STRUCTURE OF STOMATA 305 



indeed. Evaporation from the mesophyll cells does 

 however, take place even in saturated air, because the 

 living cells, owing to the process of respiration, which 

 is always going on and which liberates heat, are kept 

 at a somewhat higher temperature than the surrounding 

 air. 



Structure and Mechanism of Stomata. — We have seen 

 that if the leaf were covered by a perfectly continuous 

 layer of cuticle it would lose little or no water, but it 

 could not obtain carbon dioxide from the air. The 

 stomata allow of the carbon dioxide getting in, but they 

 also allow of water vapour getting out, and though this 

 may be useful to the plant it introduces the danger of 

 water loss so rapid that it cannot be covered by a 

 corresponding supply from the root. This danger is 

 partly met by the automatic closing of the stomatal 

 pore when there is a deficiency of water in the tissues of 

 the leaf. The guard cells (^i the stoma which surround 

 the pore, unlike the other epidermal cells, contain 

 chloroplasts (Fig. 51, C, E), and are thus able to make 

 sugar and maintain a high osmotic pressure and power 

 of absorbing water. The guard cells are firmly joined 

 to one another at the two ends, but in the centre there 

 is a spUt (the stomatal pore) between them, while the 

 side walls away from the pore abut on, and can be 

 pushed into the cavities of, the adjacent epidermal cells. 

 The cavity of the guard cells is often confined to the 

 middle part of the cell as seen in vertical section : above 

 (next the outer air) and below (next the intercellular 

 space into which the pore leads) the wall is thick and 

 cutinised. 



When the guard cells become turgid the cell cavities 

 increase in size, and since the ends and the top and 

 bottom are firmly held, the side walls increase in length, 



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