306 VEGETABLE PHYSIOLOGY. 
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oJ acid. It is stated that the quantity of oxygen absorbed in twenty -four hours varies in the leaves of 
different plants from one-half to eight times the volume of the leaves. The volume of the carbonic 
acid evolved is rather smaller than the quantity of oxygen gas taken up ; when the leaves are again 
exposed to light they give out the oxygen gas which had disappeared. 
All plants or parts of plants which are not green (fungi, roots, stems, flowers, &c.,) absorb oxygen 
and give off carbonic acid, whether exposed to light or not. 
It is to this double process that the term Respiration has generally been applied, although some 
authors have objected to it because plants possess no special respiratory organs analogous to those of 
animals. But in plants all the vegetative functions are spread over the whole of the active cellular 
tissue, instead of being localised in " systems," like the " digestive," " respirating," or other systems 
of animals, and therefore it is by no means to be concluded that a given process is not represented in 
the vegetable economy, merely because its operation is in some degree hidden, or at least complicated 
by its connexion with other processes in the same organs. Thus, in the leaves of plants, we have 
respiration, digestion, secretion, and development of new structure going on at one and the same time, 
and it is our business to endeavour to discriminate between the phenomena of these several processes, 
and to refer them to their proper laws. It probably will prove to be more correct to restrict the term 
respiration to the process analogous to the respiration of animals, which goes on in the absence of 
light, and in which oxygen is absorbed, and carbonic acid evolved. The absorption of carbonic acid 
and evolution of oxygen, going on under the influence of light, appears rather to be referrible to an 
assimilative or digestive process. 
As soon as it was known that this double action existed, it became a question whether either 
predominated over the other, or, if so, which ; whether on the whole, the plant consumed or formed a 
greater proportion of carbonic acid. Saussure's experiments gave a complete answer. When a plant 
is placed in a close vessel, with a determined volume of atmospheric air, the volume and composition 
are found unaltered after an equal number of nights and days : therefore, the plant forms as much 
carbonic acid by night as it consumes by day. But if carbonic acid be added to the atmospheric air, 
in which the plant vegetates, or the plant be watered with water containing carbonic acid, it gives off 
oxygen to the surrounding air. 
The general atmosphere is constantly supplied with a great quantity of carbonic acid by decom- 
position, combustion, the respiration of animals, emissions from volcanoes or mineral springs, &c, and 
thus plants in general are placed in a condition similar to that of the latter experiments. This continual 
influx of carbonic acid, over and above the usual proportion, is removed by the vegetable kingdom, and 
replaced by oxygen. Thus, as a general statement, we may say that plants purify the atmosphere, 
not bj' increasing the oxygen gas, but by restoring to it the oxygen removed by animals, &c, through 
the decomposition of the carbonic acid, which is in the first instance formed at the expense of the 
oxygen of the atmosphere. 
Attempts have been made to determine the respective influence of these two kinds of respiration in 
the vital processes of vegetables, by interrupting or suspending one of them. "When plants are placed 
in a dark place, where the want of light prevents their absorbing carbonic acid and evolving oxygen 
gas, their nutritive functions are disturbed and they become etiolated. They continue, however, to 
produce new shoots, at the expense of the nutritive matter contained in their older parts, just as the 
shoots are produced from tubers, like the potato ; but they are longer and weaker than shoots deve- 
loped under the influence of light, the leaves remain small and do not become green, and the natural 
characters and qualities of the juices are not brought out: bitter milky plants being sweet or tasteless 
under such conditions (celery, endive, &c.), and aromatic plants devoid of odour, &c. If, on the 
contrary, the process in which carbonic acid is consumed be stimulated, by supplying abundance of 
this gas to plants exposed freely to the sun's light, the nutrition is rendered more active. Even when 
nothing but water and carbonic acid are given them, the organic matter will increase, and the weight 
of this increase amounts to about double that of the weight of the carbon which was contained in the 
absorbed carbonic acid. 
When the process of absorbing oxygen and forming carbonic acid is interrupted by placing an entire 
plant in an atmosphere devoid of oxygen, e. g., in nitrogen gas, or under an air-pump, all the functions of 
the plant are at once paralyzed. The leaf and flower-buds are arrested in their opening and rot, the 
leaves no longer turn towards the light, do not display the alternating movements called waking and 
sleeping , and sensitive leaves lose their irritability. Even isolated organs may be killed by removing 
3, them from the influence of the atmosphere, while the root of the plant survives : for instance, roots it 
deeply buried in the earth. Plants die particularly soon when kept in the dark, in air deprived of 
oxygen gas ; Saussure killed even a Cactus, in this way, in five days. They bear such an atmosphere 
