FIG. 5. 



8 PRINCIPLES OF THE GROWTH OF TREES. 



green pulp consists of cells of various forms, with many air 

 spaces between. The cells are commonly placed very com. 

 pactly together on the upper side of the leaf, and more loosely, 

 or with air-spaces, on the lower side-hence one reason that 



leaves are usually lighter- 

 colored below. Fig. 5 is 

 a highly magnified sec- 

 tion of a leaf, showing the 

 green cells, air - spaces, 

 and epidermis above and 

 below. Leaves have also 

 breathing pores, through 

 which air is absorbed, 

 and vapor and gases are 

 given off. They are so 

 small as to require a good 

 microscope to discover 

 them; and they vary in 

 different plants from 1,000 

 to 170,000 on a square inch of surface. The apple and pear 

 have about 25,000 or 30,000, and the white lily about 60,000 to 

 the square inch. They are most- 

 ly on the lower side of the leaf. 

 Fig. 6 represents the pores on 

 an apple-leaf. Leaves are a con- 

 trivance for increasing the sur- 

 face exposed to the air and sun. 

 Professor Gray says the Wash- 

 ington elm at Cambridge was 

 estimated to bear " seven million 

 leaves, exposing a surface of 

 200, ooo square feet, or about five 

 acres of foliage." A common 

 fully grown apple-tree has from 

 three to five hundred thousand leaves, and the breathing 

 pores they all contain must be more than a thousand million. 



THE PROCESS OF GROWING. 



Water is absorbed by the roots, carrying in weak solution 

 manv earthv salts. When it is in the plant, it is denominated 



FIG. 6. 



