82 -Riley Some Interrelations of Plants and Insects. 



I need not tell the members of this Society that the old idea 

 that flowers were endowed with beauty and fragrance for our 

 particular pleasure has been effectually set aside, and that these 

 attributes have come to be looked upon in their true light, as 

 essential to the plant's existence and perpetuation ; that, in other 

 words, color, form, odor, secretions, and the general structure of 

 flowers, all have reference to insects. Nor need I dilate on the 

 need of cross-fertilization in plants generally, or the modification 

 which insect pollinizers have undergone as a consequence of this 

 need. Some of the more interesting facts are particularly well 

 exemplified in our orchids, to the philosophic study of which 

 Darwin's important work " On the Fertilization of Orchids " gave 

 a distinct impulse. But here we have adaptation of the plant 

 only, and, with scarcely an exception, most flowers, including 

 those of our orchids, may be fertilized by different insects. 

 There are, in fact, few which are dependent on a single species 

 for pollination, and, so far as I know, our Yuccas furnish the 

 only instance of this kind. It is to the fertilization of these 

 plants that I would first draw your attention. 



The Yuccas are a characteristic American group of liliaceous 

 plants, finding their home more particularly in the southern 

 United States and Mexico. There are many species which have 

 been divided even into sub-genera by Dr. Engelmann, as Sar- 

 coyucca, Clistoyucca, Chenoyucca, and Hesperoyucca ; but for 

 our present purpose they may all be included under the one 

 genus Yucca, as they all possess certain characteristics in com- 

 mon, viz, a thick, sub-mucilaginous root, which is in reality a 

 subterranean stem ; lance-shaped, evergreen leaves, narrow or 

 broad, rigid or flaccid, and with the edges either filamentose, 

 smooth, or more or less distinctly serrate. The leaves produce 

 a coarse fiber, valuable for certain kinds of fabrics, while the 

 trunks of the tree Yuccas have been used to make the toughest 

 kind of paper. The fruit of some species, as of aloifolia and 

 baccata, is fleshy and edible. It is, however, the flowers to 

 which I would draw more especial attention. They are pro- 

 duced in large panicles, and are characterized as a rule by the 

 anthers not reaching anywhere near the stigma ; so that fertiliza- 

 tion unaided can take place only by the merest accident. The 

 Yuccas show 7 great variation in detail, both in leaf, general 

 habitus, flower-stalk, flower, and fruit, from the common sessile 

 Yucca filamerUosa of our gardens to the arboreal forms, like 



