PHO TOS YN THESIS. 



93 



registering form of this apparatus is described by Copeland, in the 

 Botanical Gazette, 29, 439, but it seems liable to serious practical errors. 



It is sometimes stated in books that oxygen is given off abundantly 

 from leaves of land plants plunged under water, the bubbles collecting 

 upon such leaves being supposed to be oxygen evolved in photosynthesis. 

 In fact those bubbles are practically nothing other than air dissolved 

 i:i the water, released by the heat of the sun, as is shown by the fact 

 that they will collect as copiously upon a leaf-surface containing no 

 stomata as upon one containing them abundantly, and also they will 

 collect upon the leaves as abundantly if the vessel be set in a warm dark 

 place as in an equally warm light place. 

 A test of such bubbles would give a 

 larger proportion of oxygen than air 

 contains, because oxygen is more solu- 

 ble in water than nitrogen. 



An exact quantitative method of 

 studying the gas-exchange is given by 

 Pfeffer (see Detmer, 41 ). The principle 

 of this, with which I have had very 

 good results, may be much simplified 

 as follows (see Fig. 201 : Take two of 

 the special burettes used earlier in Ex- 

 periment 6, and close their tops by 

 rubber stoppers cut to such a length 

 that when forced tightly in they just 

 fill the space above the graduation, and 

 for additional security against leakage 

 cover them and the burette end with 

 sealing-wax. In each one place now a 

 long shoot with numerous small leaves 

 (I find Ficus repens grown in most 

 greenhouses particularly available for 

 this), to the lower end of which a very 

 fine wire is attached by means of which 

 it may be withdrawn. The wire is then 

 taken outside the burette and held in 

 place by a rubber band. Each burette is 

 then placed over a small mercury res- 

 ervoir, a very small stiff rubber tube 

 being first run up inside the burette with 

 the other end outside. The burette is FIG. 20. ARRANGEMENT FOR DE- 

 now depressed in the mercury reservoir TKRMINING THE GAS-EXCHAN<;K 

 until the mercury stands at 4 cc. above IN ~ PHOTOSYNTHESIS. One-fourth 

 the zero-mark inside and out, with end ^ ie true size - 

 of the rubber tube just at the level of the liquid. The tube is now with- 



