74 THE VEGETATIVE FUNCTIONS OF PLANTS 
entitled to credit for this discovery may be mentioned 
Jan Ingen-Housz (1730-1799), a Dutch physician, and 
Jean Senebier (1742-1809) 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 tulipifera) . 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- 
