338 Canadian Record of Science. 
To the first group belong only two elements, carbon and 
oxygen. ‘These are presented to the plant and taken up 
in the form of carbon dioxide. Oxygen is also absorbed in 
the free state, but in this respect it is concerned in the 
process of respiration, and not of digestion, and therefore 
is not to be considered in the present connection. 
Carbon dioxide is, as we know, a peculiar product of 
organic combustion, including respiration of both plants 
and animals, and when produced in excess, is as prejudicial 
to one form of life as to the other. Its elimination from the 
atmosphere in the process of vegetable growth, constitutes 
one of the most iiaportant relations in which plants stand 
towards the higher forms of animal life. During the 
Carboniferous age, when life was of a much lower type than 
now generally exists, plants attained to a luxuriance 
of growth with which but few modern plants can compare, 
and while this was the direct result of the peculiar condi- 
tions under which they were placed, it also adapted them 
to the more rapid elimination of carbon dioxide—thereby 
causing a return of oxygen to the air, and a fixation of 
the carbon, which, in course of time, became transformed 
into coal and graphite as we find them to-day. Thus the 
atmosphere became adapted to an improved type of animal 
life; the plants themselves, being brought under new con- 
ditions of environment, suffered important changes, and man 
is now enabled to convert to his own needs the transform- 
ed energy derived from the sunbeams of that remote past. 
To the second group of elements, those derived from the 
soil, belong all the others that have been enumerated. It 
should be observed here, however, that oxygen is also de- 
rived from the soil, both as water and as acids in combina- 
tion with the earthy elemenis. 
The appropriation of food is provided for by means of 
specialized organs. The gaseous elements of the air are 
absorbed by the leaves, in which specialized openings or 
mouths, called stomata, are developed. Through these, the 
gases of the atmosphere penetrate the interior structure by 
a process of diffusion, and are there absorbed by the living 
