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 CO,-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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