[Chap. Xlll SYNTHESIS OF SUGAR— PHOTOSYNTHESIS 111 



light, or to electrical energy. The first is exemplified by photosynthesis, 

 the second by the production of light by fireflies and luminous bacteria, 

 and the third by the electric discharge from an electric eel. 



Before photosynthesis can occur in an illuminated green cell the raw 

 materials, CO- and H-O, must be available. Carbon dioxide is very solu- 

 ble in water. Hence, all the green cells of the simpler algae that live 

 submerged in water, and e\'en those in the leaves of the common water 

 weed (Elociea), are in direct contact with both water and CO2. Both of 

 these substances pass readily into the cells and then to the chlorophyll 

 in the chloroplasts. 



In land plants the water moves up from the soil through the roots, 

 stems, and petioles into the \'einlets of the leaf ( Fig. 52 ) , and then passes 



.^^ 



^ ^ic ii . ' iiY"'-^^ 



Fig. 52. The vein system of a skeletonized sassafras leaf. The leaf was kept in 

 water until bacteria had digested the epidermis and mesophyll. 



from cell to cell throughout the leaf. No appreciable amount of carbon 

 dioxide gets to the leaves from the soil by way of the roots. It enters the 

 leaf from the surrounding atmosphere above the soil. Some of it may 

 pass directly through the epidermal cells to the chlorenchyma beneath. 

 Most of it passes through the stomates into the intercellular spaces of the 

 mesophyll, where it comes into contact with the wet walls of the 

 mesophyll cells that are exposed to the internal atmosphere of the leaf. 



