LEAVES 



21 



ing over the plants a large funnel and leading the bubbles 

 into a test tube, in which the presence of oxygen can then 

 be tested. 



It has been noted that photo- 

 synthesis is associated not mere- 

 ly with light but also with green 

 tissue ; and in examining the 

 structure of the leaf it was dis- 

 covered (13) that the green 

 color is due to the presence of 

 chloroplasts in the mesophyll 

 cells. It is these chloroplasts 

 that manufacture the carbo- 

 hydrates, and they obtain from 

 the light the power (energy) to 

 do it. The first visible product 

 of photosynthesis is starch, and 

 when the working cells are very 

 active starch may be observed 

 to accumulate in them] but when the process becomes 



slower or stops, as during the 

 night, this starch disappears, the 

 food being carried away for use 

 (Figs. 15 and 16).* 



A summarized statement of 

 photosynthesis is as follows : It is 

 the manufacture of carbohydrates 

 by chloroplasts in the presence of 

 light, water and carbon dioxide 

 being used, and oxygen being 

 given off as a waste product. 



FIG. 15. A bean leaf whose termi- 

 nal leaflet has been covered and 

 whose lateral leaflets have been 

 exposed to light ; the test shows 

 an absence of starch, in the 

 former and an abundance of it 

 in the latter. 



FIG. 16.-A geranium leaf, one- 

 half of which has been cov- 



ered; the test shows absence 

 of starch in the covered half. 



* Experiments should be devised to test for the accumulation of 

 starch in leaves that have been exposed for some time to a strong light, 

 and to show that this accumulation does not take place in the dark. 

 In the experiments illustrated by Figs. 15 and 16, the test for starch was 



