CHAPTER VI 



AERATING TISSUE 



The aerating tissue of the plant performs a threefold func- 

 tion: first, it permits the exchange of gases during photo- 

 synthesis; secondly, it permits the entrance of oxygen and the 

 exit of carbon dioxide during respiration; and, thirdly, it permits 

 the exit of the excess of water absorbed by the plant. 



The above functions are carried on by the stomata, the 

 water-pores, the lenticels, and the intercellular spaces of the 

 plant. The stoma functions as the chief channel for the passage 

 of C0 2 -laden air into the leaf and of oxygen-laden air from the 

 leaf to the atmosphere. The stoma also functions as an organ 

 of transpiration, since through the stoma a large part of the 

 excess water of the plant passes off into the air. 



WATER-PORES 



In certain plants the primary epidermis is provided with 

 openings resembling stomata, but unlike stomata the orifice 

 remains open, and instead of being located on the upper or 

 lower surface of the leaf, they are located on the margin of 

 leaves immediately outward from the veins. Water is given 

 off to the atmosphere from these openings. Such an opening 

 is usually designated as a water-pore. 



STOMATA 



The chief external openings of the epidermis of leaves, of 

 herbs, and of young wood stems are known as stomata. Sur- 

 rounding the stoma are two cells known as guard cells. 



Guard cells differ greatly in form, in size, in arrangement, 

 in occurrence, in association, in abundance (Plates 53, 54, and 

 55), and in color. The guard cells surrounding the stoma vary 

 in form from circular to lens-shaped. In most leaves the outline 



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