ELLIOTT: VARIATIONS OF QUERCUS. 47 



STOMATA. 



The size and number of the stomata as revealed in the 

 bleached leaves are features that enable one readily to dis- 

 tinguish between the four oaks. In all the leaves the stomata 

 were more numerous near the midrib than near the edge of 

 the leaf. By the usual method of averaging fields the count 

 of No. 2 ran up to about 2000 per sq. mm. In order to get 

 more definitely and accurately the number of stomata, 1 sq. 

 mm. of leaf surface magnified 134 diameters was projected on 

 paper and each stoma marked. (Figs. 23 to 26.) This showed 

 the following numbers per sq. mm.: No. 1, 1074; No. 2, 1252; 

 No. 3, 912 ; No. 4, 785. The stomata of No. 3 often occurred 

 in rifts and clumps while those of the other oaks seemed more 

 evenly distributed. 



Average length of guard cells : No. 1, .0188 mm. ; No. 2, .019 

 mm. ; No. 3, .019 mm. ; No. 4, .0189 mm. Average width : 

 No. 1, .005 mm. ; No. 2, .0059 mm. ; No. 3, .006 mm. ; No. 4, 

 .0059 mm. Some of the largest stomata of No. 3 were four 

 times as large as the smallest, a variability immediately no- 

 ticeable. The others were more uniform in size. 



The extreme minuteness of the guard cells and the fact that 

 the stomal opening extends no more than a third of the length 

 of the cells makes the mechanism of the stomata an interest- 

 ing problem which I think is not fully explained by papers on 

 the subject. The stomata are of the "gramineous" type, but 

 looking down on a surface view of a stoma one sees deep de- 

 pressions between the guard cells where these extend beyond 

 the ends of the stomatal opening, and the heavy inner walls 

 of the two guard cells facing each other here stand far enough 

 apart to afford considerable room for movement when the cells 

 become turgid (figs. 16 and 19, P and P'). The enlarged 

 lumen at the ends greatly increases the surface of the inner 

 heavy wall subject to the pressure of the cell sap, and these 

 walls being free to move, as just shown, would undoubtedly 

 move toward each other with the increase of turgidity. The 

 points of contact of the two guard cells at the ends of the 

 stomatal opening acting as fulcrums (fig. 16, O), the move- 

 ment of the thickened walls must exert considerable leverage 

 tending to open the stoma. (Figs. 16 and 18, r and r'.) This 

 mechanism alone would seem to suffice to open the stoma, and 

 it must be one of the most important factors, since a study 



