CHAPTER VI 

 HOW THE PLANT INCREASES ITS SUBSTANCE 



46. The Work of Leaves. The leaves are the food 

 factory of the plant. Perhaps you have never thought 

 to ask why most leaves are flat. You will find a sugges- 

 tion of the answer if you note that their flat faces 

 are usually turned toward the source of the strong- 

 est light. Look at a tree, to note the position of the 

 leaves, as seen from a distance and from among the 

 branches. This position is an advantage to the leaf in 

 carrying on its work, because it secures the greatest 

 amount of energy from the sunlight for the food-making 

 process. 



47. Structure of Leaves. A thin section of a leaf, 

 when examined under a powerful microscope, is seen 



Fig. 20. Cross-section of a leaf through a "vein," or nbro-vascular bundle. 

 Os, upper surface; us, under surface; o, layer of outside cells forming the 

 epidermis; sp, Stoma; g, water duct; wb, phloem; hlz, wood cells of nbro- 

 vascular bundle. 



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