Green Leaves at Work 10] 
the sun shines brightly upon a pool, in which 
pond-weeds are growing lustily, we can see oxy- 
gen rising in bubbles from the submerged _leaf- 
laboratories to the surface of the water. 
Oxygen-making is carried on even more actively 
by the smallest of the fresh-water alge. These 
little plants are fine green filaments, which have 
no roots and float in tangled bunches near the 
surface of still and sunny water. When the 
weather is warm and bright, the oxygen given 
out by these little alge forms great bubbles, 
which become entangled in the cobwebby meshes 
and float the plants to the surface. 
Here alge and bubbles together form a green 
scum, which froths as we look at it. One might 
readily suppose that all manner of impurities were 
festering in the water, and that evil gases and 
malaria were being distributed throughout the 
neighborhood. But, as we learned in our copy- 
book days, ‘‘ appearances are deceitful.”’ 
The frothing is caused by one of the most 
active processes of constructive life. The bubbles 
which are being cast up are life-giving oxygen, 
which enables the grass to grow and the animal's 
heart to beat. 
And the little alge, so busy and _ beneficent, 
