FUNCTION OF CHLOROPLAST 



117 



stomata if the outer air is drier, as it usually is, than 

 the air inside the leaf (Fig. 10, white arrows). 



Let us consider now what happens in the chloroplast 

 of a mesophyll cell which is suddenly well illuminated. 

 The light falling upon it is robbed of those rays of the 

 spectrum corresponding with the absorption bands 



LIGHT 



tn.t 



Fig, 1 1 . — Two mesophyll (palisade) cells. 

 The one on the left is seen in optical 

 section, the one on the right in sur- 

 face view. Above are two cells of 

 the upper epidermis, chl., chloro- 

 plast; n, nucleus; c.w., cell wall; 

 ep., epidermal cell ; int., intercellular 

 space. 



Fig. 12. — A, diagram of chlo- 

 roplast of mesophyll cell 

 to show its activity when 

 illuminated ; v, vacuole ; 

 int.. intercellular space 

 B, starch grains formed 

 in chloroplast. 



of chlorophyll. The energy so obtained is used to 

 split up the molecules of carbon dioxide provided by 

 the respiration of the protoplasm, and the carbon is 

 united with the elements of water (always present in 

 the cell) with the ultimate formation of sugar, oxygen 

 at the- same time being liberated (Fig. 12, A). Some 

 of the sugar and oxygen so formed is doubtless used 



