ABSORPTION TISSUES 117 



ovary of the flower. The lower surface of the leaf 

 shows the greatest number of stomata and the upper 

 leaf surface may be entirely devoid of them. The 

 stoma proper is an oval opening perforating the epi- 

 dermis, and is surrounded by two semicircular cells 

 termed guard cells. The guard cells are in turn sur- 

 rounded by a number of epidermal cells called sur- 

 rounding; bordering or neighboring cells. In literature 

 on vegetable histology, the term stoma includes the 

 opening in the epidermis together with the two guard 

 cells. The number and arrangement of the surround- 

 ing cells is fairly constant for a given species, and this 

 fact may often be used for purposes of identification. 

 The guard cells are modified epidermal cells and usually 

 contain a small number of chloroplasts as a proto^- 

 plasmic cell content. The stoma may be above, 

 below or on the same level as the epidermal cells. 

 The size of the breathing pore varies under different 

 conditions and is regulated by contraction and expan- 

 sion of the guard cells. Variations in the amount 

 of water in these cells are responsible for the changes 

 in size. 



Respiration. — A supply of air is just as necessary 

 for the continuance of life in plants as it is in animals. 

 The air entering the stomata or lenticels contains 

 oxygen and carbon dioxide. The oxygen is essential 

 to cell life and activity, while the carbon dioxide is 

 used by the plant in the manufacture of nutrients. 

 The respiratory processes include the absorption of 

 oxygen through the stomata or lenticels and the dis- 

 tribution of this element to . the cells through inter- 

 cellular spaces or by gradual diffusion through thin- 

 walled cells. Carbon dioxide is one of the waste 



