20 Science of Plant Life 



elongated cells standing close together, as is shown in Figure 

 12. The remainder of the mesophyll tissue is made up of 

 ovoid or irregularly shaped cells joined quite loosely, so that 

 air spaces are left between them. In fact, a much larger 

 part of the surfaces of these cells is in contact with air spaces 

 than with other cells. The air spaces within the leaf are con- 

 tinuous, and through them the oxygen and carbon dioxid of 

 the atmosphere can reach every cell in the leaf. We shall 

 see later that the differences in the epidermal and mesophyll 

 cells, and in the way they are arranged, are definitely related 

 to the different processes carried on by each of them. 



The veins. The veins in a leaf branch again and again, 

 forming a fine meshwork through all its parts. Each vein is 

 composed of a bundle of water-conducting and food-con- 

 ducting tissues surrounded by a bundle sheath. The water- 

 conducting tissues are located in the upper side of the vein. 

 These tissues are made up of long, cylindrical cells placed 

 end to end. Usually the inner walls of these cells have 

 spiral thickenings, and sometimes the end walls of the cells 

 are absorbed, leaving continuous tubes or vessels several 

 cells in length. After the growth of the cells is completed, the 

 living protoplasm within them dies, and the dead cases of the 

 cells, with their porous walls, lie within the leaf like bundles of 

 very fine pipes. Through these vessels, the water and mineral 

 salts that are absorbed by the roots pass into the leaf to 

 supply its living cells. The supplies of water and mineral 

 salts pass out through the walls of the water-conducting ves- 

 sels into the cells that adjoin them, and then from these they 

 pass to the other cells of the leaf. 



The food-conducting tissues, or vessels, lie below the water- 

 conducting vessels within the leaf veins. They provide an 



