92 PLANT PHYSIOLOGY. 



Then into two of the bottles insert under water large vigorous 

 shoots of a green plant, slip the saucers beneath them and remove 

 from the large vessel; put one of them in the bright sunlight, and 

 the other beside it but covered by a dark hood from the light (Fig, 

 19), and let the empty bottle also stand as a control in the light. At 

 evening withdraw the plants by aid of the vessel of water; lift one 

 bottle after the other cautiously from the water and replace it 

 quickly over the floating burning candle, noting the time the candle 

 will burn in each. 



{If corks just fitting tJie bottles are available, a better "way to make 

 the final test is to cork the bottles under water, invert tJiem, and 

 lower the candles into tJieni, removing the corks as little as possible. 

 It is well to use a shoot with a large quantity of leaves?) 

 The experiment may be tried without the saucers and water-vessel 

 by using preserve-jars, the stoppers of which may be screwed on air- 

 tight ; lighted candles are lowered into them after the shoots have been 

 inserted. Ordinary corks will not do for this, as they allow some gas 

 diffusion unless thoroughly covered with vaseline. 



Another very excellent method often used for testing the release of 

 oxygen in Photosynthesis is described in various books (as Detmer, 37 ; 

 MacDougal, 38). A quantity of a water-plant such as Elodea (Anacharis) 

 or Cabomba is placed in light in a glass vessel under a funnel over which 

 is a test-tube filled with water. The gas rises, guided by the funnel into 

 the tube, where it may be tested for oxygen by a glowing splinter thrust 

 into it. A rather more convenient way to apply this test is given by 

 Hansen in Flora, 86 (though his principle could be simplified). The 

 test may also be made with phosphorus (see page 42). In order to be 

 able to measure the quantity of oxygen taken up by the phosphorus, it is 

 well to use an inverted cylindrical graduate (as in Fig. 16). 



If the ends of the cut stems (in Cabomba at least) are thrust into the 

 test-tube (where they may be held tied lightly to a glass rod), the funnel 

 is needless. When the funnel is used, and indeed without it, the outer 

 vessel should be as large and with as great an air-space as is conveniently 

 possible, to allow of the free absorption of CO 2 from the air, and the 

 funnel should be lifted to allow free communication with the remainder of 

 the vessel. Moreover it is ver)' profitable to blow in CO 2 from a generator 

 from time to time. When the evolution of bubbles has stopped through 

 exhaustion of the gas from the water, it will begin again immediately if 

 the CO 2 be thus added. The gas so collected in the test-tube when 

 carefully tested will usually show not over half oxygen ; a trifle of the 

 remainder is CO 2 , while the remainder is nitrogen. Under favorable 

 conditions, however, the proportion of oxygen may greatly increase. (See 

 Pfeffer-Ewart, page 331.) The plants of course always work much better 

 when in vigorous growth than when resting during the winter. A self- 



