194 BOTANY AND PHARMACOGNOSY. 



tic, and when the cells are turgescent, as when there is an abund- 

 ance of water and root pressure is strongest, the contiguous 

 walls of the guard cells recede from each other, forming an open- 

 ing between the cells, thus permitting the exit of the excess of 

 water taken up by the plant and the exhalation of the oxygen 

 given off during assimilation, as well as the intake of the carbon 

 dioxide used in photosynthesis. The cells beneath the stomata are 

 loosely arranged, there being large intercellular spaces so that 

 carbon dioxide soon finds its way to the cells containing the 

 chloroplastids. On the other hand when the amount of water in 

 the plant is reduced below the normal and the plant shows signs 

 of wilting the guard cells expand and the stoma or pore is closed, 

 thus preventing transpiration or at least reducing it. 



The guard cells may be slightly raised above or sunk below 

 the surrounding epidermal cells, the number of the latter being 

 characteristic for certain plants. (Compare Figs. io6, 263, 286.) 



In surface view the stomata may be elliptical or circular. 

 They occur in the largest numbers on the blades of foliage leaves, 

 being more numerous on the under surface, except in aquatic 

 plants where they occur only upon the upper surface. 



Water Pores.^Near the margin of the leaf and directly 

 over the ends of conducting cells, not infrequently occur stomata, 

 in which the function of opening and closing is wanting, and 

 which contain in the cavity below the opening water and not air, 

 thus differing from true stomata (Fig. 106, D, E). These are 

 known as water pores, and they give off water in the liquid 

 form, the drops being visible on the edges of the leaves of nas- 

 turtiums, fuchsias, roses, etc., at certain times. 



Cork cells replace the epidermal cells of roots and stems 

 that persist yearafter year (Fig. 11 ^,K). They differ from the epi- 

 dermal cells in that the walls are uniformly thickened and on sur- 

 face view are polygonal in shape. The walls consist of suberin, a 

 substance allied to cutin ; in some instances they also contain ligno- 

 cellulose, forming cork stone-cells, as in asclepias and calumba 

 (Fig. 198). The young cells may contain a thin layer of proto- 

 plasm and a nucleus; they usually also contain brownish tannin, 

 or tannin-like compounds, and occasipnally crystals of cerin, or 

 calcium oxalate (Fig. 302, //). 



