146- 
Supplement to Iho " Tropicn' AgricuUiausL" [Aug. 1, 1989. 
roots, stem and leaves, ami give the .seed its 
valuable nutritive qualities. iSitice the .soil did 
not contain either carbon or nifrogeii, the plant 
must have di'awn these two elements from the air. 
It is the purpose of this article to explain the 
nature of this fixation of the carbon and nitrogen 
of the air. 
Tlie classic experiments of Priestly, in 1771, 
established the fact that plants exhale oxygen. 
Later researches made by lugenhous and by 
Tetinebier explained the decomposition of the 
carbon dioxide of the air and the evolution of 
oxygen by the le.aves under the iiifluence of light. 
The earth's atmosphere contains only three parts 
of carbon dioxide in 10,000 of air. It; is evident, 
therefore, that in order that plaius may obtain the 
carbon which they require from ;i medium so 
poorly supplied witli it, rapidity of absorption l>y 
the tissues must compensate for tiie scarcity of 
the element in tlie air. 
In the first place the absorption of carbon 
dioxide is favoured by ihe form of the leaves, 
which is such that they offer, as co-jajjaied witli 
tiieir weight, an enormous absorbing surfac". In 
a tree tiie leaves are at; tlie exti'emities of inlinitely 
ramified, flexible branches, which are agitated by 
the slightest breeze, thus facilitating contact of 
the leaves with the constantly renewed layers of 
air about them. That the absorption of carbon 
dioxide is very rapid may be shown by placing a 
leaf from which the air has been exhausted by 
means of an air-pump in an atmosphere of carbon 
dioxide in an apparatus which measures the 
change of the volume. It will be observed that 
absorption begins instantly, but that it is largely 
dependeufc upon the quantity of the water present 
in the leaf. Thus, the co-efficient of absorption of 
the carbon dioxide in old leaves of Japanese 
Euoiiymus, containing 66 3 per cent, of water was 
found to be 0*70 at 15°, while in young leaves of 
the same tree containing 75*4 per cent, of water the 
co-efficient was 0"83. A comparison, at different 
temperatures, of the co-efficient of absorption of 
carbon dioxide in tlie leaves with that in pure 
water shows the absorption in the leaves to be 
somewhat greater than in pure water. This in- 
dicates that the carbon dioxide is not simply 
dissolved in the water in leaves, but that it 
combines with the water to form a hydrate. It 
will be shown later that this fact is of great 
importance. 
The carbon dioxide which is absorbed by the 
leaves is decomposed, and the products of this 
decomposition are utilised in the formation of the 
simplest primary organic compounds, from which 
the more complex constituents of plants are 
derived. To accomplish this the ijrincipal condi- 
tion is that the leaf be perfectly healthy. 
]f it does not contain its normal proportion of 
water, i.e., if the roots do not draw trom the soil 
as much water as is given off through the leaves 
the decomposition of carbon dioxide is checked. 
Assimilation has ceased wlien, as at the end of a 
summer day, the leaves of the tobacco plant, for 
instance, are hanging down the stem, or those of 
the beet lie flat on the soil. In fact it has been 
found that the decomposition of carbon dioxide 
begins to decline even before the leaves have lost 
their turgescence. 
Light is absolutely essential to the assimilation 
of carbon by the leaves of plants. The principal 
source of this energy is, of course, the sun, but 
attempts have been made to utilise artificial light, 
especially electric liglit, for foreign plants. Sie- 
mens in England, Builey in America, and the 
author in France have made experiments of this 
character. Since there is no doubt that, with the 
increasing use of water power for the production 
of electricity, a large supply of electric light can 
be economically obtained, it is highly interesting 
to learn what its action is on plants. Ail ob- 
servers have found that rays from an arc lamp 
without a globe exert an injurious influence, 
blackening the epidermis of the leaves. During 
the autiior's experiments in 1881 the epidermis 
exposed to the direct rays became black, wdiile the 
parts jirotected by the upper leaves preserved their 
beautiful green colour. The liiie of demarcation 
was as sharp as in a photographic plate. Tlie 
in.iurious influence ceaseil as soon as tlie lamp was 
surrounded by a white glass globe through which 
the ultr.i-violet rays passed with difficulty. To 
understand the influence which the heat rays 
situated at the other extremity of the spectrum 
exert on vegetation, we must recall to mind that 
in respiration leaves, like all other plant organs, 
absorb oxygen and exhale carbon dioxide, a pro- 
cess which is precisely the opposite of that which 
occurs in assimilation. 
It must also be remembered that the activity 
of respiration increases with elevation of temper- 
ature, v>'hile rise in temperature has only a 
very slight effect on assimilation. JLaquenne 
and the author some years ago made a careful 
study of the action of botli light and heat 
rays on leaves. In this research two source of 
light were used, the Drummond light, which is 
obtained by rendering a piece of quicklime in- 
candescent by means of the oxhydrogen blowpipe, 
and the Bourbonze lamp, which is composed of a 
cylinder of platinum wire gauze, which becomes 
incandescent when heated with illuminating gas, 
the combustion of which is promoted by a strong 
cu'-rent of air. The leaves were introduced into 
tubes containing an atmosphere of known com- 
p )sition, and were placed very near the lights, but 
were protected by screens containing transparent 
liquids of varying diathermancus, properties. In 
some cases water was used, which allowed the 
light rays to pass but retained the heat rays. In 
other cases the screens were filled with benzene 
or with chloroform, which are also transparent 
but much more diathermanous than water. Ex- 
posing the leaves to the action of the Drummond 
light, which is poor in heat rays, and surrounding 
them with a screen filled with water, promoted 
reduction, the proportion of carbon dioxide in the 
tube diminish'ing, while the oxygen increased. 
When the screens were filled with chloroform, how- 
ever, and the Bourbonze lamp was used, which is 
rich in heat rays, the opposite effect v/as obtained, 
i.e., the carbon dioxide increased and the oxygen 
diminished. In this case the phenomena of res- 
piration took the place of those of assimilation. 
Passing from the study of the chemical and 
heat rays to that of the light rays in the central 
part of the spectrum, we find that the latter pro- 
duce very different effects fron; the foirmer, ' 
