yucca fertilization was for many years a vexed question; Engelmann spent much thought and observation in trying to learn its secret, and finally, over twenty years ago, called the attention of Prof. C. V. Riley, the noted entomologist, to certain moths which frequented the yucca flowers. After long and patient study and various erroneous speculations the two scientists ultimately brought the whole mystery to light; and in a recent paper, published in the Report of the Missouri Garden for 1892, Riley has fully elaborated his work of the earlier years and the observations made in the intervening time. The structure of the yucca flower
Fig. 9.—Yucca aloifolia.
is plainly outlined in Fig. 10. Long experimentation has positively shown that it is practically impossible for the sticky pollen to be transferred from the little anthers on the tips of the short stamens to the fine stigmatic tube opening only at the tip of the pistil, except by external voluntary agency. As a matter of fact, the agency is
