136 KESPIEATION OF PLANTS. 



that the spiral vessels from their structure were to be looked upon as 

 true wind-pipes or tracheae, conveying air from the stomata or pores 

 in the leaves. But although they contain aeriform matters, they have 

 been shown to be not directly concerned with the changes in the 

 atmosphere, and to have no immediate connection with the stomata. 

 The oxygen evolved by plants, appears to be derived from the carbonic 

 acid (CO 2 ), the carbon of which is appropriated, and from water 

 (HO), the hydrogen of which is assimilated. Light is necessary for 

 these decompositions, and it is probable that the alkalies taken up by 

 the roots aid the process. 



285. If the leaves of a plant are bent under an inverted tumbler of 

 water, in a pneumatic trough, and exposed to the sun, bubbles of gas 

 will soon be given off, which are found to be pure oxygen; and if the 

 water contains carbonic acid, there will be a diminution in its quantity. 

 The same leaves in darkness will not evolve any oxygen, light being 

 essential for the process. The oxygen derived from the carbonic 

 acid may be all evolved, or part of it may in its nascent state enter 

 into certain combinations within the plant. The brighter and longer 

 continued the light, the more oxygen is given off, and the greater the 

 quantity of carbon added to the plant. If a healthy plant is covered by 

 a bell jar, and exposed to light for twelve hours, oxygen will be formed, 

 and if carbonic acid be added to the air, it will gradually diminish, 

 while the oxygen will increase. During the night the action is 

 reversed, and if the plant is ieft twelve hours in darkness, the oxygen 

 will decrease, while carbonic acid will increase. 



286. The fixation of carbon probably takes place gradually, giving 

 rise at different stages to the formation of various organized com- 

 pounds. Thus, two atoms of carbonic acid, by losing one of oxygen, 

 become oxalic acid; this oxalic acid, with the aid of water, may yield 

 other acids, from which by the elimination of oxygen, and the addition 

 of the elements of water, various unazotised matters, as starch, gum, 

 and sugar, may be derived; these changes being promoted by the 

 presence of alkalies. The fixation of carbon and hydrogen from the 

 decomposition of carbonic acid and water, gives rise to the formation 

 of the various secretions found in the bark and external cells, as 

 chlorophylle, resins, oils, caoutchouc, and wax. 



287. Carbonic acid, as has been already noticed, is taken up in large 

 quantity by the roots of plants from the soil, and it is also probably 

 absorbed from the atmosphere by the leaves. In the ulterior of plants 

 it is changed in various ways, but it is in the leaves more especially 

 that its decomposition takes place. At night it is given off unchanged, 

 by what Liebig considers as a mere process of exosmose, in conse- 

 quence of the dissolved acid being no longer assimilated by the action 

 of light. Others say that carbonic acid is not produced by exhalation 

 only, but is also derived from the direct union between the oxygen of 



