ENGELMANN NOTES ON THE GENUS YUCCA. 23 



a quarter or a third of an inch thickness, resembling that of some 

 elm or willow ; a trunk of T. aloifolia, received from Dr. Melli- 

 champ, is covered with a bark of the same thickness, of a deep 

 brown color, broken up into numerous small square or angular 

 fragments, much like that of the dogwood {Cornus Jlorida). 

 The bark of a section of T. brevifolia^ sent by Dr. Parry, is 

 similar, but over half an inch thick, and still more deeply cleft. 

 The investigations of these organizations would form a worthy 

 subject for an experienced phytotomist. 



LEAVES. 



The leaves of the Yuccas are evergreen, i.e. they last at least 

 a whole year, in the low species, or several years in the arbores- 

 cent ones. They are lance-linear, abruptly narrowed above a 

 very broad, mostly membranaceous base, and usually widening 

 again near or above the middle (some narrowed-leaved species 

 are not contracted below the middle), and gradually, or rarely 

 abruptly, terminate in a horny, often sharply pointed, rarely ob- 

 tuse, sometimes soft and herbaceous spine, below which the tip 

 of the leaf is more or less concave and involute. The leaves are 

 usually more or less thick, and more or less rigid, but we find all 

 the transitions from the stiff and sharp pointed (" Spanish bayo- 

 net ") to the soft and flaccid leaf Their size in the different 

 species varies from half a foot to four feet in length, and from one 

 quarter to two and three inches in width. 



The upper side of the leaf is flat (the tip excepted), or almost 

 always more or less concave, sometimes deeply channeled, and 

 occasionally folded or plicate. The lower side is convex, and its 

 lower part bluntly keeled. The surfaces are smoothish or more 

 or less rough, and this roughness is the result of the peculiar struc- 

 ture of the cells surrounding the stomata. The lateral walls of 

 these cells are thickened, hard and transparent, and somewhat 

 elevated above the general surface ; especially in the true T. fila- 

 mentosa the edges of the upper and lower marginal cells protrude 

 over the stoma like minute, beautifully chiseled, conchoidal shields* 

 sometimes almost completely covering it. In T. brevifolia the 

 edges of other cells are also apt to protrude, and, besides, number- 

 less little knobs, similar to the marginal asperities, to be described 

 below, increase the roughness of both surfaces. I notice the same 

 appearance on the lower surface of the leaves of the T. Treculi- 

 ana and T. canaliculata^ and, less distinctly, on T. gloriosa. 



