106 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 
work in pollinating that flower. The interdependence of insect and 
plant in the case of the small Tineid genus Pronuba and the Yucca is 
one of the strangest facts brought to light by biological studies in the 
field. Without the moth few species of Yucca set seed, and the 
larvae feed only on the immature seeds of Yuccas, destroying a certain 
number in every seed-pod, but sparing others to ripen. * 
The general sterility of Yuccas in Europe is due to the absence of 
the moths, but it is possible that beetles visiting the flowers for pollen 
may occasionally deposit a few grains in the stigmatic tube. 
The flowers of Y. Whipplei differ in several characters from those of 
other Yuccas, and therefore it is now regarded as a sub-genus, Hespero- 
yucca. The filaments are attached to the lower part of the petals, 
and so when the flower is fully expanded the stamens are drawn well 
away from the ovary. The pollen is not loose and powdery as in 
Yucca proper, but distinctly glutinous. The stigma is capitate, more 
open and covered with moist papillae, ready in fact to retain any pollen 
carried to it. Thus, as Professor Riley has recorded, Y. Whipplei is 
quite likely to be pollinated by visitors other than Pronuba. Professor 
Trelease has further stated that it possesses a limited power of self- 
pollination, for in the smaller, later-opening flowers it is possible for 
the pollen to be deposited on the margin of the stigma directly from 
the anthers of closing flowers. 
This doubtless occurred in the specimen which fruited in July 1921 
in Mr. W. M. Christy's garden at Watergate, near Emsworth, only 
two pods being formed, but both containing seeds that have since 
germinated. 
It is a pity that this fine species is just too tender for all but the 
southern and western gardens of England, for its 8- to 10-feet flower- 
spike is one of the finest produced by any of the Yuccas. It would, 
however, be worth planting where a well-drained bank facing south, 
and the possibility of overhead protection in winter, could be provided. 
The narrow grey-green leaves of a healthy specimen are very beautiful 
and give it the aspect of a Dasylirion rather than of a Yucca. 
The hardiest of all Yuccas is the stemless species Y. flaccida. It is 
frequently offered as Y. filamentosa, and these two are nearly related, 
and both are variable plants and in some of their forms approach one 
another very closely. They are best regarded as distinct species, 
however, and may be distinguished by the following characters : 
Y. filamentosa. — Leaves rigid, dark green, with curling marginal 
threads. Inflorescence usually glabrous. 
Y. flaccida. — Leaves thin, flexible, the outer recurved, often 
glaucous ; marginal threads, if present, straight. Inflorescence usually 
pubescent. 
Y. flaccida is the hardier and more vigorous plant, spreading by 
means of underground stolons and soon forming dense clumps, one or 
more rosettes of which will flower every season, dying afterwards, 
but retaining its leaves green for another year. It pays, however, to 
remove the old crown that has flowered and so to make room for the 
