awe and reverence, how divinely well the milkweed and the bee understand each other. After a brief search among the blossom cl.u.s.ters I return to my seclusion with a few interesting specimens, which may serve as a text here at my desk by the open window.
Two months hence an occasional silky messenger will float away from the glistening clouds about the open milkweed pods, but who ever thanks the bees of June for them? The flower is but a bright antic.i.p.ation--an expression of hope in the being of the parent plant. It has but one mission. All its fragrance, all its nectar, all its beauty of form and hue are but means towards the consummation of the eternal edict of creation--"Increase and multiply." To that end we owe all the infinite forms, designs, tints, decorations, perfumes, mechanisms, and other seemingly inexplicable attributes. Its threshold must bear its own peculiar welcome to its insect, or perhaps to its humming-bird friend, or counterpart; its nectaries must both tempt and reward his coming, and its petals a.s.sist his comfortable tarrying.
Next to the floral orchids, the mechanism of our milkweed blossom is perhaps the most complex and remarkable, and ill.u.s.trates as perfectly as any of the orchid examples given in Darwin"s n.o.ble work the absolute divine intention of the dependence of a plant species upon the visits of an insect.
Our milkweed flower is a deeply planned contrivance to insure such an end. It fills the air with enticing fragrance. Its nectaries are stored with sweets, and I fancy each opening bud keenly alert with conscious solicitude for its affinity. Though many other flowers manage imperfectly to perpetuate their kind in the default of insect intervention, the milkweed, like most of the orchids, is helpless and incapable of such resource. Inclose this budded umbel in tarlatan gauze and it will bloom days after its fellow-blooms have fallen, antic.i.p.ating its consummation, but no pods will be seen upon this cl.u.s.ter.
What a singular decree has Nature declared with reference to the milkweed! She says, in plainest terms, "Your pollen must be removed on the leg of an insect, preferably a bee, or your kind shall perish from the face of the earth." And what is the deep-laid plan by which this end is a.s.sured? My specimens here on the desk will disclose it all.
Here are two bees, a fly, and a beetle, each hanging dead by its legs from a flower, an extreme sacrificial penalty, which is singularly frequent, but which was certainly not exacted nor contemplated in the design of the flower. A careful search among almost any good-sized cl.u.s.ter of milkweeds will show us many such prisoners. As in all flowers, the pollen of the milkweed blossom must come in contact with its stigma before fruition is possible. In this peculiar family of plants, however, the pollen is distinct in character, and closely suggests the orchids in its consistency and disposition. The yellow powdery substance with which we are all familiar in ordinary flowers is here absent, the pollen being collected in two club-shaped or, more properly, spatula-shaped ma.s.ses, linked in pairs at their slender prolonged tips, each of which terminates in a sticky disc-shaped appendage united in V-shape below. These pollen ma.s.ses are concealed in pockets (B) around the cylindrical centre of the flower, the discs only being exposed at the surface, at five equidistant points around its rim, where they lie in wait for the first unwary foot that shall touch them. A glance at the two views of this central portion of the flower, as it appears through my magnifying-gla.s.s--the honey-horns and sepals having been removed--will, I think, indicate its peculiar anatomy or mechanism. No _stigma_ is to be seen in the flower, the stigmatic surface which is to receive the pollen being concealed within five compartments, each of which is protected by a raised tent-like covering, cleft along its entire apex by a fine fissure (A). _Outside of each of these, and entirely separated from the stigma in the cavity_, lie the pollen ma.s.ses within their pockets, each pair uniting at the rim below in V-shape, the union at the lower limit of the fissure.
[Ill.u.s.tration]
[Ill.u.s.tration]