Page:Popular Science Monthly Volume 41.djvu/189

From Wikisource
Jump to navigation Jump to search
This page has been validated.
177
THE YUCCA MOTH AND YUCCA POLLINATION.

the tongue are so often separated at tip, and so weakly joined throughout, as to raise the question, in connection with a somewhat imperfect alimentary canal, as to whether the moth feeds at all, and to suggest that the rather strong tongue, otherwise, assists pollination.

No sooner is the ovipositor withdrawn into the abdomen than the moth runs up to the top of the pistil, thrusts the pollen into the stigmatic opening, and works her head rapidly—the motion

PSM V41 D189 Flower of yucca with petals removed.jpg
Fig. 7.—Flower of Yucca, with near petals removed to show normal position of Pronuba in ovipositing.

being mostly up and down and lasting several seconds. She works with a vigor that would indicate combined pleasure and purpose, and makes every effort to force the pollen into the tube, thrusting it ordinarily from the base of one of the three primary clefts of the style. After the more vigorous motions of thrusting the pollen into the tube, she frequently rests in comparative quiet, working her tongue in the tube sometimes for four or five minutes together, but ordinarily the act of pollination ceases with the few vigorous thrusts already described. The importance of this act will be better appreciated when I state that numerous experiments in artificial or brush pollination have shown that effective fertilization in Yucca filamentosa is by no means an easy matter, and that it rarely takes place as effectively as through the actions of Pronuba.

This carrying of the pollen to the stigma generally follows every act of oviposition, so that where ten or a dozen eggs are consigned to a single pistil, the stigma will be so many times bepollened.

The egg of Pronuba is an extremely delicate, thread-like structure, averaging 1·5 millimetre in length and less than 0·1 millimetre (Fig. 9, c) in diameter, tapering at the base and en-