Il8 THE GREEN PLANT CELL 



in the respiration of the colourless protoplasm, but, 

 under favourable conditions, the rate of photosynthesis 

 far exceeds the rate of respiration in a green cell. This 

 means in the first place that far more carbon dioxide 

 is used than can be obtained from the process of respira- 

 tion, the pressure of the gas in the cell decreases and 

 fresh supplies enter from the air of the intercellular 

 spaces. This in turn causes an inflow of carbon dioxide 

 through the stomata. 



The oxygen formed as a by-product of photosynthesis 

 increases the pressure of this gas in the cell, and is given 

 off at the cell surface into the intercellular spaces and 

 thence diffuses through the stomata into the open air, 

 so that well illuminated leaves are constantly giving 

 off free oxygen. The sugar continuously produced 

 by the chloroplasts diffuses into the cell sap. Some of 

 it passes into the bundles (veins) of the leaf and along 

 the bundles to other parts of the plant. But more 

 sugar is produced than can be removed from the meso- 

 phyll by this means, and the concentration of sugar in 

 the cell continuously rises during the day. 



Formation of Starch in the Chloroplasts. In most 

 green plants, though by no means in all, the excess of 

 sugar after a certain degree of concentration is reached, 

 is converted into starch by condensation, and this 

 forms small grains in the substance of the chloroplast : 



C 6 H 12 6 --= (C 6 H 10 5 ) n + ^H 2 



A foliage leaf of a starch-forming plant at the close 

 of a bright day when large amounts of sugar have 

 been formed shows its chloroplasts packed with starch 

 grains (Fig. 12, B), as can easily be seen by examining 

 a section of the leaf under the high power. The starch 

 grains appear as bright granules in the substance of 



