236 PLANT AND ORGANIC CHEMISTRY 
Darwin 1 has shown that the insectivorous plants, by means 
of their modified leaves, absorb complex compounds, and that 
these are of importance in their nutrition. Flies and other small 
insects may often be found clasped in the tentacles of the 
Drosera , and in those experiments small pieces of meat, when 
placed on the leaves, were dissolved after a time by the secre¬ 
tions of the leaf glands and absorbed. 
Hydrogen is absorbed by all plants in combination in the 
form of water or ammonia and its compounds, or in the com¬ 
plex substances mentioned above. 
Oxygen is taken up by plants, free or in combination in water 
or in salts. The free oxygen is especially concerned in destruc¬ 
tive metabolic processes. The large quantities of this gas ab¬ 
sorbed by plants, and especially by fungi, show conclusively 
its consumption in metabolic processes. 
The process known as the respiration of plants is the absorp¬ 
tion of oxygen and the exhalation of carbon-dioxide. 
The researches of Garreau 2 show that two distinct processes 
are in operation when leaves are exposed to the fight: in the one 
oxygen is absorbed and carbon-dioxide is exhaled; in the other, 
carbon-dioxide is absorbed and oxygen is exhaled. When the 
leaves are exposed to a very bright sunlight, carbon-dioxide is 
absorbed and oxygen is exhaled, and the activity of these pro¬ 
cesses is so much greater than the absorption of oxygen and the 
exhalation of carbon-dioxide, that it appears as if the former 
only were in operation. 
Gases, like solids, can be assimilated only in solution, and as 
they are soluble in water, the cell walls of submerged plants 
may absorb them, and the sap near the surface of land plants 
will dissolve the gases from the atmosphere. The sap of plants 
contains, in solution, carbon-dioxide, oxygen, and also a certain 
amount of free nitrogen. That this nitrogen does not enter into 
the metabolism of the plant seems completely decided by the 
experiments 3 of Lawes, Gilbert, and Pugh; but the more recent 
experiments of Atwater 4 and Hellriegel 5 should be compared 
1 Insectivorous Plants. 2 Ann. d. Sei. Nat., ser. 3, t. xv. 
3 Phil. Trans., i860. 4 Amer. Chem. Jour., viii, Nos. 5 and 6. 
6 Zeit. d. Ver. j. d. Rübenzucker Industrie, Nov., 1886. 
