22 TRANS. ST. LOUIS ACAD. SCIENCE. 



favorable circumstances make short trunks, sometimes of only a 

 few years' duration (T. Jilamentosa, and still more T. angusti- 

 folia)^ and others, again, among them most notably T. baccata, 

 are absolutely stemless near their northern limits, while the farther 

 south we meet them, the higher and more tree-like their trunk 

 grows. 



The primary axis of the Yuccas is terminated by the inflores- 

 cence and its apex dies with it. The plant is then rejuvenated by 

 lateral buds, either from the same axis or from the subterranean 

 rootstock. In the first instance the buds appear about the time 

 of the maturity of the fruit, in the trunk-bearing forms from the 

 axils of the uppermost, in the stemless ones from those of the low- 

 est leaves. A single subterminal bud will soon simulate the 

 direct continuation of the main axis ; several buds will produce 

 branches in the trunk-bearing species, while in the stemless ones 

 they will give the plant a cespitose appearance. From Dr. Mel- 

 lichamp's observations, it seems that the caulescent Yuccas show 

 certain differences in the place where the bud appears ; he noticed 

 the young bud of T. aloifolia, from exactly the uppermost axil, 

 at the base of the inflorescence, while in T. gloriosa it sprung 

 from between the uppermost and the next lower series of leaves. 

 In a Y. jilamentosa in my garden, I observed several buds in the 

 axils of the highest leaves developed two years in succession, so 

 that a short branching trunk was formed, while after the third 

 year the vitality of this trunk seems to have died out, and the 

 plant was rejuvenated by shoots from the subterranean root- 

 stock. In other forms, which probably belong to the same species, 

 I find only rarel.y, in very vigorous garden specimens, a bud from 

 the uppermost axils, while almost always they branch from below 

 the crown of leaves. But observations of this kind, relating to 

 the biology of these plants, have been made too seldom to permit 

 yet the deduction of general laws. 



The Yucca trunk has a light fibrous wood, which exnibits dis- 

 tinct marks of concentric arrangement, so that in an old trunk of 

 T. Treculiapa, of two and a half feet in diameter, I can count 

 twenty layers in a space of two and a half inches, or one and a 

 half lines to the layer; the trunks certainly grow in thickness as 

 they get older. Another peculiarity of old Yucca trunks is their 

 thick, corky bark ; the above mentioned T. Treculiana, sent by 

 Mr. Lindheimer, has an irregular., rather scaly, dark gray bark of 



