316 THE AMEBIC AN NATURALIST [Vol.XLII 



girders extending through the leaf in a dorso-ventral 

 direction. The epidermis bulges out along the lines of 

 attachment of the girders and sinks in between them. 

 The epidermal cells bordering the grooves thus formed 

 grow out into long interlocking teeth. The teeth get 

 longer and the groove narrower as the leaf develops, thus 

 protecting the stomata from the dry air. 



In the very young seedling the leaf is somewhat tri- 

 angular in cross section (Fig. 21) with a vascular bundle 

 in the center and one in each lateral angle and a bundle 

 of mechanical tissue in the midrib. The epidermis is 

 cutinized at this early stage. 



I kept a seedling in a greenhouse of the University of 

 Pennsylvania about two years. The leaves did not grow 

 stout as in plants from the natural habitat and I think the 

 increased moisture and diminished insolation affected the 

 thickness of the leaves (Fig. 22). The leaves were very 

 thin; the mechanical tissue uniting with the vascular 

 bundles formed beams connecting the dorsal with the 

 ventral epidermis between the bands of stomata, but the 

 grooves only began to develop. It seems remarkable that 

 a plant so adapted to xerophytic conditions should be 

 able so quickly to adjust itself to a moist atmosphere, 

 and I think the subject merits further investigation. 



1903. Bray, W. L. "Tissues of some plants of the Sotol region." Bull. 



Torrey Bot. Club, p. 30. 

 1890. Drude, O. "Handbuch der Pflanzen geographic " (Die Drachen- 



baumgruppe). 



1890. Gilson, E. "La suberine et les cellules du liege." La Cellule, 



VI, p. 63. 

 1 903. Hansgirg. ' ( Phyllobiologie. ' ' 



1903-1905. Karsten and Schenk. " Vegetationsbilder. " Ser. 1, Pis. 28, 

 33, 34, 46; Ser. 2, Pis. 19, 22, 59; Ser. 3, Pis. 25, 26. 



1895. Ludwig, F. "Lehrbuch der Biologie der Pflanzen." 



1896. Mulford, A. I. "A Study of the Agaves of the U. S." Rep. Mo. 



Bot. Garden, '96. 

 1902. Trelease, W. "The Yucca?." Eep. Mo. Bot. Garden, no. 13. 



