118 THE WORK OF PLANTS 
surface of the water. Here the plant can get more light, 
more air, and so do more work of various kinds. 
The leaves of garden herbs and shrubs, trees, and other 
land plants do the same kind of work. But since they do 
not grow in water they do not show the signs of this 
work as water plants do. At least, we cannot see the 
sions of it because the gas is so much like the air in its 
nature. Perhaps you have put a lettuce leaf or a leaf 
of some other land plant under water, and have been 
told that the bubbles which rise from the leaf in the 
water are a sign that the leaf is doing work in starch- 
making. But this brings the land plant into an unfa- 
vourable environment, and it soon dies. Not all the 
bubbles given off are of the same kind of gas as that 
given off by the water plant, so that this must be 
regarded as a misleading experiment.! 
1 When the leaf of a land plant is placed in water there is always 
a thin layer of air over the surface of the leaf. If the water is 
exposed to the sunlight, there is a rise in the temperature which causes 
the air around the leaf to expand, and some of it rises in the form of 
bubbles. This may continue for a considerable time. Some of the air 
inside the leaf is also crowded out because of the change in tempera- 
ture. This air that is rising from the leaf because of the change in 
temperature is not the same kind of gas that rises from the water plant 
ey from the pond scum. We cannot distinguish between the two 
kinds of gas as they rise together from the land plant in the water. 
Therefore it is no sign that the plant is doing the work, but only an 
evidence that a change in temperature ig going on which expands the 
air and causes some of it to be freed from the surface of the leaf. The 
same thing can be seen if we place a piece of broken crockery or a dry 
