LEAVES 



557 



796-798) there are large subsidiary cells behind the dumbbell-shaped 

 guard cells, in which the lumina are narrow at the center and enlarged 

 at the ends. In the conifers (fig. 1039) and in Equisetum, the guard cells 

 and the adjoining epidermal cells are incurved below the epidermal level, 

 the stomata thus lying at the base of pits; the walls are very thick and 

 heavily cutinized, while the outer ridges are unusually prominent. In 

 cycads the walls may be lignified instead of cutinized. The sporophytes 

 of mosses and of Anthoceros have stomata much like those of higher 

 plants, except that the inner cutin ridges usually are wanting and that 

 the guard cells vary in number from one to four. In the gametophytes 

 of the Marchantiaceae there are capacious air chambers, which com- 



FiG. 799. A section through the thallus of Marchantia polymorpha, showing an 

 air chamber (a), an air pore (/) with its surrounding cells, the chlorenchyma composed 

 of short alga-like filaments (c), and a tissue of closely packed cells (s) in which the chloro- 

 plasts are sparse; the latter tissue is rich in water; highly magnified. From COULTER 

 (Part I). 



municate with the exterior through simple air pores that sometimes are 

 enclosed by chimney-like tiers of cells (as in Marchantia, fig. 799). 



Variations associated with habitat. In many xerophytes the stomata 

 occur in pits; sometimes, as in Dianthus (fig. 800), there is one stoma at 

 the base of each pit, while in other cases (Nerium, Begonia) the stomata 

 occur in groups. In Populus pyramidalis the pits of the upper leaf sur- 

 face are deeper than are those of the lower. In most xerophytes the 

 guard cell walls are very heavily cutinized (fig. 801); sometimes the 

 walls are thickened uniformly, but more frequently there are projecting 

 ridges of remarkable shapes and dimensions. ' In Nipa fruticans, for 

 example, the ventral walls project in such a way as to form a most 

 tortuous passageway between the leaf interior and the outside air (fig. 

 802). Both in heavy cutinization and in depression below the surface 



