THE YUCCEAE. 87 



absolute dependence upon vegetative methods of propaga- 

 tion; though they continue to flower profusely, and because 

 of the unusual if aberrant period over which their bloom- 

 ing extends they now and then fruit, and they are shown 

 to be so fertile under skilful artificial pollination that there 

 is little reason to doubt that they would fruit regularly if 

 they bloomed when the Pronuba was about ; while over 

 the great territory lying between the Atlantic and Pacific 

 and the big bend of the Missouri river and central Mexico, 

 the other Yuccas have held so close a relation with their 

 pollinators as to be very fruitful under all ordinary circum- 

 stances. The ecological facts stated, however, are con- 

 sistent with the morphological suggestion that Y. gloriosa 

 may be a hybrid between Y. aloifolia and Y. filamentosa, 

 and the two considerations appear to constitute so strong 

 an argument for the acceptance of the a priori theory 

 advanced, as to throw the burden of proof upon any who 

 would still regard gloriosa as a species in the ordinary 

 sense, though for purposes of classification it, as well as 

 recurvifolia and flexilis, may continue to be treated as 

 species.* 



222. Leaves crowded, regularly and rigidly arcuate. 



Y. DE SMETIANA Baker, Gard. Chron. 187O : 1217. Joura. 

 Linn. Soc. Bot. 18 : 222. Kew Bull. 1892 :8. 



? Y. Helkinsi Hort. 



Caulescent, at length with a trunk 2 or 3 m. high. Leaves rigid, evenly 

 and stiffly recurved, becoming .4 m. long and 25 mm. or more wide, pur- 

 ple tinged, entire or slightly rough-margined at base, not pungent. Flow- 

 ers and fruit unknown. Plate 48. 



A garden plant ascribed to Mexico, which when small is 

 very suggestive in appearance of a lily because of its 

 crowded arching not at all concave leaves : quite unlike any 

 other Yucca, and perhaps not of this genus. No positive 

 record exists of the source of the plants of this species cul- 



* The substance of these conclusions was presented at the Denver 

 meeting of the Botanical Society of America, in August 1901. 



