74 THE VEGETATIVE FUNCTIONS OF PLANTS 



entitled to credit for this discovery may be mentioned 

 Jan Ingen-Housz (i 730-1 799), a Dutch physician, and 

 Jean Senebier (i 742-1 809) and Nicolas Theodore de 

 Saussure (1767-1845), two Frenchmen. Joseph Priestley 

 an Englishman, also contributed to this work, both 



Fig. 55. — Leaves of the tulip tree {Liriodendron tuUpifera). At left, 

 from a large mature tree; at right, from a young sapling. Average sized 

 leaves were chosen from each tree. Greatly reduced. 



directly and indirectly, by his discovery, in 1774, of the 

 gas oxygen, and his experiments on the purification of the 

 air by green plants. 



73. The Significance of Leaves. — While the formation 

 of carbohydrate food may take place in any cell con- 



