THE YUCCEAE. 123 



of moisture, and their leaves are well guarded against 

 undue transpiration. Like other arboreous Liliaceae, their 

 larger representatives produce the impression of being the 

 culmination of a vegetative type perhaps formerly of wide 

 distribution, but now barely able to hold its own except 

 in desert regions where competition between plants is 

 less than elsewhere, while structural adaptation enables 

 them to endure the rigors of this last resort, in a 

 sense, therefore, recalling the bald cypress (Taxodium) 

 among conifers, which for similar reasons has betaken 

 itself to the other extreme of deep swamps. I know of no 

 ecological explanation of the filif erous shedding of the leaf- 

 margins of many species. 



The dissemination arrangements of the Yucceae are of 

 the more highly specialized types. Many species, consti- 

 tuting the genus ffesperaloe, Hesperoyucca, and the capsular 

 section of Yucca, are wind-disseminated, with thin flat 

 seeds lifted from time to time out of the suberect capsules 

 by gusts of wind. In Clistoyucca the indehiscent mature 

 fruit is spongy and light and apparently adapted to being 

 blown about by the desert winds after the manner of blad- 

 der-fruits or tumble-weeds. Yucca gloriosa and Y. recur- 

 vifolia possess fruits which do not dehisce, though their 

 seeds are thin and flat ; nor do they become edible in ripen- 

 ing, but dry to a firm almost wooden consistency, 

 out of harmony with any usual mode of dissemination. 

 All of the baccate species of Yucca and the two species of 

 Samuela have fleshy edible fruits at maturity, and their 

 abundant endosperm suggests an adaptation to the dry 

 regions, in which all of them, so far as known, live, with 

 the exception of Y. aloifolia, and, perhaps, Y. elephantipes. 

 That they have been derived from thin-seeded capsular 

 species seems more probable than the reverse, and the 

 coreless fruit of the seaside Y. aloifolia suggests its 

 independent fruit specialization rather than a genetic con- 



