Page:Popular Science Monthly Volume 41.djvu/188

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176
THE POPULAR SCIENCE MONTHLY.

position, as a rule, is accomplished in some other flower than that from which the pollen was gathered, and that cross-fertilization is thus secured.

Once fully equipped with this important commodity, she may be seen either crawling over or resting within the flower, generally PSM V41 D188 Pronuba yccasella female.jpgFig. 6.—Pronuba yuccasella, female, in the act of gathering pollen from the anthers; five times enlarged. with the head toward the base. From time to time she makes a sudden dart and deftly runs around the stamens, and anon takes a position with the body between and the legs straddling two of them, her head being usually turned toward the stigma. As the terminal halves of the stamens are always more or less recurved, she generally has to retreat between two of them until the tip of her abdomen can reach the pistil (Fig. 7). As soon as a favorable point is reached—generally just below the middle—she rests motionless for a short time, when the abdomen is slightly raised and the lance-like ovipositor is thrust into the soft tissue, held there the best part of a minute, while the egg is conducted to its destination, and then withdrawn by a series of up-and-down motions. Fig. 8 is a transverse section of the young fruit at this stage of the growth, indicating the manner in which it is punctured at a, a, and how the egg is conveyed into the ovarian cell at b, while Fig. 9 shows a longitudinal section of the pistil at a, the puncture of the ovipositor at b, and the egg within the ovarian cell at c.

The stigmatic liquor is not nectarian, and the flower secretes but a small amount of nectar at the base of the petals; and while these facts serve to disprove any positive value of their nectar in the pollination of the yucca flowers, they add to the importance of Pronuba by showing that the acts of collecting pollen and transferring it to the stigma do not result in any food compensation, as I was at first inclined to suppose. In other words, there is no nectar to allure other nectar-loving insects and cause them to go to the stigma; but, on the contrary, those which are drawn to the plant by the slight amount of nectar are led in the very opposite direction, viz., to the base of the style or of the flower. It is also an interesting fact that I have never noticed Pronuba feeding, as contradistinguished from pollinizing, for the motions of the tongue of Lepidoptera when feeding are quite characteristic and easily recognized. Indeed, the two pieces which form