STOMATA OF MNIUM 



453 



entire absence of hinge-areas, the thickness of the dorsal, and the 

 thinness of the ventral walls of the guard-cells ; all these characters 

 clearly indicate that the mechanism, in this case, is altogether different 



Fig. 171. 



Diagram showing a stoma of Milium cuspidatum in the open (heavy lines) and in the 

 closed (faint lines! condition. T.S. For explanation see text. 



from anything that we have previously encountered. Attention may 

 next be directed to the appended table of measurements : 



Total length of the stoma, - 

 width 



Width of a single guard-cell, 

 front-cavity, 

 pore-passage, 



Stoma open. 



/* 



51 



42 



17 



9 



8 



Stoma nearly 

 closed. 



51 



42 

 20 



9 



2 



As is quite evident from these figures, the total length and 

 breadth of the stoma are always the same, whether the pore is open or 

 closed. Closure or contraction of the pore depends entirely upon an 

 increase in the width of the individual guard-cells. In the closed 

 condition of the stoma, the cross-sectional outline of each guard-cell is 

 approximately elliptical. When turgor increases, this outline tends to 

 become more nearly circular. But since the thick dorsal and outer 

 (upper) walls are practically immoveable, this change in shape can 

 only be effected by a straightening out of the bulging thin ventral 

 walls ; as these straighten themselves, they draw apart and thus 

 enlarge the pore. Here the alterations in the shape of the guard-cells, 

 upon which the movements of the stoma depend, are not confined to 

 the plane parallel to the leaf surface ; displacements also occur in 

 planes which are perpendicular to that surface. 



A mechanism very like that of Milium had previously been 

 described by Schwendener in the case of Hclleborus. The changes of 

 form which take place in the latter instance, are indicated in the 

 adjoining diagram (Tig 172). Here, as in Fig. 171, the heavy lines 



