74 PLANT GROWTH 



Figure 15 shows a leaf highly magnified to make clear the 

 relative size, shape, and arrangement of its cells. The epi- 

 dermis has three chief functions: first, to protect the inner 

 cells of the leaf against mechanical injury, for which it has 

 tough surface walls, supported in most leaves by more than 

 a million walls per square inch; second, it prevents the exces- 

 sive loss of water, for which it has a layer of a waxy substance 

 called cutin on the outside; and third, it must allow the gases 

 necessary for photosynthesis and respiration to enter and 

 leave the leaf. For this purpose it has many small openings 

 called stomata, which make continuous channels for the air 

 from the atmosphere to the air of the intercellular spaces of 

 the leaf. 



A stoma is a small opening made by two curved cells (see 

 Fig. 15) called guard cells. The walls facing each other are 

 thicker than the remainder of the wall of each guard cell. 

 When they absorb water the pressure causes more expansion 

 of the thinner walls than of the thicker walls, in such a way 

 that the opening becomes larger. When a guard cell loses 

 water the stoma gradually closes as the pressure within the 

 guard cells decreases. When soil water is deficient this may 

 continue until the stoma is entirely closed. 



The guard cells, which contain chloroplasts and make 

 sugar, have more concentrated contents than the epidermal 

 cells, which do not have chloroplasts. This difference causes 

 the water to diffuse to the guard cells, and so maintains a 

 high pressure in them during the day, except as explained 

 above. At night most stomata close for at least a part of the 

 time, and some of them remain closed all night. 



Stomata vary in size, distribution, and number, but are 

 always microscopic, and may be as small as one-ten-thou- 

 sandth of an inch wide and five times as long. A very com- 

 mon size is three to five times the size just given. The total 

 area of stomatal openings is usually about one to two per 

 cent of the leaf area. Recent researches have shown that 



