HOW LEAVES ARE FORMED, 



CHAPTER XXXVII. 



THE FUNCTIONS OF THE LEAVE&- 



Leaves consist of a woody and a cellular part. The 

 woody part consists of a framework of ribs and veins upon 

 which is spread cellular tissue. These serve not only to 

 strengthen and support the leaf, but also to introduce and 

 distribute the ascending sap through the veinlets and cellu- 

 lar tissue. The cellular portion is the green pulp, and is 

 nearly the same as the green layer of the stem. So that 

 the leaf may be considered as an extension or expansion of 

 the fibrous and green layers of the outer covering of the 

 stem, and the whole of it is covered by a skin or epidermis 

 like that of the stem. 



The green pulp of the leaf consists of cells of various 

 forms loosely arranged, and leaving many irregular spaces 

 between them; these spaces form air passages which com- 

 municate with each other throughout the whole leaf. The 

 green color is due to minute green grains which lie loosely 

 in the cells. This coloring matter is known as chlorophyll, 

 or the green of the leaf. It is this green matter, when de- 

 composed into the primary colors of yellow and blue, which 

 is supposed to give the rich yellow color to the butter of 

 cows, and the yellow color of leaves which have ripened 

 and faded in the fall of the year. The green tissue of leaves 

 differs on the upper and lower sides of the leaves; the for- 

 mer being of a darker green, because of the close contact of 

 the cells; the under side being light green because of the 

 many open spaces between the loosely placed cells. 



The leaves are provided with a vast number of pores 

 called "gtomates," which afford communication between the 

 passages among the cells and the air. Through these, the 

 vapor of water, air and gases, can readily escape or enter 

 as the case may be. A pair of cells acting as valves guard 

 the opening of each pore; and when dried, these contract 

 and close the opening so as to promptly arrest the escape of 



