576 PHYSIOLOGY. 



&c. by day is probably reabsorbed and decomposed in the green 

 parts before it arrives at the surface of the leaves. 



According to De Saussure, if a plant is kept in a perfectly closed jar 

 containing a measured quantity of atmospheric air, for several days and 

 nights (an equal number of each), no change is found in the volume or 

 composition of the air ; the plant has exhaled oxygen by day and absorbed 

 it by night, and exhaled carbonic dioxide by night and decomposed it by 

 day, in equal proportion. But if this plant is watered with solution of 

 carbonic dioxide, or this gas be added to the air, the quantity of oxygen 

 in the air becomes increased. Under ordinary circumstances the leaves 

 decompose by day much more carbonic dioxide than they exhale by night. 

 The disengagement of oxygen has been observed in some aquatic plants 

 to go on in the dark for some hours after exposure to the sun. The sun's 

 light is thus stored away in the plant and rendered available in some form 

 or other when wanted. 



If plants are placed under such circumstances that they cannot decom- 

 pose carbonic dioxide and exhale oxygen (hy excluding light from them, 

 or by confining them in vessels deprived of carbonic dioxide), they never 

 acquire proper development ; no green colour appears (they are etiolated), 

 little or no woody matter is formed in the walls of the cells, and the 

 whole energy is consumed in pushing out weak watery shoots j scarcely 

 any of the peculiar resinous, milky, or other secretions are produced ; and 

 plants can only subsist under these circumstances when supplied with or- 

 ganic nutriment 



We see this when shoots are developed from Potato-tubers in the dark, 

 in the cultivation of Celery and other blanched plants, &c. But in some 

 cases it would seem that plants not only have the power of acquiring 

 carbon by their surfaces, but that they have also the power of growing 

 in an atmosphere deprived of carbonic dioxide provided they can assimi- 

 late the carbon from the carbonic dioxide circulating in their own tissues 

 (Saussure, Corenwinder). 



A moderate addition of carbonic dioxide to the food of a plant, with 

 free access of light and air, is mostlv accompanied by acceleration of the 

 nutrient processes and a more abundant liberation of oxygen. 



Many green plants will flourish in sunlight on water and carbonic di- 

 oxide alone. Saussure found that the organic matter of plants increased 

 in the proportion of 2 to 1 of the carbon contained in the carbonic 

 dioxide ; the elements of water being combined with the carbon. 



Effect of Nitrogen and of a want of Air. When plants are placed 

 in pure nitrogen gas, or in vacuo, all the functions of vegetation 

 are arrested ; not only do the chemical actions above noticed cease, 

 but irritability, like that of the Sensitive-plants &c., is lost, and 

 the plant decays. Even shoots separately enclosed suffer in the 

 same way. The death occurs especially early when the plant is 

 kept in the dark. 



This accounts in some degree for the injury resulting from roots grow- 

 ing down too deeply into the ground, as is often observed with fruit-trees. 



Assimilation and Respiration. It appears, therefore, that there 

 are two opposed sets of operations in which plants have close and 



