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THE AM KMC AX XATURAL1ST 



[Vol. XLII 



In Dasylerion wheeled a second similar constriction 

 occurs half way down this passage, so that in cross section 

 it appears like Dasylerion texanum. In this species the 

 epidermal cells are produced into papilke. 



Going back to the simple type of stoma found in Yucca 

 aloifolia we can trace variation in another direction. In 

 the stomata of the lower side of the leaf of Yucca recurvi- 

 folia the four sides of the supra-stomal passage are pro- 

 duced into lips ; those parallel to the long axis of the leaf 

 being beneath the others (Figs. 7 and 8). On the upper 

 side of the leaf the lips are not as well developed, and 

 are all in the same plane, as are also those of Yucca 

 australis and Y. treculiana. 



In Yucca radiosa (Fig. 9) and Y. constrict a only two 

 of the lips are developed, so that we have an opening 

 somewhat like the ones in Agave americana rotated 

 through a right angle. In Yucca rostrata the lips have 

 developed so as to close the opening in the middle, leaving 

 only an opening at each end (Fig. 10). 



In Y. glauca (Fig. 11) four additional lips have de- 

 veloped above the two as in Y. radiosa, and the accessory 

 cells have sunken considerably beneath the level of the 

 other epidermal cells. 



In Agave victoria regina (Fig. 12) the presence of six 

 lips is accompanied by the meeting of one pair and by the 

 division of the supra-stomal passage into two dead air 

 spaces (Fig. 13). Here the epidermal cells secrete an 

 immensely thick layer of reserve or water-storing ( ?) 

 cellulose under the cuticle (Fig. 13). The leaf is very 

 succulent and the vascular bundles are confined to the 

 middle layer. 



In Agave schotti the shape of the supra-stomal passage 

 changes greatly as we go inward. On the surface it is 

 elongated at right angles to the long axis of the leaf, 

 while farther down it is elongated parallel to the long 

 axis (Fig. 14). 



Besides being sunken singly between the level of the 



