YAH. See Dioscorea and Sweet Potato. Circular 21, 

 Div. of Bot., U. S. Dept. Agric, has valuable cultural 

 notes on the introduction of West Indian Yams (Dios- 

 coreas) to subtropical agriculture in the U. S. 



YARBOW. Consult Achillea. 



YATE. See Eucalyptus occidentalis. 



YELLOW ROOT SHRUB. Xanthorrhiza. 



YELLOW-WOOD. Cladrastis tinctoria. 



YERBA BUENA. Micromeria Douglasii. 



YEW. See Taxus. 



YOUTH-AND-OLD-AGE. Zinnia. 



its forms, and glauca are hardy in the northern states, 

 and Y. Treculeana shows considerable resistance to 

 frost. The tender species are kept in the cactus house. 

 Well-drained sandy loam suits them best, but with good 

 drainage they are tolerant of a large range of soil and 

 exposure. Prop, by seeds, offsets, stem -cuttings, and 

 the rhizomes that several species produce, which may 

 be cut into short lengths and rooted in the cutting bench. 

 Y. Treculeana blooms usually in March in plant houses, 

 as when wild, and the Mexican species when brought to 

 flower are usually spring bloomers, but they often refuse 

 to flower for long periods and then suddenly and unex- 

 pectedly produce an abundance of simultaneous bloom, 

 even on the smaller plants. Of the hardy species, 

 Y. glanca flowers in June and it is quickly followed 

 by Y. filamentosa and Y. flaccida, while the forms of 

 Y. gloriosa, which usually flower only at intervals of 

 several years, bloom from late August to so late in the 

 autumn as to be cut down by frost. 



t 





\\m\\T'i/W/ 



2761. Yucca Whipple 



rtfCCA (Indian 'name for the Manihot, erroneously 

 applied by Gerarde). Lili&cece. About a dozen species, 

 chiefly of the arid North American table-land and con- 

 fined to the United States, Mexico and adjacent islands. 

 Evergreens with long, narrow, usually spiny-pointed 

 leaves and panicles of large white nocturnal flowers 

 frequently shaded with green or purple. 



Y. filamentosa, flaccida, baccata, gloriosa in some of 



Most species may be fertilized if fresh pollen is trans- 

 ferred directly from the anther to the stigmatic cavity of 

 a newly opened flower, preferably one seated directly on 

 the main shaft, where nutrition is more certain. Y.aloi- 

 folia commonly fruits freely, but the others rarely fruit 

 spontaneously in cultivation except Y. filamentosa and 

 }~. flaccida, which are pollinated by a small white moth 

 {Pranuba yuccasella) that accompanies them when cul- 



126 



(1999) 



