THE LEAF | 51 
From the above it will be seen that when a plant organ, 
from lack of chlorophyll or light, is not carrying on photosyn- 
thesis, it will, owing to respiration, absorb oxygen and give off 
carbon dioxide. When photosynthesis is active in green tissues, 
more oxygen is liberated by photosynthesis than is used in res- 
piration, and more carbon dioxide is employed in photosynthe- 
sis than is produced by respiration, with the result that these 
tissues will absorb carbon dioxide and give 
off oxygen. | 
Whether leaves in light give off carbon 
dioxide or oxygen will naturally depend on 
whether more carbon dioxide is produced by 
respiration or is used in photosynthesis. With 
very feeble light the respiration of leaves 
may produce more carbon dioxide than is 
used in photosynthesis, and so the leaves 
absorb oxygen and give off carbon dioxide 
even when they are performing photosynthe- 
sis slowly. On an average day, however, the 
products of photosynthesis must be built up 
in the green tissues much more rapidly than 
they are broken down by respiration. A sur- p38 \ethod of 
plus must be accumulated to supply mate-  gemonstrating res- 
vial for the respiration of the organs lacking _piration of seed 
chlorophyll, and also for the respiration of 
the green tissues themselves at night. In addition, building 
material must be produced for the growth of the plant body. 
Demonstration. If germinating seeds are inclosed in a vessel 
containing ordinary air, it will be found that the oxygen is ab- 
sorbed and replaced by carbon dioxide. The same phenomenon 
can be shown by using green plants if they are kept in the dark. 
A very simple method of demonstrating the exchange of gases 
in respiration is that shown in Fig. 38. A few germinating seeds 
are placed in the bottom of a test tube and held in place by a 
small amount of cotton. The test tube is then inverted over a 
solution of potassium hydroxide. The carbon dioxide that is 












