THE EVOLUTION OF THE PLANT BODY 



'hi 



layers consist of closely packed cylindrical cells, each gorged with 

 a dozen or more chloroplasts; it is this tissue which gives leaves 

 their green color. The parenchyma cells found between these and 

 the lower epidermis are more irregular in shape, have fewer 

 chloroplasts and are surrounded by connecting air spaces into 

 which the stomates open. The veins are vascular channels, made 

 up of elongated conductive cells, which form a circulatory net- 

 work among the parenchyma cells; the terminal portions of 

 each vein are in actual contact with a parenchyma cell. 



SOLAR ENERGY 



RE SP/ RAT/ ON 



PHOTOSYNTHESiS 

 TRANSP/RAT/ON 



PROTEfN SYNTHES/S O'^ ^^^'^y ^eu) 



Fig. 16. — Numerous activities take place in a living leaf during daylight hours. 



Every day, from sunrise to sunset, leaves are scenes of con- 

 tinuous activity (fig. 16). Carbon dioxide, diffusing in through 

 the stomates from the atmosphere, passes through the air spaces 

 of the parenchyma and eventually enters a chloroplast-bearing 

 cell. At the same time water, flowing upward through the vas- 

 cular channels from the roots, passes into smaller and smaller 

 veins until it too enters a chloroplast-bearing cell. With these 

 raw materials, secured from the distant air and the more distant 

 soil, each green cell then carries on the same process that took 

 place in the unicellular desmid plant. Glucose difluses out of 

 the photosynthesizing cell through other channels in the same 

 veins and in time passes into every cell of the tree, to be used for 

 respiration, protein and protoplasm building, or storage. The 



