Leaves and Their Structures 21 



elaborate system of channels by which the surplus foods 

 manufactured in the leaf are distributed throughout the 



Figs. 15 and 16. Upper and lower epidermis of vinca leaf. 



plant. The foods pass from the mesophyll cells into this 

 food-conducting tissue, and then down through the petiole 

 of the leaf to the living cells of the stem and roots. 



In the smaller veins the bimdle sheath is a layer of meso- 

 phyll cells. In the larger veins it contains one or more layers 

 of thick-walled cells, which act as a mechanical or supporting 

 tissue. The mechanical tissue is rigid and gives stiffness to 

 the leaf. 



Cells, tissues, and organs. We see, then, that the actual 

 work of the plant is done in its cells, of which there are many 

 millions, and it is the sum of the hfe and work of all these 

 cells that makes up the Hfe and work of the plant as a whole. 

 All cells carry on certain fundamental Hfe processes like 

 respiration and the assimilation of food, but most cells are 

 especially adapted to some particular work that is carried on 

 for the benefit of the plant as a whole. Cells that have the 

 same special function are similar in structure and are generaUy 



