LEAVES 



53 



grass stems are fed to stock. As to the contribution of 

 stems to clothing, linen is the most important item. Linen 

 is principally composed of fibers derived from the stems of 

 a plant called flax. 



Rubber is another important product of stems. It is 

 derived from the milky juice of certain tropical trees. 

 This juice in appearance is like the juice of the common 

 milkweed. 



17. Leaves. — Often you have had a leaf in your hand. 

 You have noticed the stem-like part. That is the petiole. 

 You have noticed the broad 

 green part. That is the 

 blade. You have noticed 

 that the blade is supported 

 by a sort of fine frame- 

 work. That framework is 

 composed of what are called 

 the veins. The largest vein, 

 if there is a largest, is the 

 midrib. (See Figure 7.) 

 Extensions of the veins 

 run from the leaf down 

 through the stem of the 

 plant clear to the ends of 

 the roots. Inside the stem 

 and the root these exten- 

 sions are known by other fig. 7 

 names than veins, but they 

 have the same structure. 

 The veins and their extensions form the paths along which 

 movement inside the plant principally occurs. On the 



Leaf of poplar showing petiole, 

 blade, veins, and midrib. Also a bud 

 in the axil. 



