314 THE AMEBIC AN NATURALIST [Vol. XLII 



imagine a lamella removed from the leaf when turgid, 

 the cells composing it would be rounded on their free 

 surfaces. If such a lamella were placed on another so 

 that some of its cells rested on the apices of the cells of 

 the one underneath, we should get a condition similar to 

 that in Nolina texana (Figs. 18 and 19) and allied forms. 

 Fig. 18 is drawn from a thick longitudinal section of the 

 leaf, and the irregularities in the lamelhe obscure their 

 distinctness. The lamellae are additionally supported by 

 columns of elongated cystoliths running parallel to the 

 long axis of the leaf and connecting one lamella with 

 another. 



Mechanical and Vascular Tissue.— The leaves of Yucca 

 and its allies are covered by a very thick cutinized epi- 

 dermis which is quite stiff and supports the soft tissue 

 beneath, thus forming an exoskeleton. In smooth leaves 

 the bundles are distributed through the interior of the 

 leaf, in thick leaves being arranged in several rows 

 (Fig. 25). Each bundle is composed of vascular tissue 

 supported by a large amount of mechanical tissue, and 

 thus the bundles form an internal skeleton. In those 

 leaves in which grooves occur the ridge between two 

 grooves is heavily braced within with mechanical tissue 

 (Fig. 23). These braces often extend inward until each 

 fuses with a bundle (Fig. 23) and in some species all the 

 bundles are attached to the inner edges of these braces 

 (Fig. 15). In Dasylerion glaucophyllum and some 

 species of Xolina (Fig. 20) the braces of one side of the 

 leaf usually fuse in the middle with those of the other 

 side, making a very rigid structure. 



Relation of Habitat to Structure of Stoma.— The com- 

 plexity in structure of stoma in these forms lead Dr. Bray 

 to suggest that I see whether there was any correlation 

 between the structure of the stomata and the aridity of 

 habitat. I found that there was in all except those that 

 had the supra-stomal passage much elongated in the long 

 axis of the leaf. Yucca aloifolia, Y. gloriosa and their 



