Fig. 2. The Parts of a Flower
The calyx usually encloses the bud, and may be tubular, or composed of separate leaves or sepals, as in a rose. The corolla, or colored portion, may consist of several petals, as in the rose, or of a single one, as in the morning-glory. At the centre is the pistil, one or more, which forms the ultimate fruit. The pistil is divided into three parts, ovary, style, and stigma. Surrounding the pistil are the stamens, few or many, the anther at the extremity containing the powdery pollen.
Although these physiological features have been familiar to observers for thousands of years, the several functions involved were scarcely dreamed of until within a comparatively recent period.
In the writings of ancient Greeks and Romans we find suggestive references to sexes in flowers, but it was not until the close of the seventeenth century that the existence of sex was generally recognized.
Fig. 3. Historical Series, Showing the Progress of Discovery of Flower Fertilization
In 1682 Nehemias Grew announced to the scientific world that it was necessary for the pollen of a flower to reach the stigma or summit of the pistil in order to insure the fruit. I have indicated his claim pictorially at A (Fig. 3), in the series of historical progression. So radical was this "theory" considered that it precipitated a lively discussion among the wiseheads, which was prolonged for fifty years, and only finally settled by Linnæus, who reaffirmed the facts declared by Grew, and verified them by such absolute proof that no further doubts could be entertained. The inference of these early authorities regarding this process of pollination is perfectly clear from their statements. The stamens in most flowers were seen to surround the pistil, "and of course the presumption was that they naturally shed the pollen upon the stigma," as illustrated at B in my series. The construction of most flowers certainly seemed designed to fulfil this end. But there were other considerations which had been ignored, and the existence of color, fragrance, honey, and insect association still continued to challenge the wisdom of the more philosophic seekers. How remarkable were some of those early speculations in regard to "honey," or, more properly, nectar! Patrick Blair, for instance, claimed that "honey absorbed the pollen," and thus fertilized the ovary. Pontidera thought that its office was to keep the ovary in a moist condition. Another botanist argued that it was "useless material thrown off in process of growth." Krunitz noted that "bee-visited meadows were most healthy," and his inference was that "honey was injurious to the flowers, and that bees were useful in carrying it off"! The great Linnæus confessed himself puzzled as to its function.
For a period of fifty years the progress of interpretation was completely arrested. The flowers remained without a champion until 1787, when Sprengel began his investigations, based upon the unsolved mysteries of color and markings of petals, fragrance, nectar, and visiting insects. The prevalent idea of the insect being a mere idle accessory to the flower found no favor with him. He chose to believe that some deep plan must lie beneath this universal association. At the inception of this conviction he chanced to observe in the flower of the wild geranium (G. sylvaticum) a fact which only an inspired vision could have detected—that the minute hairs at the base of the petal, while disclosing the nectar to insects, completely protected it from rain. Investigation showed the same conditions in many other flowers, and the inference he drew was further strengthened by the remarkable discovery of his "honey-guides" in a long list of blossoms, by which the various decorations of spots, rings, and converging veins upon the petals indicated the location of the nectar.
His labors were now concentrated on the work of interpretation, until at length his researches, covering a period of two or three years, were given to the world. In a volume bearing the following victorious title, "The Secrets of Nature in Forms and Fertilization of Flowers Discovered," he presented a vast chronicle of astonishing facts. The previous discoveries of Grew and Linnæus were right so far as they went—viz., "the pollen must reach the stigma"—but those learned authorities had missed the true secret of the process. In proof of which Sprengel showed that in a great many flowers, as I have shown at C (Fig. 3), this deposit of pollen is naturally impossible, owing to the relative position of the floral parts, and that the pollen could not reach the stigma except by artificial aid. He then announced his startling theory: