Part 13

This radical assumption of fertilization in the individual flower, which lay at the base of Sprengel's theory, thus so completely exposed as false, discredited his entire work. The good was condemned with the bad, and the noble volume was lost in comparative oblivion--only to be finally resurrected and its full value and significance revealed by the keen scientific insight of Darwin (1859). From the new stand-point of evolution through natural selection the facts in Sprengel's work took on a most important significance. Darwin now reaffirmed the Sprengel theory so far as the necessity of the insect was concerned, but showed that all those perplexing floral conditions which had disproved Sprengel's assumption, instead of having for their object the conveying of pollen to the stigma of the same flower, implied its transfer to the stigma of another, cross-fertilization being the evident design, or evolved and perpetuated advantage.

This solution was made logical and tenable only on the assumption that such evolved conditions, insuring cross-fertilization, were of distinct advantage to the flower in the competitive struggle for existence, and that all cross-fertilized flowers were thus the final result of natural selection.

The early ancestors of this flower were self-fertilized; a chance seedling at length, among other continual variations, showed the singular variation of ripening its stigma in advance of its pollen--or other condition insuring cross-fertilization--thus acquiring a strain of fresh vigor. The seedlings of this flower, coming now into competition with the existing weaker self-fertilized forms, by the increased vigor won in the struggle of their immediate surroundings, and inheriting the peculiarity of their parent, showed flowers possessing the same cross-fertilizing device. The seeds from these, again scattering, continued the unequal struggle in a larger and larger field and in increasing numbers, continually crowding out all their less vigorous competitors of the same species, at length to become entire masters of the field and the only representatives left to perpetuate the line of descent.

Thus we find in almost every flower we meet some astonishing development by which this cross-fertilization is effected, by which the transferrence of the pollen from one flower to the stigma of another is assured, largely through the agency of insects, frequently by the wind and water, occasionally by birds. In many cases this is assured by the pollen-bearing flowers and stigmatic flowers being entirely distinct, as in cucumbers and Indian-corn; perhaps on different plants, as in the palms and willows; again by the pollen maturing and disseminating before the stigma is mature, as already mentioned, and vice versa.

From these, the simplest forms, we pass on to more and more complicated conditions, anomalies of form and structure--devices, mechanisms, that are past belief did we not observe them in actuality with our own eyes, as well as the absolutely convincing demonstration of the intention embodied: exploding flowers, shooting flowers, flower-traps, stamen embraces, pollen showers, pollen plasters, pollen necklaces, and floral pyrotechnics--all demonstrations in the floral etiquette of welcome and au revoir to insects.

From the simplest and regular types of flowers, as in the buttercup, we pass on to more and more involved and unsymmetrical forms, as the columbine, monk's-hood, larkspur, aristolochia, and thus finally to the most highly specialized or involved forms of all, as seen in the orchid--the multifarious, multiversant orchid; the beautiful orchid; the ugly orchid; the fragrant orchid; the fetid orchid; the graceful, homely, grotesque, uncanny, mimetic, and, until the year 1859, the absolutely non-committal and inexplicable flower; the blossom which had waited through the ages for Darwin, its chosen interpreter, ere she yielded her secret to humanity.

And what is an orchid? How are we to know that this blossom which we plucked is an orchid? The average reader will exclaim, "Because it is an air-plant"--the essential requisite, it would seem, in the popular mind. Of over 3000 known species of orchids, it is true a great majority are air-plants, or epiphytes--growing upon trees and other plants, obtaining their sustenance from the air, and not truly parasitic; but of the fifty-odd native species of the northeastern United States, not one is of this character, all growing in the ground, like other plants. It is only by the botanical structure of the flowers that the orchid may be readily distinguished, the epiphytic character being of little significance botanically.

A brief glance at this structural peculiarity may properly precede our more elaborate consideration of a few species of these remarkable flowers.

The orchids are usually very irregular, and six-parted. The ovary is one-celled, and becomes a pod containing an enormous yield of minute, almost spore-like, seeds (Fig. 3) in some species, as in the vanilla pod, to the number of a million, and in one species of the maxillaria, as has been carefully computed, 1,750,000.

The pollen, unlike ordinary flowers, is gathered together in waxy masses of varying consistency, variously formed and disposed in the blossom, its grains being connected with elastic cobwebby threads, which occasionally permit the entire mass to be stretched to four or five times its length, and recover its original shape when released. This is noticeable specially in the O. spectabilis, later described. The grains thus united are readily disentangled from their mass when brought into contact with a viscid object, as, for instance, the stigma.

But the most significant botanical contrast and distinction is found in the union of the style and stamens in one organ, called the column (Fig. 2), the stigma and the pollen being thus disposed upon a single common stalk. The contrast to the ordinary flower will be readily appreciated by comparison of the accompanying diagrams (Fig. 1).

When, therefore, we find a blossom with the anthers or pollen receptacle united to a stalk upon which the stigma is also placed, we have an orchid.

The order is further remarkable, as Darwin first demonstrated in his wonderful volume "The Fertilization of Orchids," in that the entire group, with very few exceptions, are absolutely dependent upon insects for their perpetuation through seed. They possess no possible resource for self-fertilization in the neglect of these insect sponsors.

Many of our common wild flowers, as perfectly and effectually planned for cross-fertilization as the orchids, do retain the reserve power of final self-fertilization if unfertilized by foreign pollen.

But the orchid has lost such power, and in the progress of evolution has gradually adapted itself to the insect, often to a particular species of insect, its sole sponsor, which natural selection has again gradually modified in relation to the flower.

The above work by Darwin was mostly concerned with foreign species, generally under artificial cultivation, and so startling were the disclosures concerning these hitherto sphinx-like floral beings that a most extensive bibliography soon attested the widespread inspiration and interest awakened by its pages.

But it is by no means necessary to visit the tropics or the conservatory for examples of these wonders. Our own Asa Gray, one of Darwin's instant proselytes, was prompt to demonstrate that the commonest of our native American species might afford revelations quite as astonishing as those exotic species which Darwin had described.

During a period of many years the writer has devoted much study to our native species of orchids from this evolutionary stand-point of their cross-fertilization tendencies. Of the following examples, selected from his list, some are elaborations of previous descriptions of Gray and others, though pictorially and descriptively the result of direct original study from nature; others are from actual observation of the insects at work on the flowers; and others still, original demonstrations based upon analogy and the obvious intention of the floral construction, the action of the insect--its head or tongue--having been artificially imitated by pins, bristles, or other probe-like bodies.

How many an enthusiastic flower-hunter has plucked his fragrant bouquet of the beautiful Arethusa, in its sedgy haunt, without a suspicion of the beautiful secret which lay beneath its singular form! Indeed, how many a learned botanist, long perfectly familiar with its peculiarities of shape and structure, has been entirely content with this simple fact, nor cared to seek further for its interpretation! But

"All may have the flower now, For all have got the seed."

With Darwin as our guide and the insect as our key--an open sesame--the hidden treasure is revealed. It is now quite possible, as Darwin demonstrated, to look upon a flower for the first time and from its structure foretell the method of its intended cross-fertilization; nay, more, possibly the kind, or even the species, of insect to which this cross-fertilization is intrusted.

Let us look at our Arethusa. The writer has never happened to observe an insect at work upon this flower, but the intention of its structure is so plain that by a mere examination we may safely prophesy not only what must happen when the insect seeks its nectar, but with equal assurance the kind of insect thus invited and expected. I have indicated a group of the orchids in their usual marshy haunt, and in Fig. 4, separately, a series of diagrams presents sections of the flower, natural size and duly indexed, which renders detailed description hardly necessary. The column is here quite elongated, forked at the tip, the space between the forks occupied by the anther, which is hinged to the upper division. This anther lid is closed tightly, with the sticky mass of pollen hidden behind it in the cavity. The stigma is on the external inner side of the lower division, and thus distinctly separated from the pollen. The "lip" is extended forward as a hospitable threshold to the insect. And to what insect might we assume this invitation of color, fragrance, nectar, and threshold to be extended?

Let us consider the flower simply as a device to insure its own cross-fertilization. The insect is welcomed; it must alight and sip the nectar; in departing it must bear away this pollen upon its body, and convey it to the next Arethusa blossom which it visits, and leave it upon its stigma. These are the conditions expressed; and how admirably they are fulfilled we may observe when we examine flower after flower of a group, and find their nectaries drained, their anther cells empty, and pollen upon all their stigmas. The nectar is here secreted in a well--not very deep--and the depth of this nectar from the entrance is of great significance among all the flowers, having distinct reference to the length of the tongue which is expected to sip it. In the Arethusa, it is true, the butterfly or moth might sip at the throat of the flower, but the long tongues of these insects might permit the nectary to be drained without bringing their bodies in contact with the stigma. Smaller insects might creep into the nectary and sip without the intended fulfilment. It is clear that to neither of such visitors is the welcome extended. What, then, are the conditions embodied? The insect must have a tongue of such a length that, when in the act of sipping, its head must pass beyond the anther well into the opening of the flower. Its body must be sufficiently large to come in contact with the anther. Such requisites are perfectly fulfilled by the humblebee, and we may well hazard the prophecy that the Bombus is the welcomed affinity of the flower.

The diagrams (Fig. 4) sufficiently illustrate the efficacy of the beautiful plan involved. At A the bee is seen sipping the nectar. His forward movement thus far to this point has only seemed to press the edge of the anther inward, and thus keep it even more effectually closed. As the bee retires (B), the backward motion opens the lid, and the sticky pollen is thus brought against the insect's back, where it adheres in a solid mass. He now flies to the next Arethusa blossom, enters it as before, and in retiring slides his back against the receptive viscid stigma, which retains a portion of the pollen, and thus effects the cross-fertilization (C). Professor Gray surmised that the pollen was withdrawn on the insect's head, and it might be so withdrawn, but in other allied orchids of the tribe Arethusae, however, in which the structure is very similar, the pollen is deposited on the thorax, and such is probably the fact in this species. In either case cross-fertilization would be effected. Nothing else is possible in the flower, and whether it is Bombus or not that effects it, the method is sufficiently evident.

Having thus had one initiation into this most enticing realm of riddles, each successive orchid whose structure we examine from this stand-point becomes a most interesting, perhaps a fresh, problem, whose assumed solution may often be verified by studying the insect in its haunts. Darwin thus foretold the precise manner of the cross-fertilization of Habenaria mascula, and also the insect agent, simply by the structural prophecy of the flower itself.

Suppose, for example, an unknown orchid blossom to be placed in our hands. Its nectary tube is five inches in length, and as slender as a knitting-needle. The nectar is secreted far within its lip. The evolution of the long nectary implies an adaptation to an insect's tongue of equal length. What insect has a tongue five inches long, and sufficiently slender to probe this nectary? The sphinx-moth only. Hence we infer the sphinx-moth to be the insect complement to the blossom, and we may correctly infer, moreover, that the flower is thus a night-bloomer. Examination of the flower, with the form of this moth in mind, will show other adaptations to the insect's form in the position of pollen and stigma, looking to the flower's cross-fertilization. In some cases this is effected by the aid of the insect's tongue; in others, by its eyes.

In our own native orchids we have a remarkable example of the latter form in the Habenaria orbiculata, whose structure and mechanism have also been admirably described by Asa Gray.

All orchid-hunters know this most exceptional example of our local flora, and the thrill of delight experienced when one first encounters it in the mountain wilderness, its typical haunt, is an event to date from--its two great, glistening, fluted leaves, sometimes as large as a dinner-plate, spreading flat upon the mould, and surmounted by the slender leafless stalk, with its terminal loose raceme of greenish-white bloom.