802 ABSORPTION AND EXHALATION OF GASES BY LEAVES. 
exhalation of carbon dioxide by day and night, as constituting 
vegetable respiration ; and the exhalation of oxygen by day, as 
connected with assimilation ; while the supporters of Pepys’ 
views regard the exhalation of oxygen gas as vegetable respira- 
tion. Pepys says that oxygen is given off by the leaves both 
by night and day, but in a greatly accelerated degree during 
the day ; by most observers, however, no evolution of oxygen 
has been traced at night. 
It will be seen from the above abstract of the opinions of 
different physiologists, that various ideas have been entertained 
by them as to the action of the leaves and other green organs 
under different degrees of light ; and also upon the character of 
such changes. Generally, it may be stated,—that all agree as 
to the evolution of oxygen by the leaves and other green parts 
of plants under the influence of solar ight with the fixation of 
carbon, i.e. the deoxidation of assimilable materials, to which 
process the term assimilation is applied in this volume in accord- 
ance with the views now commonly entertained by botanists ; 
while that of respiration is here used to denote the absorption 
of oxygen and evolution of carbon dioxide, which takes place 
both by night and day, but is most evident by night, because 
the large quantity of oxygen given off during the day in the 
process of assimilation obscures the former change. In certain 
plants the sun’s light appears to be stored up in some unknown 
way for future use, so that we find some aquatic plants after 
exposure to its influence disengage bubbles of oxygen in the - 
dark. 
Whatever views we may entertain, all admit that this evo- 
lution of oxygen gas by day has a most important influence in 
Nature. This will be at once evident when it is remembered 
that it is the only known process by which oxygen gas,—so 
essential to our existence, and which is constantly being re- 
moved from the atmosphere we breathe, by the respiration of 
man and other animals, by the process of combustion, by oxi- 
dation of mineral matter, and by other processes that are con- 
stantly goimg on upon the globe,—is restored to it in a free 
condition. Thus we see that, ‘the two great organised king- 
doms of nature are made to co-operate in the execution of the 
same design ; each ministering to the other, and preserving that 
due balance in the constitution of the atmosphere which adapts 
it to the welfare and activity of every order of beings, and which 
would soon be destroyed were the operations of either of them 
to be suspended. It is impossible to contemplate so special an 
adjustment of opposite effects without admiring this beautiful 
dispensation of Providence, extending over so vast a scale of 
being, and demonstrating the unity of plan upon which the 
whole system of organised creation has been devised.’ 
In alike manner, plants purify the water in which they grow, 
and render it habitable by certain animals. We all know by early 
Se a ————— 
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