ENGELMANN — NOTES ON THE GENUS YUCCA. 21 

 ROOTSTOCK. 



While the rootlets of Yucca annually spring from the youngest 

 part of the rootstock, and decay again after a season, the root- 

 stock itself increases often to a large size and irregularly 

 branched shape. We have very few^ data about the form of this 

 organ ; in fact, the only definite information, accessible to me, has 

 been imparted by Mr. Lindheimer, who, with persistent zeal, has 

 dug up from the often hard and stony soil of West Texas the dif- 

 ferent species accessible to him. He informed me that T. an- 

 gustifolia usually exhibits a perpendicular rootstock of a finger's 

 thickness, and two or three feet long, " rising from" (it is evi- 

 dent from what is stated above, that it is rather " descending to") 

 a long horizontal simple or branching part, one or one and 

 a half inches thick, exhibiting many knobs and buds of future 

 shoots. T. rupicola has a rootstock consisting of a few thick, 

 cylindric, horizontal branches, one to two feet long. The tree- 

 like T. Trcculiana has few short, thick, club-shaped, horizon- 

 tal branches to its rootstock, sometimes only a single, short and 

 very stout knob, which does not seem to readily sprout out. It 

 will be interesting to study these conditions in other species in 

 their native localities. 



The rootstock of all the Yuccas is, under the name of "Amole," 

 an important article in a Mexican household, being everywhere 

 used as a substitute for soap, as it is replete with mucilaginous 

 and saponaceous matter, probably a substance analogous to the 

 saponine of the Saponaria root. It is curious to learn that the 

 negroes of the coast of South Carolina repeatedly destroyed Dr. 

 Mellichamp's carefully observed clumps of Yuccas, in order to 

 obtain the saponaceous rootstock. How may the knowledge of 

 its quality have reached them? Perhaps from the West Indies. 



TRUNK. 



The trunk of the Yuccas either remains entirely below the sur- 

 face, or it takes different degrees of development above ground. 

 Heretofore, specific characters were partly based on such differ- 

 ences, but we know now that only few species are regularly and 

 always acaulescent i^T. rupicola)^ while others, when in a per- 

 fect, or flower-bearing state, always have trunks ( T. aloifolia and 

 T. Treculiana^ though this species was first described as stem- 

 less) ; a certain number, usually counted as acaulescent, under 



