CHAPTER V 

 LEAVES 



Development of Leaves.^ — ^Leaves appear at the growing 

 point of a stem, as lateral protuberances (Fig. 15) consist- 

 ing at first of a shapeless mass of cells. We call this group of 

 cells the primordial leaf. By further cell division and dif- 

 ferentiation (becoming different from each other) of these 

 few cells the adult leaf arises. In the embryos of seeds 

 the first few leaves are already formed, and even in this 

 early stage may bear some resemblance in shape to the 

 adult leaves. 



Parts of Leaf. — Most leaves have two distinct parts: 

 hlade and petiole (leaf stalk). Some leaves, as those of peas 

 and beans, have two small, leaf-like structures at the base 

 of the petiole. These are stipules (Fig. 19). The petiole is 

 sometimes absent, the blade being mounted directly on the 

 stem. Such a leaf is said to be sessile. Vascular bundles run 

 from the stem out through the petiole into the blade, where 

 they branch to form the network of veins. The veins not 

 only carry water, solutes and food materials, but also form 

 a framework for the softer tissue of the leaf. 



Kinds of Leaves. — It is possible to classify leaves in many 

 different ways. Common green leaves that we are all 

 familiar with are usually called foliage leaves. They are the 

 chief food-making organs of all ordinary plants. However, 

 there are many leaves that do not possess green coloring 

 matter (chlorophyll) and hence, have no food-making power. 

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