THE VEGETABLE CELL. 85 



cular kinds of gas, the term respiration. Many Lave regarded 

 the term as inapt, because plants have no organ of respiration, 

 and the like. Let us not contest words, but enquire in what 

 relation these processes stand to each other and to the life of the 

 plant. 



Plants, from what has been said, have a double respiration, one 

 consuming carbonic acid and exhaling oxygen by day in the green 

 coloured organs, and one connected v/ith a consumption of oxygen 

 and a formation of carbonic acid in the green organs by nighty, and 

 in those not green by day and night. 



The question, which of these processes predominates, whether, 

 on the whole, the plant consumes or forms a greater quantity of 

 carbonic acid, whether consequently the respiration of plants is 

 on the whole a deoxidating or an oxidating process, is again fully 

 cleared up by Saussure's experiments. 



When a plant is confined in a definite volume of air, the air 

 is found unaltered in volume and composition after an equal num- 

 ber of days and nights ; thus the plant has formed just as much 

 carbonic acid by night as it has consumed in the day. But if 

 carbonic acid is added to the atmospheric air in which the plant 

 vegetates, or the plant is caused to absorb water containing car- 

 bonic acid, it exhales oxygen into the surrounding air. 



There can be no doubt that plants in open air are in the same 

 position as those in the last experiment. A very considerable 

 quantity of carbonic acid is continually being added to the atmo- 

 sphere through putrefaction, combustion, the respiration of animals, 

 volcanic eruptions, mineral sources, &c. ; this constant addition of 

 carbonic acid above the usual amount, is again removed from the 

 air by plants and replaced by oxygen. Consequently, plants do 

 not purify the air by increasing the proportion of oxygen in it (if 

 we do not take into account that carbonic acid which is not formed 

 at the expense of oxygen of the air, such as that derived from vol- 

 canic sources), but by the removal of the carbonic acid constantly 

 flowing into the atmosphere, formed at the expense of atmospheric 

 oxygen. 



In order to become acquainted with the influence which these 

 two kinds of respiration exercise upon the vital operations of 

 plants, we must investigate the phenomena that present them- 

 selves when one or other of these breathing processes is inter- 

 rupted. 



When plants are prevented, by keeping them from the lights 

 from absorbing carbonic acid and exhaling oxygen, their nutrition 

 suffers and they become etiolated. They do continue to form new 

 shoots at the expense of the nutriment contained in their older 

 parts ; these are even larger than those developed under the in- 

 fluence of light, but weak and soft; the leaves remain small and 

 do not become green, the normal qualities of the saps are not pro- 

 duced, bitter, milky plants remain sweet, &c. Some plants will 



