200 PART III. PHYSIOLOGY. [ 45 



distributed osmotically in the form of diffusible compounds, pro- 

 bably amides, to the adjacent parenchyinatous tissues, and it is 

 probable, though not yet ascertained, that here again a proteolytic 

 enzyme is involved. 



Respiration. This term is applied to the gaseous interchange, 

 consisting in the absorption of free oxygen and the evolution of 

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

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

 garded as the external expression of the oxidative catabolic processes 

 going on in the tissue of the plant. This gaseous interchange goes 

 on over the whole surface of the body ; but in those parts jvvhich 

 possess stomata or lenticels, it is mainly conducted through these 

 apertures. 



Respiration seems to be somewhat diminished under the in- 

 fluence 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 0C. ; 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 de- 

 finite 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 2? may be unity, or less or more than unity, ac- 

 cording to the nature of the plant under experiment, and is not 

 affected either by temperature or by light. 



Respiration can be demonstrated by placing a quantity of germinating 

 seeds, or opening flower-buds, in an air-tight glass receiver (somewhat as 

 in Fig. 129), through which a current of air is drawn previously freed 

 from CO 2 by passing through solution of caustic potash. On examining 

 the gas drawn from the receiver, by passing it through a clear solution of 

 lime-water, it will be found that the lime-water becomes turbid in con- 

 sequence of the formation of calcium carbonate, the CO 2 in the gas with- 

 drawn combining with the lime. 



5. The Products of Metabolism. The relation between the 

 anabolism and the catabolism of the plant may be generally stated 

 thus, that the construction of organic substance in the former is 

 greater than the decomposition of it in the latter, so that on the 



