CELL-CONTEXTS AND FORMS OF CELLS. 279 



ing or pore between them, which leads to a cavity beneath it. The 

 two cells of the stoma are known as GUARD CELLS (Fig. 147, G). 

 The adjoining walls of the guard cells are alike in transverse sec- 

 tion, but the cells vary in shape in different plants. The guard 

 cells are more or less elastic, and when the cells are turgescent, as 

 when there is an abundance of water and root pressure is strongest, 

 the contiguous walls of the cells recede from each other, forming 

 an opening between them, 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. 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 flatten and the open- 

 ing or pore is closed (Fig. 214). The cells beneath the stoma are 

 loosely arranged, so that the air containing carbon dioxide may 

 be readily diffused to the cells containing the chloroplastids. 



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. 147, 211-218.) 



Stomata 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. 147, 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 nasturtiums, 

 fuchsias, roses, etc., at certain times. 



Plant Hairs.- -The epidermal cells are sometimes specially 

 modified centrifugally, giving rise to papilke, to which the velvety 

 appearance of the petals of flowers is due ; in other cases this 

 modification is in the form of hairs or trichomes (Figs. 148-155). 

 These may be unicellular or multicellular, and in addition the 

 latter may be glandular or non-glandular. Glandular hairs possess 

 a head-like apex, consisting of one or more cells, and they secrete 

 oil, mucilage, and other substances (Figs. 124, 125, 149, 15)- 



