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. alotfolia 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 Smert1ana Baker, Gard. Chron. 1870: 1217. Journ. 
Linn. Soc. Bot. 18 : 222. Kew Bull. 1892 :8. 
? Y. Helkinsi Hort. 
Caulescent, at length with a trunk 2 or 3m. 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. 
