84 FOUNDATIONS OF BIOLOGY 



The veins, as we have seen, are merely the extensions into 

 the leaf of the chief elements of the vascular bundles of the 

 stem. They form the framework of the leaf, as well as the 

 system of ramifying highways for the transportation of 

 materials between the blade as a whole and the stem. 

 In cross-section the larger veins show the essential features 

 of the vascular bundles seen in the stem, lacking, however, 

 the cambium. 



The chief tissue of the leaf is the chlorenchyma, consisting 

 of chlorophyll-bearing cells. Immediately under the upper 

 epidermis these cells are arranged in a definite layer known as 

 the PALISADE LAYER. Below this region the cells are more or 

 less irregularly disposed so that there are larger and smaller 

 AIR SPACES between them. These air spaces form a prac- 

 tically continuous system of passages and thereby facilitate 

 the interchange of oxygen, carbon dioxide, and water vapor 

 between the leaf cells and the outer world through the 

 stomata. 



The cytoplasm within the thin walls of the chlorenchyma 

 cells forms merely a lining in which are situated the 

 nucleus and numerous specialized, disc-shaped, cytoplasmic 

 bodies, the chloroplastids, which bear the chlorophyll and 

 therefore appear green. It will be recalled that these are the 

 essential agents of photosynthesis. The center of the cell 

 is occupied by a large vacuole of cell sap. This sap is usually 

 under considerable pressure, which accounts for the close 

 application of the cytoplasm to the inside of the cell wall and 

 produces the turgor characteristic not only of the chloren- 

 chyma cells, but also of many other types of plant cells as well. 



C. PHYSIOLOGY 



We have now outlined the essential structure of a general- 

 ized Flowering Plant, with the exception of the parts modi- 



