LABORATORY STUDIES 99 
funnel and its tube completely. 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, supporting 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 CO, can reach the Spirogyra and photo- 
synthesis will soon cease. If CO, is passed into 
the water occasionally, taking care not to let any experi- 
bubbles enter the funnel, the activity of the process ™e™* ()- 
is increased. 
(e) Ina 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 photosynthesis. 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 different colors in front of the dish 
containing this plant, in all cases waiting long enough to 
avoid the effect of the changing volume of the enclosed gas due 
to changes of temperature. 
(f) Place two potted geranium (Pelargonium) plants, 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 COs. 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 
