No. 497] ON XEBOPHTTIC ADAPTATIONS 



309 



dide of zinc. A freezing microtome might have been 

 of service. Celloidin did not penetrate thoroughly in six 

 weeks and was abandoned. Thin paraffine sections were 

 cut, but where bundles were isolated in succulent tissue 

 they usually tore through part of the latter. The ex- 

 perimental side of the subject was not touched. 



Yucca and its allies are mostly xerophytic plants that 

 have been modified along various lines to adapt them to 

 their xerophytic habitat. Boot, stem and leaf are suc- 

 culent, thus serving for water-storage tissue. The leaves 

 are closely set in a rosette, thus protecting one another 

 against too great insolation and transpiration. In 

 Agave, Hesperaloe, Dasylerion, Nolina and some species 

 of Yucca the stalk is very short, thus enabling the leaves 

 to shade the ground over the larger roots and protect 

 them from drying. The leaves are especially modified: 

 the epidermis is greatly thickened and heavily cutinized, 

 the skeleton more or less rigid, the stomata sunken 

 beneath the surface, either singly or in grooves, and the 

 assimilation tissue many layers thick. 



The Respiration System. — The simplest type of stoma 

 in this group is probably found in Yucca aloifolia tenux- 

 folia (Figs. 1 and 2). In this species the guard cells 

 are not sunken much beneath the general epidermal sur- 

 face, but the very thick epidermis is pierced by an air 

 passage nearly square in cross section (Fig. 1), leading 

 to the slit between the guard cells. Beneath the guard 

 cells is an intercellular air space of considerable size 

 leading into small intercellular air spaces. The epi- 

 dermis is heavily cutinized and the cutin extends inward 

 through the stoma and lines the upper part of the air 

 space beneath. The assimilation tissue is palisade near 

 the epidermis ; but the cells show a tendency to arrange 

 themselves radially around the vascular bundles. There 

 are large intercellular air spaces in the interior of the 

 leaf. Stomata of like character were found in Yucca 

 aloifolia, Y. aloifolia marginata, Y. aloifolia conspicua, 



