9 



PLANT PHYSIOLOGY. 



keep the physical conditions as nearly as practicable the same, 

 pose to bright light for a day, and test for starch. 







EX- 



FIG. 18. ARRANGEMENTS FOR DEPRIVING LEAVES OF CARBON DIOXIDE. 



< )ne-fourth the true size. 



This experiment may also be tried, and for some reasons more advan- 

 tageously, by using entire potted plants placed in bell-jars sealed by vas- 

 eline to ground-glass plates (Darwin and Acton, 29; also Fig. 18, left). 

 Through rubber stoppers should pass calcium chloride tubes containing 

 soda lime and sawdust respectively, and a dish of soda lime should be 

 placed in one bell-jar. Otherwise the experiment proceeds as above. 



It is also possible to test easily whether CO 2 enters the leaves through 

 the stomata or the epidermis. If a thin leaf containing stomata upon 

 one surface only (such as Primula) be selected and the plant kept in dark- 

 ness for a day, and the surface containing the stomata be coated with 

 vaseline, no starch is made as shown by the usual test, while a neighbor- 

 ing leaf coated with vaseline on the upper surface makes it abundantly. 

 Or, of the two halves of the under surface, one may be coated and the 

 other left free, with the same result. 



The need for carbon dioxide, and hence its probable 

 absorption, in photosynthesis, shov/n by Experiment 28, sug- 



