THE LEAF 47 
that all parts of the leaf have a plentiful supply of air) and 
then testing for starch. The leaf or the part of the leaf which 
was in the dark will remain white, while that which was in the 
light will turn blue (Fig. 36). 
The necessity for chlorophyll is shown by the fact that 
starch is not formed in those parts of variegated leaves which 
lack chlorophyll. 
Leaves will not produce starch in an at- 
mosphere that lacks carbon dioxide. If a plant 
is placed in an atmosphere containing a known 
quantity of carbon dioxide, it will be found 
that carbon dioxide is absorbed and that the 
carbon appears in the leaf in the carbohy- 
drates formed as a result of photosynthesis. 
That water is used in photosynthesis is 
shown by the fact that the carbohydrates 
produced by this process contain hydrogen, 
which must have been derived from water. ror cer oe Aten er 
That oxygen is given off in photosynthesis tested with iodine 
can be shown very simply by the use of certain for starch after the 
water plants which during photosynthesis give part showing as a 
off bubbles of gas. This can be collected by the ‘St@™ and that out- 
. : : side of the circle 
arrangement shown in Fig. 37. If a glowing Ped eeu 
splinter is inserted into the gas, the brightness ,. 4, light and the 
of the glow is greatly increased, thus indicat-  pemainder kept in 
ing a high percentage of oxygen. the dark 
Relation to animals. Photosynthesis is the 
source of all the food of animals. Animals do not possess the 
power of manufacturing food from simple inorganic compounds, 
but must obtain food that has already been elaborated. 
In respiration, animals take up oxygen and give off carbon 
dioxide, so that if there were no plants to separate oxygen 
from carbon dioxide, the supply of oxygen would be used up 
and animals cease to exist. 
From what has been said it will be seen that, indirectly, pho- 
tosynthesis furnishes man with the food he eats and the oxygen 

