NOTES ON THE GENUS YUCCA. 281 
bark; the above-mentioned Y. 7Jreculiana, sent by Mr. Lindheimer, has an irregular, rather 
sealy, dark gray bark of a quarter or a third of an inch thickness, resembling that of some [23] 
elm or willow; a trunk of Y. aloifolia, received from Dr. Mellichamp, 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 florida). The bark of a section of Y. brevifolia, 
sent by Dr. Parry, is similar, but over half an inch thick, and still more deeply cleft. The inves- 
tigations 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 arborescent 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 obtuse, 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 bayonet”) 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 channelled, 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 structure of the cells surrounding the stomata. The lateral walls of these 
cells are thickened, hard, and transparent, and somewhat elevated above the general surface ; espe- 
cially in the true Y. filamentosa the edges of the upper and lower marginal cells protrude over the 
stoma like minute, beautifully chiselled, conchoidal shields, sometimes almost completely covering 
it. In V. brevifolia the edges of other cells are also apt to protrude, and, besides, numberless 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 Y. 7reculiana and 
Y. canaliculata, and less distinctly on Y. gloriosa. 
The color of the leaves varies from deep or fresh green through dull green to light glau- [24] 
cous. Of great interest and diagnostic importance is the edge of the leaf. In some species 
(such as Y. alotfolia, Y. brevifolia, Y. rupicola) it is rough, or, as it is usually termed, serrulate, and 
remains unaltered through life. The teeth consist of small, irregular, isolated cartilaginous knobs, 
each consisting of quite a large number of colorless prismatic or clavate cells, arranged in fan-shaped 
or straight bundles. These are the “serrulate” or rough-edged Yuccas. 
Others have “smooth-edged” leaves, (Y. gloriosa, Y. Treculiana) ; the edge, at first green, and 
often roughened with very delicate and deciduous asperities, soon becomes discolored and _ brittle, 
and in old leaves is apt to crumble off, or sometimes to detach itself in a few short fibres, thus 
approaching the next form. 
The “ filamentose,” or fibrous-edged Yuccas (Y. jilainentosa, Y. angustifolia, Y. baccata) consti- 
tute the third class. In these the fibrous system of the leaves is much stronger and tougher than 
in the last, and, the parenchymatous tissue soon withering on the edge, the marcescent marginal 
fibres detach themselves as more or less numerous, delicate or coarse, straight or often curled 
threads, of a whitish or ashy or reddish color. In the young leaf they are most conspicuous, 
especially near the involute point of the leaf, but in old ones they sometimes become obsolete. 
8 A fourth form of leaves is described in the books as this to be a factitious species, made up of Yucca flowers 
having marginal spines, and Y. spinosa, HBK. Nov. Gen. (similar to those of Y. Treculiana) and the spiny leaves of 
I. 289, from Mexico, is quoted as the representative of this Dasylirion acrotriche 
type. But the inspection of the Berlin Herbarium proves 
