556 



ECOLOGY 



Usually the guard cells are adjoined or surrounded by subsidiary cells 

 (figs. 796-798), which sometimes differ from the other epidermal cells. 

 Most guard cells differ from the adjoining epidermal cells in possessing 

 chloroplasts, starch grains, and abundant cytoplasm with a prominent 

 nucleus. In cross section (fig. 795) the guard cells reveal some com- 

 plexity of structure. Toward the ventral (slit) side, the upper parts of 



the walls project in sharp cutin- 

 ized ridges that almost meet 

 when the stoma is closed. In 

 most cases the corresponding 

 lower walls project similarly but 

 less prominently, and the space 

 between them is greater. In the 

 median region is a third but 

 rounded pair of projections with 

 slightly thickened walls; since 

 stomatal closure is effected by the 

 meeting of these median projec- 

 tions, the very narrow slit be- 

 tween them (known as the central 

 slit) might properly be regarded 

 as the true stoma. The narrow 

 spaces between the ridges above 

 and below are known, respec- 

 tively, as the outer and the inner 

 slits or openings, while the larger 



797 



FIGS. 796-798. Stomata from the leaf 

 of a grass (Poa) : 796, a stoma from the blue 

 grass (Poa pratensis), as seen in surface view, 

 showing the guard cells (g) with their dumb- 

 bell-shaped lumina, the chloroplasts being 

 confined to the enlarged portions; note the 

 subsidiary cells (b) with their prominent 

 nuclei (w); 797, a median cross section of 

 the stoma of Poa annua, showing the narrow 

 portion of the guard-cell lumina (g) and the 

 relatively large median portion of the sub- 

 sidiary cells (b); 798, a cross section through 



the end portion of the stoma of Poa annua, 

 showing the enlarged part of the guard-cell 

 lumina (g), the terminal portion of the sub- 

 sidiary cells (6) being relatively small ; highly 

 magnified. 797 and 798 from HABER- 



LANDT. 



spaces separating them from the 

 central slit are the outer and the 

 inner vestibules. The dorsal 

 walls are of cellulose and are 

 much thinner than are the ventral 

 walls, frequently they are connected with the subsidiary cells in such a 

 way that the walls of the latter may swing back and forth like hinges, 

 moving the guard cells with them (figs. 800, 806). Below the stoma 

 is a rather large air cavity. 



Structural variations of stomata. Taxonomic variations. Many 

 stomatal variations have no obvious relation to external conditions, as in 

 grasses and conifers, where a certain structural plan appears to charac- 

 terize an entire family, regardless of environment. In grasses (figs. 



