PHOTOSYNTHESIS 



97 



gen and the oxygen are in the same proportions as in water, it 

 is possible that a part of the material is derived from the water 

 present in the leaves, and this is actually known to be true. 

 Seeking a source of supply for the carbon, and noting the irregu- 

 larity of its occurrence in the soil, the most obvious possibility 

 is that it is derived from the carbon dioxide of the atmosphere, 

 which presents the following experimental problem: 



Do green plants use the carbon dioxide of the atmosphere in 

 the formation of the photo synthale? 



This may be tested most directly by analysis of the air- in which 

 plants carrying on photosynthesis have been confined. But such 

 analysis, because of the smallness of amount of the carbon dioxide 

 concerned, is difficult; and in practice the matter may much more 

 easily be tested by the indirect method of placing equivalent green 

 tissues in light under conditions precisely alike except that carbon 

 dioxide of the air is allowed access to one, but is kept from the other, 

 and noting whether any photosynthate, typified by starch, is formed. 



Experiment. Take two two-neck Wolff bottles each of at least 

 a liter capacity, and on each grind one neck flat on top, preferably 

 making the ground tops the same height from the table. Cover the 

 bottom of one bottle with a layer of a carbon-dioxide absorbent, prefer- 

 ably fresh soda-lime, and the bottom of the other with a layer of chalk, 

 which is physically equivalent, but non-absorbing. Stopper the un- 

 ground necks, and cover the ground surfaces with soft wax, described 

 under Manipulation in Part III ; bring the waxed necks close together, 

 and press down upon them the two halves of a large leaf of a good 

 starch-forming plant previously emptied of starch by a day or two 

 of darkness, and attach the leaf firmly in place under glass slips clamped 

 by wire springs, as shown in the figure (Fig. 22). After two or three 



Fig. 22. — Arrangement for making an air-tight connection of a leaf with 

 necks of bottles. 



Spring clips hold glass slips against the leaf which rests on soft wax against the necks 



of the bottles. 



hours in bright light, remove the leaf, scrape off the wax, blanch the 

 leaf, and apply the iodine test. 



