STOMATA OF THE GLUMIFLORAE 



451 



phyllodes of Acacia leprosa, A. farinosa, and A. acinacea, and in the 

 leaves of Melaleuca uncinate/,, Hakea suaveolens, etc. It should be 

 stated that they are generally restricted to the older organs ; in young 

 leaves and phyllodes the guard-cells are asymmetrical in structure and 

 display the ordinary or Amaryllis-type of mechanism. 



The peculiar stomata of the Gramineak and Gypekaceae represent 

 a special modification of the ordinary symmetrical type. Here each 

 guard-cell is shaped like a dumb-bell, the ends being dilated and 

 connected together by a narrow middle-piece, the upper and lower 

 walls of which are very thick-walled throughout, so that the cavity is, 

 in this portion of the guard-cell, reduced to an insignificant transversely 



Fig. 169. 



Stoma of Poa annua. A. Surface view. B. T.S. through the dilated ends of the 

 guard-cells. C. T.S. through the middle of the guard-cells. 



elongated slit (Fig. 169 c). Towards either end, the thickened strips 

 rapidly become narrower and finally become reduced to the same 

 thickness as the thin walls enclosing the dilated terminal portions of 

 the guard-cell (Fig. 169 a). The forces responsible for the movements 

 of a stoma of this type must obviously be altogether located in the 

 dilated ends of the guard-cells except in so far as the well-developed 

 subsidiary cells, which are always present in such cases, may play a 

 part in the process. Schwendener has, in fact, shown that a rise of 

 turgor in the guard-cells of a stoma is followed by an increase in the 

 volume of their dilated ends. As a result, the total width of the two 

 ends of the stoma increases ; points such as m and n in the adjoining 

 diagram (Fig. 170) draw apart from one another, pari passu with 

 more distally situated points, such as a and b, because of the 

 comparative rigidity of the intervening stretch of thickened wall 

 (m-a and n-b). Obviously a further consequence is an increase in the 

 angle mon, and hence in the width of the pore-passage s. As seen 

 in surface view, the pore-passage is shaped like a hexagon, elongated in 



