[Illustration: Fig. 7]
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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.
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[Illustration: Fig. 8]
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.
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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.
[Illustration: Fig. 9]
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.]
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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.
[Illustration: Fig. 10]
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.)
[Illustration: Fig. 11]
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.