Chapter Eleven 

 LEAF STRUCTURE 



The leaves are outgrowths of the stem, consisting, usu- 

 ally, of a slender stalk, the petiole, frequently having at its 

 base small appendages called stipules, and the broadened 

 conspicuous portion called the blade. The leaves begin to 

 develop in the buds by the protrusion of a small portion of 

 the dividing tissue of the stem tip. All the leaf cells retain 

 this ability to divide for some time, so that a leaf grows in 

 every region. For this reason repeated spraying is necessary 

 to keep the growing leaves covered. 



Leaves are of many shapes or forms, but two general 

 classes should be considered. Simple leaves, such as the 

 zinnia, have the blade in one piece. In compound leaves the 

 blade is completely divided into leaflets, either pinnately as 

 in the tomato (Plates I and II) or palmately as in white 

 clover. Thus, to determine which are simple leaves on a 

 branch and which are compound leaves you must distinguish 

 between petiole and stem. A stem ends in a more or less 

 conspicuous bud, which never occurs at the tip of a com- 

 pound leaf. 



The veins of leaves consist of strengthening tissues and 

 conducting tissues which are continuations of the conduct- 

 ing tissues of the root and stem, and they function in the 

 same way. They branch and rebranch until the entire leaf 

 is penetrated with a microscopic network of the cells of con- 

 duction. They are divided into two classes: parallel vena- 

 tion when they run in nearly parallel lines, as in grasses; and 

 net venation when they branch in many directions, as in 

 most broad leaves. 



The epidermis is the outer covering of the leaf, consisting 

 of a single layer of flattened cells on both sides of the leaf. 



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