Diagram C & D
But now Sprengel himself was met with most discouraging opposition to his theory, showing that he had guessed but half the secret after all. Flowers by the hundreds were brought to his notice, like that shown in Diagram C, in which the insect could not transfer the pollen from anther to stigma, as the stigma is closed when the pollen is ripe, and like that in Diagram D, which does not open until the pollen is shed. For seventy years this astonishing fact puzzled the world, and was at last solved by the great Darwin, who showed that nearly all flowers shun their own pollen, and are so constructed, by thousands of singular devices, that the insect shall bring to each the pollen of another flower of the same species, and thus effect what is known as cross-fertilization.
We must then look at all flowers as expressions of welcome to some insect—day-flowering blossoms mostly to bees and butterflies, and night-bloomers to moths. And not only expressions of welcome, but each with some perfect little plan of its own to make this insect guest the bearer of its pollen to the stigma of another flower of the same species. And how endless are the plans and devices to insure this beautiful scheme! Some flowers make it certain by keeping the stigma closed tight until all its pollen is shed; others place the anther so far away from the stigma as to make pollen contact impossible; others actually imprison these pollen-bringing insects until they can send them away with fresh pollen all over their bodies.
Take almost any flower we chance to meet, and it will show us a mystery of form which the insect alone can explain.
Here is one, growing just outside my door—a blossom "known" even to every child, and certainly to every reader of the "Round Table"—the pretty bluets, or Houstonia, whose galaxy of white or blue stars tints whole spring meadows like a light snowfall. We have "known" it all our lives. Perhaps we may have chanced to observe that the flowers are not all constructed alike, but the chances are that we have seen them all our lives without discovering this fact. If we pluck a few from this dense cluster beside the path, we observe that the throat of each is swollen larger than the tube beneath, and is almost closed by four tiny yellow anthers [(Fig. 1)]. The next and the next clump may show us similar flowers; but after a little search we are sure of finding a cluster in which a new form appears, as shown in Fig. 2, in which the anthers at the opening are missing, and their place supplied with a little forked stigma! The tube below is larger than the first flower for about two-thirds its length, when it suddenly contracts, and if we cut it open we find the four anthers secreted near the wide base of the tube. What does it mean, this riddle of the bluets? For hundreds of years it puzzled the early botanists, only finally to be solved by Darwin. This is simply the little plan which the Houstonia has perfected to insure its cross-fertilization by an insect, to compel an insect to carry its pollen from one flower and deposit it upon the stigma of another. Once realizing this as the secret, we can readily see how perfectly the intention is fulfilled.
In order to make it clear I have drawn a progressive series of pictures which hardly require description. The flowers are visited by small bees, butterflies, and other insects. At the left is an insect just alighting on a clump of the blossoms of the high-anther form indicated below it. The black probe represents the insect's tongue, which, as it seeks the nectar at the bottom of the tube, gets dusted at its thickened top with the pollen from the anthers. We next see the insect flying away, the probe beneath indicating the condition of its tongue. It next alights on clump No. 2, in which the flowers happen to be of the high-stigma form, as shown below. The tongue now being inserted, brings the pollen against the high stigma, and fertilizes the flower, while at the same time its tip comes in contact with the low anthers, and gets pollen from them. We next see the insect flying to clump No. 3, the condition of its tongue being shown below. Clump No. 3 happens to be of the first low-stigma form of flowers, and as the tongue is inserted the pollen at its tip is carried directly to the low stigma, and this flower is fertilized from the pollen from the anthers on the same level in the previous flower. And thus the riddle is solved by the insect. From clump to clump he flies, and through his help each one of the pale blue blooms is sure to get its food, each flower fertilized by the pollen of another.
1st Clump.—Flower enlarged. Insect's Tongue inserted.
Pollen high on Insect's Tongue after withdrawal from Blossom.
2d Clump.—Flower enlarged. Pollen thrust against high Stigma at top and touching Pollen below.
Pollen at Base of Insect's Tongue after withdrawal from Blossom.
3d Clump.—Flower enlarged. Pollen thrust against low Stigma.
Another beautiful provision is seen in the difference in size of the pollen-grain of the two flowers, those of the high anthers being much larger than those from the lower anthers. These larger grains are intended for the high stigma, which they are sure of reaching, while those of smaller size, on the top of the tongue, which should happen to be wiped off on the high stigma, are too small to be effective for fertilization.