74 THE VEGETATIVE PUNCTIONS OF PLANTS 



entitled to credit for this discovery may be mentioned 

 Jan Ingen-Housz (1730-1799), a Dutch physician, and 

 Jean Senebier (i 742-1809) and Nicolas Theodore de 

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

 an Englishman, also contributed to this work, both 



Fig. 55. — ^Leaves of the tulip tree {Liriodendroh tidipifera). At left, 

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

 leaves were chosen from each tree. Greatly reduced. (Photo by the 

 author.) 



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- 



