THE LEAVES 



55 



the garden. A very thin, white scum forms on the 

 top of the clear water. This is caused by the carbon- 

 dioxide in the air. How httle of this gas there is, is 

 shown by the thinness of the scum. 



7. The chief aim of a plant. You can see now why 

 a tree requires to spread out many leaves in order tO' 

 get as much of this air food as it needs. You can see, 

 too, why there is such a struggle among the crowded 

 grasses and among the trees in the thick " bush " to 

 get up into the air. They are all seeking for a gas 

 which is not plentiful. The great aim of a tree in 

 putting out branches and leaves is to get as much 

 food out of the air as possible. 



Rose branch in shade of north fence ; the leaves are far apart as 

 if taking long steps to the sunlight. 



8. The leaves v/ithout light cannot make air-food. 



Just as a ship carries as much sail as it can, so a tree 

 carries as many leaves as it can. If you could spread 



Branch (from same rosebush as in fig. 33) that has overtopped the 

 fence and got into the light. 



out all the leaves of a great tree on the ground, side 

 by side, you would be astonished at the great amount 

 of space covered. But there is something needed by 



