No. 497] ON XER0PHYT1C ADAPTATIONS 315 



varieties have a supra-stomal passage nearly square in 

 cross section (Fig. 1) that goes straight down to the 

 stoma, and these species inhabit coast lands which, 

 although hot, are humid. In Y. austral is, Y. treculiana 

 and Y. recurvifolia the supra-stomal passage is guarded 

 by four lips (Fig. 7) and these species inhabit a region 

 from the Gulf coast to the dry interior of the United 

 States and Mexico. In Y. radiosa and Y. constricta the 

 supra-stomal passage is closed in the middle (Fig. 9) and 

 these species inhabit southern Arizona, western Texas 

 and adjacent country. In Y. rostrata two widely sepa- 

 rated openings lead into the supra-stomal passage (Fig. 

 10), and we find this plant in the deserts of northern 

 Mexico. In Agave victoria regina we find a similar 

 structure with the addition of four small lips (Fig. 12) 

 and we find this plant also in the deserts of northern 

 Mexico (Coahuila). In Yucca glauca we find a structure 

 similar to the preceding save that the two lower lips 

 are slightly separated (Fig. 11), and this plant inhabits 

 the slightly less arid lands from South Dakota to New 

 Mexico and North Texas. In Agave schotti the supra- 

 stomal passage is compressed in one axis (Fig. 14) and a 

 little lower down is compressed at right angles to the 

 first (Fig. 14, dotted line) and we find this plant in the 

 deserts of southern Arizona. Finally, the Nolinas, whose 

 stomata are placed in deep grooves guarded by inter- 

 locking teeth, cling to crevices in the rocks in western 

 Texas and northern Mexico, where not only is the air 

 dry and the sun hot, but there is extremely little soil to 

 hold a store of moisture. 



Development of Leaf in Nolina Texana.— The very 

 young leaf in the adult plant is not deeply grooved as in 

 Fig. 19 or 20 and the mechanical tissue is not well 

 developed. The formation of grooves is associated with 

 the formation of ribs of mechanical tissue supporting the 

 epidermis between the bands of stomata, and by growth 

 inward uniting with the vascular bundles, thus forming 



