800 ABSORPTION AND EXHALATION OF GASES BY LEAVES, 
The amount of nitrogen found in plants is greater, however, 
than can be accounted for by the quantity of nitrogen supplied 
to the soil by rain, and is doubtless partly due to the absorption 
of ammonia from the soil, as also, probably, partly by the leaves, 
according to Sachs and Meyer, whose observations have been 
confirmed by Schloésing. 
The absorption and exhalation of carbon dioxide and oxyge 
by the leaves vary according to the circumstances in which they 
are placed. Thus, when the green leaves of a healthy plant 
are exposed to sunlight, all experiments show that carbon di- 
oxide is absorbed from the atmosphere and decomposed, leaving 
its carbon, which is the result of the decomposition, behind, and 
evolving its oxygen. It is in this way that by far the largest — 
proportion of carbon, which, as will be presently shown, forms 
so large a part of plants, is taken up by them. 
The evolution of oxygen by the green leaves and also by other 
green organs may be readily observed taking place in the form 
of bubbles, when a submersed aquatic plant or some freshly 
gathered leaves are placed in water exposed to the direct rays of 
the sun. No such evolution of oxygen takes place unless the 
water contains carbon dioxide, and not, therefore, in pure freshly 
distilled water, or in that which has been recently boiled. It has 
been found, also, that there is a constant relation between the 
amount of carbon dioxide absorbed and the oxygen exhaled. 
These experiments prove therefore, not only the exhalation of 
oxygen by the leaves, but also that part of it must be derived 
from the decomposition of the absorbed carbon dioxide. These 
changes do not take place in the deep-seated tissues of the plant, 
nor in the epidermal cells, but in those only immediately beneath 
the latter. This decomposition of carbon dioxide is effected by 
the influence of chlorophyll ; for when leaves are not green, as 
is the case in many parasitic plants and in those which are more 
or less blanched, they, like the other parts of a plant in a simi- 
lar condition, are incapable of assimilating, and must therefore 
procure their nutriment from already assimilated materials. 
This absorption of carbon dioxide with fixation of carbon and 
evolution of oxygen is in direct proportion to the intensity of 
the light to which the plants are exposed ; but the experiments 
of Draper, Hunt, and others, show that the different rays of the 
spectrum have a varying influence in promoting such a decom- 
position. The results obtained by Draper by exposing the 
green parts of plants to the different rays of the spectrum were, 
that no oxygen was set free by them when they were in the 
violet and indigo rays; ‘00 to *53 only when in the extreme red ; 
1 in the blue ; 4:10 in the green and blue; 43°75 in the yellow 
and green ; and 24°75 in the red and orange. Hence he con- 
cluded, that the illuminating or yellow rays have the greatest 
effect in promoting decomposition of carbon dioxide, those 
nearest them much less so, and the heating and chemical rays 
