4 The American Naturalist. (January, 
and discovered (1779) that light was an essential factor in 
restoring air, and that by the aid of sunlight he could per- 
form in a few hours the experiments which took Priestley five 
or six days. 
In 1800 Senebier added the discovery that plants obtain all — 
their carbon from carbon dioxide, but fell into the error of 
supposing that part, at least, of this gas was taken up by the 
plant through its roots, an error that has proved extremely 
tenacious, existing in our text books to the present day, 
although repeatedly and fully disproven. : 
. e have now arrived at the time of De Saussure (1804), 
who, with his superior chemical knowledge, placed the whole 
subject in excellent shape. He distinguished between carbon 
assimilation and true respiration. He dealt with the subject : 
quantitatively, and showed that there was a definite relation | 
between the carbon dioxide taken up by the plant and the 
oxygen evolved by the action of light.. He clearly pointed | 
out that the presence of oxygen was as essential to the growth 2 
of plants as to animals, the most active parts, such as green l 
leaves, opening flowers, ete., requiring the most, and that this 
requirement had no relation to the presence of light. . 
De Saussure also pointed out that while plants receive their | 
supply of carbon dioxide for assimilation, and oxygen for res- 
piration directly from the atmosphere, yet the nitrogen, which _ 
is an essential constituent of their organization and by far the 
most abundant gas in the atmosphere is not utilized by plants | 
in the gaseous form. 
Having now established that plants contain gases, that these 
gases are the same as those of the atmosphere surrounding the © 
plant, that oxygen and carbon dioxide are made use of in _ 
their gaseous forms in the life processes of plants, while nitro- ` 
gen as a gas is not actively connected with the life of plants, — 
and having established these facts with a wealth of accurate — 
experiment and logical deduction that permitted no doubt of 
the truth, it was left to De Saussure’s successors to elaborate 
the structure which he had so ably built, without being called 
upon to again readjust the foundations. 
