530 THE POTATO 



cells, and are continuous one with the other 

 throughout the leaf. The air spaces are in 

 direct communication with the outside air 

 through tiny openings known as stomata. These 

 openings on the surface often occur on both sides 

 of the leaf, but in most crop plants they are more 

 numerous on the under side, or may be entirely 

 wanting on the upper. They are very important 

 to the plant, since it is through them that the 

 plant food derived from the atmosphere finds 

 ingress to the leaf, which is really the plant's 

 Starch factory. They also serve the very impor- 

 tant function of allowing the escape of the watery 

 vapor and the oxygen which is released during 

 the period of active starch formation. Neces- 

 sary as are these openings on the leaf surface, they 

 are also sources of great danger to the plant. The 

 rest of the surface of the leaf is so thickened or 

 waterproofed with the substance known as cutrae 

 that water cannot enter nor escape through it, 

 neither can germs from the atmosphere find en- 

 trance into the leaf. When germs or other fungi 

 secure admission to the tissues of the plant it is 

 usually either through the stomata or through 

 wounds upon the leaf or other part of the plant. 

 It is for this reason that insect attacks are so often 

 followed by fungous and germ diseases. Simi- 

 larly wounds which plants accidentally receive or 

 which are made by pruning are also often followed 

 by diseased conditions. 



"The crude sap of plants, which is essentially 

 the soil water with the gases and soluble minerals 

 of the soil dissolved in it, is taken up by the roots 

 and conveyed through the stem to the leaves. 

 The air which has been admitted to the leaf 

 through the stomata supplies the carbon dioxide. 



