Chapter 6
From this companionable point of view let us now look again at the strange curved stamen of the sage. Why this peculiar formation of the long curved arm pivoted on its stalk? Considered in the abstract, it can have no possible meaning; but taken in association with the insect to which it is shaped, how perfect is its adaptation, how instantly intelligible it becomes! Every one is familiar with the sage of the country garden, its lavender flowers arranged in whorls in a long cluster at the tips of the stems. One of these flowers, a young one from the top of the cluster, is shown at A (Fig. 4), in section, the long thread-like pistil starting from the ovary, and curving upward beneath the arch of the flower, with its forked stigma barely protruding (B). There are two of the queer stamens, one on each side of the opening of the blossom, and situated as shown, their anthers concealed in the hood above, and only their lower extremity appears below, the minute growth near it being one of the rudiments of two former stamens which have become aborted. If we take a flower from the lower portion of the cluster (D), we find that the thread-like pistil has been elongated nearly a third of an inch, its forked stigma now hanging directly at the threshold of the flower. The object of this will be clearly demonstrated if we closely observe this bee upon the blossoms. He has now reached the top of the cluster among the younger blossoms. He creeps up the outstretched platform of the flower, and has barely thrust his head within its tube when down comes the pair of clappers on his back (C). Presently he backs out, bearing a generous dab of yellow pollen, which is further increased from each subsequent flower. He has now finished this cluster, and flies to the next, alighting as usual on the lowermost tier of bloom. In them the elongated stigma now hangs directly in his path, and comes in contact with the pollen on his back as the insect sips the nectar. Cross-fertilization is thus insured; and, moreover, cross-fertilization not only from a distinct flower, but from a separate cluster, or even a separate plant. For in these older stigmatic flowers the anther as it comes down upon his back is seen to be withered, having shed its pollen several days since, the supply of pollen on the bee's body being sufficient to fertilize all the stigmas in the cluster, until a new supply is obtained from the pollen-bearing blossoms above. And thus he continues his rounds.
The sage is a representative of the large botanical order known as the Mint family, the labiates, or gaping two-lipped flowers, the arched hood here answering to the upper lip, the spreading base forming the lower lip, which is usually designed as a convenient threshold for the insects while sipping the nectar deep within the tube. This mechanism of the sage is but one of many curious and various contrivances in the Mint family, all designed for the same end, the intercrossing of the flowers.
While each family of plants is apt to favor some particular general plan, the modifications in the various species seem almost without limit.
Let us now look at the Heath family. The family of the heath, cranberry, pyrola, Andromeda, and mountain-laurel--how do these blossoms welcome their insect friends? This group is particularly distinguished by the unusual exception in the form of its anthers, which open by pores at their tips, instead of the ordinary side fissures. Two or three forms of these anthers are shown in my row of stamens (Fig. 1).
Seen thus in their detached condition, how incomprehensible and grotesque do they appear! And yet, when viewed at home, in their bell-shaped corollas, their hospitable expression and greeting are seen to be quite as expressive and rational as those of the sage. Take the mountain-laurel, for instance; what a singular exhibition is this which we may observe on any twilight evening in the laurel copse, the dense clusters of pink-white bloom waited upon by soft-winged fluttering moths, and ever and anon celebrating its cordial spirit by a mimic display of pyrotechnics as the anthers hurl aloft their tiny showers of pollen!
Every one is familiar with the curious construction of this flower, with its ten radiating stamens, each with its anther snugly tucked away in a pouch at the rim of its saucer-shaped corolla. Thus they appear in the freshly opened flower, and thus will they remain and wither if the flower is brought indoors and placed in a vase upon our mantel. Why? Because the hope of the blossom's life is not fulfilled in these artificial conditions; its natural counterpart, the insect, has failed to respond to its summons.
But the twilight cluster in the woods may tell us a pretty story.
Here a tiny moth hovers above the tempting chalice, and now settles upon it with eager tongue extended for the nectar at its centre. What an immediate and expressive welcome! No sooner has this little feathery body touched the filaments than the eager anthers are released from their pockets, and, springing inwards, clasp their little visitor, at the same time decorating him with their compliments of webby pollen (A, Fig. 5).
The nectary now drained of its sweets, the moth creeps or flutters to a second blossom, and its pollen-dusted body thus coming in contact with its stigma, cross-fertilization is accomplished. The pollen of the laurel differs from that of most of the Heath blooms, its grains being more or less adherent by a cobwebby connective which permeates the mass as indicated in my magnified representation (B, Fig. 5).
It is probable that an accessory cross-fertilization frequently results from a mass of the pollen falling directly upon the stigma of a neighboring blossom, or even upon its own stigma, but even in the latter case, as has been absolutely demonstrated as a general law by the experiments of Darwin, the pollen from a separate flower is almost invariably prepotent, and leads to the most perfect fruition, and thus to the survival of the fittest--the cross-fertilized. And, in any event, the insect is to be credited for the release of the tiny catapults by which the pollen is discharged. But the laurel may be considered as an exceptional example of the Heath family. Let us look at a more perfect type of the order to which it belongs, the globular blossom of the Andromeda (_A. ligustrina_).
Only a short walk from my studio door in the country I recently observed its singular reception to the tiny black-and-white banded bee, which seems to be its especial companion, none the less constant and forgiving in spite of a hospitality which, from the human stand-point, would certainly seem rather discouraging. Fancy a morning call upon your particular friend. You knock at the door, and are immediately greeted at the threshold with a quart of sulphur thrown into your face. Yet this is precisely the experience of this patient little insect, which manifests no disposition to retaliate with the concealed weapon which on much less provocation he is quick to employ. Here he comes, eager for the fray. He alights upon one of the tiny bells scarce half the size of his body. Creeping down beneath it, he inserts his tongue into the narrowed opening. Instantly a copious shower of dust is poured down upon his face and body. But he has been used to it all his life, and by heredity he knows that this is Andromeda's peculiar whim, and is content to humor it for the sweet recompense which she bestows. The nectar drained, the insect, as dusty as a miller, visits another flower, but before he enters must of necessity first pay his toll of pollen to the drooping stigma which barely protrudes beneath the blossom's throat, and the expectant seed-pod above welcomes the good tidings with visions of fruition.
And how beautiful is the minute mechanical adaptation by which this end is accomplished! This species of Andromeda is a shrub of about four feet in height, its blossoms being borne in close panicled clusters at the summit of the branches. The individual flower is hardly more than an eighth of an inch in diameter. From one of three blossoms I made the accompanying series of three sectional drawings (Fig. 6). The first shows the remarkable interior arrangement of the ten stamens surrounding the pistil. The second presents a sectional view of these stamens, showing their peculiar S-shaped filaments and ring of anthers--one of the latter being shown separate at the right, with its two pores and exposed pollen. The freshly opened blossom discloses the entire ring of anthers in perfect equilibrium, each with its two orifices closed by close contact with the style, thus retaining the pollen. It will readily be seen that an insect's tongue, as indicated by the needle, in probing between them in search for nectar, must needs dislocate one or more of the anthers, and thus release their dusty contents, while the position of the stigma below is such as to escape all contact.
In most flowers, with the exception of the orchids, the stamens and pollen are plainly visible; but who ever sees the anthers of the blue-flag? Surely none but the analytical botanist and the companion insect to whom it is so artfully adjusted and so demonstrative. This insect is likely to be either a bumblebee or a species of large fly. In apt illustration of Sprengel's theory of the "path-finder" or honey-guide, the insect does not alight at the centre of the flower, but upon one of the three large drooping sepals, whose veins, converging to the narrow trough above, indicate the path to the nectar. Closely overarching this portion is a long and narrow curved roof--one of three divisions to the style, each surmounting its veined sepals. Beneath this our visiting bee disappears, and a glance at my sectional drawing shows what happens. Concealed within, against the ridge-pole, as it were, the anther awaits his coming, and in his passage to and from the nectar below spreads its pollen over his head and back. Having backed out of this segment of the blossom (A, Fig. 7), he proceeds to the next; but the shelf-like stigma awaits him at the door, and scrapes off or rubs off a few grains of the pollen from his back (B). Thus he continues until the third segment is reached, from which he carries away a fresh load of pollen to another flower. It will be seen that only the outer side of this appendage is stigmatic, and that it is thus naturally impossible for the blue-flag to self-fertilize--only one instance of thousands in which the anther and stigma, though placed in the closest proximity, and apparently even in contact--seemingly with the _design_ of self-fertilization--are actually more perfectly separated functionally than if in separate flowers, the insect alone consummating their affinity.
In some flowers this separation is effected, as I have shown, by their maturing at different periods; in others, as in the iris, by mere mechanical means; while in a long list of plants, as in the willow, poplar, hemp, oak, and nettle, the cross-fertilization is absolutely necessitated by the fact of the staminate and stigmatic flowers being either separated on the same stalk or on different plants, the pollen being carried by insects or the wind. We may see a pretty illustration of this in the little wild flower known as the devil's-bit (_Chamælirium luteum,_), whose long, white, tapering spire of feathery bloom may often be seen rising above the sedges in the swamp. Two years ago I chanced upon a little colony of four or five plants at the edge of a bog. The flowers, all of them, were mere petals and stamens (B, Fig. 8). I looked in vain for a single stigmatic plant or flower; but far across the swamp, a thousand feet distant, I at length discovered a single spire, composed entirely of pistillate flowers, as shown in A (Fig. 8), and my magnifying-glass clearly revealed the pollen upon their stigmas--doubtless a welcome message brought from the isolated affinity afar by some winged sponsor, to whom the peculiar fragrance of the flower offers a special attraction, and thus to whom the fortunes of the devil's-bit have been committed.
The presence of fragrance and honey in a dioecious flower may be accepted in the abstract as almost conclusive of an insect affinity, as in most flowers of this class, notably the beech, pine, dock, grasses, etc., the wind is the fertilizing agent, and there is absence alike of conspicuous color, fragrance, and nectar--attributes which refer alone to insects, or possibly humming-birds in certain species.
Look where we will among the blossoms, we find the same beautiful plan of intercommunion and reciprocity everywhere demonstrated. The means appear without limit in their evolved--rather, I should say, involved--ingenuity. Pluck the first flower that you meet in your stroll to-morrow, and it will tell you a new story.
Only a few days since, while out on a drive, I passed a luxuriant clump of the plant known as "horse-balm." I had known it all my life, and twenty years previously had made a careful analytical drawing of the mere botanical specimen. What could it say to me now in my more questioning mood? Its queer little yellow-fringed flowers hung in profusion from their spreading terminal racemes. I recalled their singular shape, and the two outstretched stamens protruding from their gaping corolla, and could distinctly see them as I sat in the carriage. I had never chanced to read of this flower in the literature of cross-fertilization, and murmuring, half aloud, "What pretty mystery is yours, my Collinsonia?" prepared to investigate.
What I observed is pictured severally at Fig. 9, the flowers being shown from above, showing the two spreading stamens and the decidedly exceptional unsymmetrical position of the long style extending to the side. A small nectar-seeking bumblebee had approached, and in alighting upon the fringed platform grasped the filaments for support, and thus clapped the pollen against his sides. Reasoning from analogy, it would of course be absolutely clear that this pollen has thus been deposited where it will come in contact with the stigma of another flower. So, of course, it proved. In the bee's continual visits to the several flowers he came at length to the younger blooms, where the forked stigmas were turned directly to the front, while the immature stamens were still curled up in the flower tubes. Even the unopened buds showed a number of species where the early matured stigma actually protruded through a tiny orifice in precisely the right position to strike the pollen-dusted body of the bee, as he forced his tongue through the tiny aperture.[A]
[Footnote A: In numerous instances observed since the above was written I have noted the larger bumblebees upon the blossom. These insects have a different method of approach, hanging beneath the flower, the anthers being clapped against their thorax at the juncture of the wings, instead of the abdomen, as in the smaller bee.]
If their dainty mechanism excite our wonder, what shall be said of the revelations in the great order of the Compositæ, where each so-called flower, as in the dandelion, daisy, cone-flower, marigold, is really a dense cluster of minute flowers, each as perfect in its construction as in the examples already mentioned, each with its own peculiar plan designed to insure the transfer of its own pollen to the stigma of its neighbor, while excluding it from its own?
All summer long the cone-flower, Fig. 10 (_Rudbeckia hirta_), blooms in our fields, but how few of us imagine the strange processes which are being enacted in that purple cone! Let us examine it closely. If we pluck one of the blossom's heads and keep it in a vase over-night, we shall probably see on the following morning a tiny yellow ring of pollen encircling the outer edge of the cone. In this way only are we likely to see the ring in its perfection, as in a state of nature the wind and insects rarely permit it to remain.
If we now with a sharp knife make a vertical section, as shown at A (Fig. 3), we may observe the conical receptacle studded with its embryo seeds, each bearing a tiny tubular blossom. Three distinct forms of these flowers are to be seen. The lower and older ones are conspicuous by their double feathery tails, the next by their extended anthers bearing the pollen at their extremity, and above these again the buds in all stages of growth. These various states are indicated in Fig. 11.
As in all the Compositæ, the anthers are here united in a tube, the pollen being discharged within. At the base of this anther-tube rises the pistil, which gradually elongates, and like a piston forces out the pollen at the top. Small insects in creeping over the cone quickly dislodge it. In the next stage the anthers have withered, the flower-tube elongated, and the top of the two-parted pistil begins to protrude, and at length expands its tips, disclosing at the centre the stigmatic surface, which has until now been protected by close contact. (See section.)
A glance at Fig. 11 will reveal the plan involved. The ring of pollen is inevitably scattered to the stigmas of the neighboring flowers, and cross-fertilization continually insured. Similar contrivances are to be found in most of the Compositæ, through the same method being variously applied.
Perhaps even more remarkable than any of the foregoing, which are more or less automatic in their movements, is the truly astonishing and seemingly conscious mechanism displayed in the wild arum of Great Britain--the "lords and ladies" of the village lanes, the foreign counterpart of our well-known jack-in-the-pulpit, or Indian-turnip, with its purple-streaked canopy, and sleek "preacher" standing erect beneath it. A representation of this arum is shown in Fig. 12, and a cross section at A, properly indexed.
How confidently would the superficial--nay, even careful--examination of one of the old-time botanists have interpreted its structure: "How simple and perfect the structure! Observe how the anthers are placed so that pollen shall naturally fall directly on the stigmas and fertilize them!" Such would indeed appear to be intended, until it is actually discovered that the _stigmas have withered_ when the pollen is shed--a device which, acting in association with the little ring of hairs, tells a strange story. It is not my fortune to have seen one of these singular blossoms, but from the description of the process of fertilization given in Hermann Müller's wonderful work, aided by a botanical illustration of the structure of the flower, I am readily enabled to picture the progressive stages of the mechanism.
In the first stage (B, Fig. 13) small flies with bodies dusted with pollen from a previous arum blossom (for insects, as a rule, remain faithful or partial to one species of flowers while it is in bloom) are entering the narrowed tube, easily passing through the drooping fringe of hairs. Nectar is secreted by the stigmas, and here the flies assemble, thus dusting them with pollen. Their appetite temporarily satisfied, the insects seek escape, but find their exit effectually barred by the intruding fringe of hairs (C). In this second stage the stigmas, having now been fertilized, have withered, at the same time exuding a fresh supply of nectar, which again attracts the flies, whereupon, as shown at D, the anthers open and discharge their pollen upon the insects. In the fourth stage (E), all the functions of the flower having now been fulfilled, the fringe of hairs withers, and the imprisoned pollen-laden flies are permitted to escape to another flower, where the beautiful scheme is again enacted.
In a paper of this kind it is of course possible only to hint at a few representative examples of floral mechanisms, but these would be indeed incomplete without a closing reference to that wonderful tribe of flowers with which the theory of cross-fertilization will ever be memorably associated. I have previously alluded to the absolute dependence of the red clover upon the bumblebee. This instance may be considered somewhat exceptional, though numerous parallel cases are known. Among ordinary flowers this intervention of the insect is largely a _preferable_ intention, and though almost invariably fulfilled, a large proportion of flowers still retain, as a _dernier ressort_, the power of at least partial self-fertilization and perpetuity in the absence or neglect of their insect counterpart.
The numerous and conclusive demonstrations of Darwin, however, have proved that in the competition for existence such self-fertilized offspring quickly yield before the progeny of cross-fertilization.
But the distinctive feature of the orchids lies in the fact that this dependence on the insect is wellnigh universally absolute. Here are a great host of plants which are doomed to extinction if for any reason their insect sponsors should permanently neglect them. The principal botanical feature which differentiates the orchid from other plants lies in the construction of the floral organs, the pistil, stigma, and anthers here being united into a distinct part known as the column. The pollen is, moreover, peculiar, being collected into more or less compact masses, and variously concealed in the flower. Some of these are club-shaped, with a viscid extremity, others of the consistency of a sticking-plaster, and all are hidden from external view in pouches and pockets, from which they never emerge unless withdrawn on the body of an insect. The various devices by which this removal is insured are most astonishing and awe-inspiring. Nor is it necessary to go to the conservatory for a tropical specimen, as is commonly supposed. An orchid is an orchid wherever it grows, and our native list of some fifty species will afford examples of as strange mechanical adaptations as are to be found among Darwin's pages. Indeed, a few of our American species are there described. One example will suffice for present illustration--the sweet-pogonia or grass-pink of our sedgy swamps (_Pogonia ophioglossoides_). Its solitary rosy blossom, nodding on its slender stem above the sedges, is always a welcome episode to the sauntering botanist, and its perfume, suggesting ripe red raspberries, is unique in the wild bouquet. One of these flowers is shown in profile at Fig. 14, its various parts indexed. Concealed behind the petals is the column, elsewhere indicated from various points of view. Attracted by its color and fragrance, the insect seeks the flower; its outstretched fringy lip offers a cordial invitation at its threshold, and conducts its visitor directly to the sweets above. In his entrance, as seen at D (Fig. 15), the narrowed passage compresses his back against the underside of the column, forcing his head and back against the stigma. The effect of this inward pressure, as will be seen, only serves to force the anther more firmly within its pocket; but as the insect, having drained the nectar, now backs out, note the result. The lip of the anther catches upon the back, swings outward on its hinge, and deposits its sticky pollen all over the insect's back, returning to its original position after his departure. In another moment he is seen upon another blossom, as at D again, his pollen-laden back now coming in contact with the stigma, and the intention of the blossom is accomplished; for without this assistance from the insect the little lid remains close within its pocket, and the pollen is thus retained.