726 PART IV. — THE PHYSIOLOGY OF PLANTS. 



carbohydrates. Again, the decompositions effected by organised 

 ferments (e.g. alcoholic fermentation) commonly involve an evolu- 

 tion of carbon dioxide without any corresponding absorption of 

 oxygen. 



I Respiration. This term is applied to the gaseous interchange, 

 pconsisting in the absorption^ of free oxyg;en and the evolution of 

 jcarbon dioxide, which takes place (with but few exceptions) be- 

 tween the living plant and the atmosphere, and which may be 

 cegarded as the external expression of the oxidative catabolic 

 process going on in the tissue of the plant. This gaseous inter- 

 change goes on over the whole surface of the body ; but in those 

 parts which possess stomata or lenticels, it is mainly conducted 

 through these apertures. 



Respiration seems to be somewhat diminished under the in- 

 fiuence of bright light; but its activity is promoted by a rising 

 temperature, and to some extent by greater moistness of the air. 

 The relation to temperature is such that respiration takes place 

 at temperatures even slightly below 0°C. ; that it increases in 

 intensity with a rise of temperature, but in greater proportion, 

 up to an optimum of 40°-45° ; and then sinks as the temperature 

 further rises until the fatal degree is reached. 



The relation of the volume of the gases absorbed and evolved 

 in respiration, that is, of oxygen and carbon dioxide, is a matter 

 of importance. It may be generally stated that the relation is 

 definite and constant for any given plant, or for any part of it, at 

 a given stage of development, all other conditions being constant : 

 the proportion ^^- may be unity, or less or more than unity, 

 according to the nature of the plant under experiment, and is 

 not affected either by temperature or by light. 



As illustrating the possible extremes of variation in the proportion, the 

 following cases may be mentioned. In succulent plants, when in darkness, 

 (see p. 725) oxygen is absorbed, but no carbon dioxide is evolved: and on the 

 'other hand, when Yeast and Bacteria, or germinating seeds, or parts of plants 

 (fruits, leaves, etc.), are kept in an atmosphere destitute of free oxygen, they 

 evolve carbon dioxide : again, at periods, such as a certain stage in the germi- 

 nation of seeds, the opening of buds and flowers, when the activity of respira- 

 tion is intense, the proportion '^ is at a maximum, though it rarely ex- 

 ceeds unity, that is, the evolution of COg is relatively large ; whereas during 

 periods of more normal respiratory activity, the proportion of carbon dioxide 

 evolved is smaller. The variations in the proportion ^3 are the expression 

 of the varying nature of the catabolic processes going on during dififerent stages 

 in the life of the plant or of any of its parts. 



