The process of carbohydrate formation by chlorophyl is called photo- 

 synthesis, from two Greek words meaning "light" (compare ^y^o/ograph) 

 and "a putting together". It is easy to show that in the absence of light, 

 chlorophyl is inactive and photosynthesis is suspended. Moreover, if a 

 plant is kept in darkness for a longer period, the chlorophyl begins to dis- 

 appear, and in the end the leaves will turn yellow or even white. We use 

 this fact in the blanching of celery. We also know that the outer, exposed, 

 leaves of a head of lettuce or cabbage are much greener than the inner 

 leaves. 



Experiments have shown that plants can carry on this work under 

 artificial light. By the use of strong electric light during the night, lettuce 

 plants have been hastened in their growth and development, and brought 



PHOTOSYNTHESIS IN A LEAF 



Palisade cells receive water from the roots by way of fine tubules, and carbon diox- 

 ide by osmosis from the surrounding air spaces. Under the action of sunlight, the 

 chlorophyl combines carbon, oxygen and hydrogen from water and carbon dioxide 

 into sugar or starch molecules, and an excess of oxygen passes out of the cells by 

 osmosis 



139 



