

1920] BROWN— STOMA TA 299 



more or .less cutinization. Individual cells varied in form and size 

 from the base to the apex of the shoot. Those in the apical region 

 were much elongated (fig. 6), some of them measuring 175 m- 

 Their lateral walls were straight, and their shorter end walls were 

 wavy in outline. In the basal region the epidermal cells were 

 not longer than 85 ju, and nearly all of the walls were slightly wavy 

 when seen from the surface of the shoot (fig. 5). 



However striking the changes in external form and gross 

 structure due to etiolation may appear, the internal changes were 

 even more interesting. When the surface of the etiolated shoot 

 was examined under low magnification, numerous small elevations 

 were observed which reflected the light. Closer investigation 

 showed these structures to be minute papillae, each one bearing a 

 stoma at the apex (figs. 13, 25, 27). Their nature was better seen 

 in longitudinal and transverse sections of the shoot (figs. 25, 26), 

 and they were usually found to be epidermal, the entire structure 

 arising in most cases from a single cell; in others, chiefly from a 

 single cell, but augmented to some extent by division of neighbor- 

 ing cells of the cortex. The papillary initial appeared to be 

 analogous to a stoma mother cell. Evidently the stimulus to 

 division had continued to act on the stoma mother cell and its 

 progeny for some time and had met with a response, even after 



1 



cutinization of the surface cell walls had occurred, for in one 

 instance the resulting structure, unable to grow outward, had 

 pushed inward among the undifferentiated cells of the cortex 

 (fig. 28). The first division of the papillary initial was either a 

 vertical one, as in the ordinary process of stoma formation (figs. 

 7-9), or else the cell became papillate and divided by an oblique 

 wall (figs. 18, 19). The lower of the two cells thus formed then 

 divided by a vertical wall, and the upper cell followed with a trans- 

 verse division (fig. 2.0). After this the walls were chiefly oblique. 

 Eventually two guard cells were cut off, which, seen from the 

 surface, resembled the guard cells of a normal stoma (fig. 27), 



but which were more or less wedge-shaped in transverse sections of 

 the shoot (figs. 21-25). The mature papilla might be compared 

 to a hydathode in surface view, but in internal structure and in 

 origin it was very different; the tissue was not glandular, no 



