8 



PRINCIPLES OF THE GROWTH OF TKZES. 



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 1 70, ooo 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 

 many earthy salts. When it is in the plant, it is denominated 



FIG. 6. 



