Leaves and Their Structures 31 



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 ves- 

 sels, several cells in length. After the growth of the cells is com- 

 pleted, the living protoplasm within them dies, and the dead 

 cases of the cells, with their porous walls, lie like bundles of very 

 fine pipes within the leaf. Through these vessels the water and 

 mineral salts that are absorbed by the roots pass into the leaf and 

 supply its living cells. The supplies of water and mineral salts 

 pass out through the walls of the water-conducting vessels into 

 the cells that adjoin them, and then from these they pass to other 

 cells of the leaf. 



The food-conducting tissues or vessels he below the water-con- 

 ducting vessels within the leaf veins. They provide an elaborate 

 system of channels by which the surplus foods manufactured in 

 the leaf are distributed throughout the plant. The foods pass 

 from the mesophyll cells into these food-conducting tissues, and 

 then down through the petiole of the leaf to the living cells of the 

 stem and roots. The conductive tissues, or bundles, may be 

 readily studied in the petioles of celery leaves. 



In the smaller veins the bundle sheath is a layer of mesophyll 

 cells. In the larger veins it contains one or more layers of thick- 

 walled elongated cells, which act as a mechanical or supporting 

 tissue. The mechanical tissue is rigid and gives stiffness to the 

 leaf. 



