made the basis for the primary division of the genus into three groups: the Singulifloræ, the Geminifloræ, and the Paniculatæ. In all cases the flower-scape rises from the apex of the main axis of the plant; all the vital energy of often many years' growth is centered there, and the plant throws up its blossom-stalk as the supreme effort of its existence, and, when the fruit has ripened, dies—a strange phenomenon, and almost without parallel in any other so extensive group. In the Singulifloræ, is the simplest type of inflorescence. The flowers are loosely spiked, each one in the axil of a bract. To this group belongs our one Northern agave, the little Agave Virginica, which grows from Maryland and southern Indiana southwestward into Texas. The Geminifloræ have the flowers borne in pairs, and densely spiked along the scape. Variations which show transition between both these simpler groups and the third occur. The Paniculatæ have the scape more or less branching, often in the fashion of a candelabrum, each branch terminating in a dense cluster of flowers. These are the typical agaves, the crowning glory of the genus. The familiar Agave Americana is a representative of the Paniculatæ, and so also the plant shown in the accompanying photograph (Fig. 7), Agave Salmiana, a magnificent species that blossomed in the Missouri Botanical Garden in the summer of 1892. A splendid agave that commemorates the founder of the Missouri Garden is the Agave Shawii, dedicated by Engelmann to Henry Shaw; and in turn the labors of Engelmann have been fitly honored in the dedication to him of a most striking type that he once presented to the Missouri Garden. It blossomed there in the summer of 1891, and when it had been clearly proved a new species it was duly christened by Director Trelease Agave Engelmanni (Fig. 5). The structure of the agave flower is extremely unique in several particulars, but further detail can not be entered into. Almost every step taken in the investigation of the genus gives additional emphasis to the first impression, that it is one of the master marvels of plant life.
It remains to add some passing notes on the wonderfully beautiful genus which the lily family contributes to our group, the yuccas. A glorious floral offering to the arid Southwest highlands they certainly are, and scientifically their structure is in many ways scarcely less remarkable than that of the cacti and agaves. But the consideration of these points will be passed over here in order to call up more particularly the phenomenon that makes the yucca an astounding mystery to naturalist and philosopher, the manner of its cross-fertilization. For the fact is, we have here an extensive genus entirely incapable, save under most rarely extraordinary circumstances, of self-fertilization, and entirely dependent on one moth that fertilizes the flowers in order to insure food supply for its larvæ in the ripening seeds. The problem of
