48 EPIDERMIS 



are regulated by the relative degree of turgor in the guard cells, 

 which in turn causes slight alterations in their shape. When the 

 guard cells are turgid, the width of the pore is at a maximum 

 while closing of the aperture occurs when the turgor of the cells 

 decreases. A discussion of the variation in the construction of 

 the walls of guard cells and of the physiological factors influenc- 

 ing the turgor movements of these cells, however, is beyond the 

 scope of this book (cf. Ilaberlandt pp. 445-477). Guard cells 

 originate by the anticlinal division of certain protoderm cells 

 into two dissimilar daughter cells. In the simplest condition, one 

 of these cells functions as an initial cell and by an anticlinal 

 division directly produces the two guard cells; the other daughter 

 cell meanwhile differentiates into an ordinary epidermal cell. 

 But many deviations from this simple type of stomatal develop- 

 ment occur, especially with reference to the formation of the 

 subsidiary cells. The latter differ in form and arrangement from 

 the neighboring epidermal cells and are believed to cooperate 

 physiologically with the guard cells in regulating the width of 

 the stoma (for information regarding the various modes of 

 stomatal development cf. De Bary, pp. 39-45, Porterfield, 1!).'}7, 

 and Yarbrough, 1934). 



A study will be made first of the structure of the uniseriate 

 epidermis. Later in this exercise a brief explanation and direc- 

 tions for study of the multiple epidermis and of t)ichomes will 

 be given. 



II. Material for the Study of the Uniseriate Epidermis— 



1. The bull)-sc{ile of the onion (Allium Cepa). Keniove, with 

 foi'ccps. a small strip of epidermis from the outer or abaxial 

 surface of the bulb-scjile and mount it carefully in w.ilci-. rnder 

 low magnification, note the rather orderly arrangement of the 

 "rectangular" or tabular epidermal cells. Stomata. which may 

 be abortive or "abnormaT' in appearance, are occasionally pres- 

 ent in this mntei-ial. Fndei- high magnification, it will be seen 

 that the radi<d walls of the epidermal cells are i)rovided with 

 nuinei'ons small simple pits. Because of the comparatively small 

 i-adial depth of the epidermal cells, nuclei are i-eatlily seen. Tiie 

 cytoi)lasm is highly vacuolat(^ and often is actively streaming or 

 moving within many of the cells. Small greenish-yellow bodies, 



