THE LEAF - 125 
in a flask of water, and collecting the gas given off. 
Elodea is probably the best plant for the purpose, 
but, if this is not available, watercress, pond-weed 
(potamogeton) or eallitriche may be used instead. 
Obtain a flask with a fairly long neck, put in the weed, 
and then quite fill it with water. Bubble a little carbon 
dioxide into the water and then cork with a rubber 
cork through which passes a tube bent at right angles 
twice. The part of the tube inside the flask should 
reach to the bottom of the neck. Now place the open 
end in a vessel of water. As the gas is given off from 
the weed it will rise to the top of the neek and foree 
the water out through the tube (Fig. 86). Under 
favourable cireumstanees, as for instance on a warm, 
sunny day, the neck may become full of gas in a few 
hours. Now remove the cork and plunge in a glowing 
splinter. This does not usually burst into flame as 
is the case in pure oxygen, but glows more brightly, 
merely showing that the gas collected contains an 
excess of oxygen. In photosynthesis, then, a consider- 
able amount of oxygen is evolved. 
RESPIRATION, 
Respiration is not peculiar to the leaf: indeed, it 
is in connection with the leaf that it is most difficult 
to discover. But the subject is introduced here for 
convenience of comparison with photosynthesis, and 
because certain apparatus may be used for the investi- 
vation of both processes. Respiration is a process 
common to plants and animals. 
As already indicated, green plants are the only 
collectors of energy. In photosynthesis, the carbon is 
separated from the oxygen, and by this very separation 
energy is stored. The carbon combines with other 
substances to form organic compounds, and it is the 
oxidation of these compounds that sets energy free 
to do work. Animals derive all their energy from 
