LABORATORY STUDIES 99 



funnel and its tube completel}'. Over the latter invert a test 

 tube filled with water. Now raise the funnel as high as it will 

 go without lifting the edge of the test tube above the surface 

 of the water, supi)orting it on a small block. Place the whole 

 in the sunlight. As photosynthesis goes on the oxygen given off 

 by the pond scum collects in the test tube and may 

 be tested in various ways, e.g. by carefully re- 

 moving the test tube, inverting it and inserting 

 a glowing splinter which will burst into flame if 

 sufficient oxygen is present. The diameter of the 

 funnel must be considerably less than that of the 

 jar or no CO2 can reach the Spirogyra and photo- 

 synthesis will soon cease. If CO 2 is passed into 

 the water occasionally, taking care not to let any 

 bubbles enter the funnel, the activity of the process 

 is increased. 



(e) In a similar way the oxygen evolved in photosynthesis by 

 Philotria (Elodea) may be collected by inserting the cut ends of 

 several plants into the mouth of an inverted test tube filled 

 with water and placing the whole dish in the sunlight. Care 

 must be taken, however, not to confuse two phenomena here, viz. 

 the rapid outflow of bubbles at first, due to the expansion of the 

 gas already present in the stem as it heats, and the much slower 

 evolution of oxygen by photosj-nthesis. After the first outrush 

 of gas due to the expansion by heat is past the relative 

 amount of photosynthesis can be told with a fair degree of 

 accuracy by counting the number of bubbles of oxygen evolved 

 per minute under the different conditions. Be sure, however, 

 to keep the water well supplied with CO2. Test now the effect 

 of placing glass plates of difi"erent colors in front of the dish 

 containing this j)lant, in all cases waiting long enough to 

 avoid the effect of the changing volume of the enclosed gas due 

 to changes of temperature. 



(/) Place two potted geranium (Pehirgonium) ]ilants. prefer- 

 ably with plain, not variegated leaves, in the dark until their 

 leaves contain no starch. Now place them under bell jars, 

 sealing one air tight with sealing wax or by other means, first 

 placing under the jar a dish containing a strong solution of 

 KOH to absorb all CO2. Leave a small air space under the 

 edge of the other bell jar so as to permit the entry of air 

 containing CO2. After an hour or so place both plants in the 



