8 



PRIN-CIPLES OF THE GROWTH OF TREES. 



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Fig. s- 



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 

 i\ ^ which air is absorbed, 



cv' V -M. Vj. I / and vapor and gases are 



- " ' '"■'"'"'' '■■'' '"-' 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 

 200,000 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 



Pig. 6. 



