206 Plant Physiology 



metric methods should be carried out in an accurate man- 

 ner by the use of special apparatus. 



For demonstration purposes the evolution of oxygen 

 from cut stems of water plants (such as Elodea or Ca- 

 bomba) is the simplest indication of photosynthesis ; but 

 this may not be applied to land plants. During photo- 

 synthesis gas escapes from the large air chambers through 

 the cut stems, and with vigorous action a slow stream of 

 bubbles may arise. It is then necessary to employ a 

 method whereby these bubbles may be caught so that the 

 gas may be simply identified. 



A funnel may be inverted over a quantity of clean, 

 growing sprigs of Elodea, or water weed, in a deep vessel 

 or aquarium. Over the funnel is inverted a test-tube of 

 water (Fig. 58) for the collection of the oxygen. In order 

 that there may be free access of carbon dioxid, the funnel 

 should be much smaller than the vessel and should rest on 

 supports several inches above the bottom; while the 

 water should be spring or well-aerated tap water, or should 

 contain a supply of CO 2 introduced from a generator. 

 The gas caught in the tube may be tested by proper ma- 

 nipulation (see Laboratory experiments, p. 221), with an 

 oxygen absorbent, preferably pyrogallate of potassium. 



If the gas is collected under favorable conditions, it will 

 consist largely of oxygen, about four fifths ; the remainder 

 consisting of other gases formed in the plant and of nitro- 

 gen from the air, which must, of course, diffuse into the 

 air spaces. Having determined that oxygen is the chief 

 part of the gas given off as bubbles from water weeds, 

 during photosynthesis, the simple bubble-counting method 

 may be employed in determining relatively the rate of 



