194 BOTAXY 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 p'ermitting the exit of the excess of 

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

 given ofif 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 

 tlu' i^lnit is reduced below the normal and the plant shows signs 

 of wilting the guard cells fiatten and the stoma or pore is closed. 



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.) 



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. 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. 



Periderm. The epidermis is not adapted for the protection 

 of the perennial plant organs on account of its thin, frequently 

 delicate structure and its inability to continue with the increase in 

 thickness of stems and roots. Hence it becomes replaced by the 

 periderm, which consists of a lasting tissue, the cork and of a meri- 

 stematic tissue, the piiellogen, which reproduces the cork when 

 it becomes torn or destroyed by the continued growth in thick- 

 ness of stems or roots. Cork is not only of sub-epidermal origin, 

 but may occur deeper in the cortex, or even inside the endodermis. 

 In the latter case, as in roots, it owes its existence to the activity 

 of the pericambium. Superficial, i.e., hypodermal cork, is ex- 

 tremely rare in roots. 



