The writers found that the humblebees were the principal agents effecting cross-pollination. It was observed that the bee in visiting the flower allowed itself to rest on the tips of the extending stamen and pistil, which, being of the same length, came in contact with both sides of the body just in front of the hind legs, these being left perfectly free. The weight of the bee springs down both stamen and pistil.
Professor Todd’s theory in regard to the pollination of this plant is as follows: “The weight of the bee so springs down the flower, that it is quite difficult, on account of the large, flexible corolla, to see just what is done, but repeated observations led me, quite satisfactorily, to this conclusion. The bee seeks the pollen—for the flowers have neither nectar nor odor—and this she uniformly gets from the four shorter stamens; never, so far as I could determine, from the larger one. This she does by seizing each one, near its base, between her mandibles, and with a sort of milking motion crowds the pollen out of the terminal pores; meanwhile, by the movements of her feet, the larger stamen is repeatedly sprung backwards, and as often throws a cloud of pollen on one side of her body; this in a right-handed flower. When she passes to a left-handed flower, which, as was explained above, is very likely not to be on the same plant, the pollen is carried directly to the pistil of that flower, and so on. We have here, therefore, a novel apparatus for cross-fertilization, quite distinct from those that have been most commonly noticed.”
A considerable quantity of pollen may be thrown from the terminal pores of the large stamen upon tapping it. It thus seems quite possible that some pollen is thrown upon the side of the insect, as described by Professor Todd. All the meaning of Mr. Meehan’s[K] statement is not clear to the writers, but he says, in speaking of Professor Todd’s results: “In regard to the manner in which the pollen is extracted, he found that ‘this she does by seizing each anther near its base between her mandibles, and, with a sort of milking motion, crowds the pollen out of the terminal pores.’ If this were the general way, there would be no necessity for any pollen being ejected from the long stamens, for the stigma would surely receive some during the ‘milking’ process; and the pore at the apex in the long anther is beyond the line of the stigma, so that on ejection from the pore the pollen would go still farther beyond.”
It seems that this statement is of considerable importance for S. rostratum as well as for C. marilandica. Professor Todd very evidently overlooked the fact that, in securing the pollen from the small stamens and transferring it to the hind legs, the sides of the insect are sure to be well dusted with pollen from these stamens. In the case of Apis mellifica, as noted above, there is no certainty that in visiting the flower the same side will be turned toward the stamen or pistil. Even in the case of large insects, such as Bombus, it would seem that the probability that the stigma will be supplied with pollen from the large stamen exclusively is very small. It seems improbable that S. rostratum should depend exclusively upon such an uncertain method of pollination as the projection, by the jarring of a stamen, of a puff of pollen upon the side of an insect, and the subsequent transfer of this pollen to the stigma of a flower of a different type. Of course, it is not improbable that a part of the pollen is furnished by the large stamen, as suggested by Professor Todd, but that fertilization should be effected exclusively by this means seems highly improbable.
The pollen from the large stamen has been shown to be fertile in a certain number of cases, but unfortunately opportunity was not offered for experiments on the fertility of pollen from the small stamens. A rather hasty microscopic examination of fresh, unstained pollen from the large and small stamens reveals no very striking difference in form.
In C. marilandica, Meehan[L] found that the large, strong stamens on each side of the pistil served only as a platform upon which the insect could rest while procuring the pollen from the small stamens. He found that the lower stamens, while filled with pollen, did not dehisce of their own account, nor were they opened by the insect.[M]
The lower stamens and the pistil of the Solanum under consideration serve the purpose of a platform when the flowers are visited by the larger bees. It seems to the writers that this is not improbably the function of the greatest importance of the observed arrangements of the stamen and pistil in S. rostratum. In C. marilandica, the pollen for fertilization, as well as for the attraction of the insect visitor, is furnished by the small stamens, while the pollen produced by the large stamens appears to have no function.[N] The condition is not so specialized in the species of Solanum under consideration. Here the pollen produced by the small anthers serves for the attraction of insects and, as it seems to the writers, for fertilization, while the large stamen, in connection with the pistil, serves as a support for the visiting insect, and possibly furnishes some pollen for cross-fertilization.[O]
In reference to the relative amount of pollen produced by a large and small stamen, Halstead has given a note, in his paper in the Botanical Gazette.[P] The material in the hands of the writers at the time of the writing of this paper is not suitable for a verification of Mr. Halstead’s results; consequently they are simply quoted on his authority. Even if the amount of pollen produced by the large anther is no greater than that produced by one of the smaller, it is still very considerable, as may be readily seen by tapping it out on a glass slip. He says:
“The single large stamen of Solanum rostratum, with its beak-like appearance, is a giant among its fellows, but does not exceed them in the production of pollen, for, while three or four times larger than the others, its thecæ are reduced to narrow, curved lines of mother-cells. The ordinary stamens, on the other hand, possess unusually large cavities in which the pollen is borne. The giant stamen, in cross-section, is shown at a, in fig. 3, while a similar section of an ordinary stamen is shown at b. The almost infertile condition of the large stamen reminds one of the structure of the stamens of the cultivated potatoes. In these, while large and plump, there is almost no pollen-bearing layer, and usually no apical pore opens for the discharge of pollen.”
In C. marilandica, as Meehan has shown, autogamy is impossible, while in S. rostratum autogamy may possibly sometimes take place.