IRRITABILITY 209 



cally unstable substances. Some of its constituents and 

 organs bear also definite physical relations to light. Light 

 is neither constant nor uniform in amount, its direction and 

 intensity change, hence its action upon the same organ or 

 organism is different, may even be opposite, at different 

 times. Light affects the living protoplasm through the 

 chemical processes and physical conditions of the cell 

 at all influenced by light. Sunlight and the ordinary ar- 

 tificial lights are composed of rays of different sorts which 

 separately affect the substances, living and lifeless, exposed 

 to them. The effect of ordinary white light is then the sum 

 of the effects of its constituent rays. These rays, visible and 

 invisible, fall into three classes, the thermal or heat, the 

 luminous or light, and the actinic or chemical rays. Their 

 effects are partly suggested by their names. The luminous 

 rays, being the ones most concerned in food-manufacture, 

 affect the protoplasm in ways which we have already studied 

 (see pp. 53-57). The heat rays we have also examined in 

 their relation to transpiration, etc. (pp. 136-151), and we 

 shall study them further in the succeeding section of this 

 chapter (pp. 219-222). We have here to consider mainly 

 the influence of the visible and invisible (" ultra-violet") 

 chemical rays. 



Some idea as to why and how actinic rays affect living 

 protoplasm so strongly in other words, why living proto- 

 plasm is so very sensitive to light may be gained by con- 

 sidering how powerfully those rays influence lifeless sub- 

 stances and many processes taking place under the more 

 readily understood conditions prevailing outside the living 

 organism. For example, * olive oil oxidizes when exposed to 

 light, oxalic acid in solution will break up into carbon- 

 dioxide and formic acid in the light, alcoholic solutions of 

 chlorophyll decompose more rapidly in light than in dark- 

 ness, and many enzyms are destroyed by exposure to light. 

 These are all organic compounds occurring in plant cells. 

 The most significant in the list are the enzyms. Enzyms 

 accomplish the conversion of insoluble substances in the 



* Davenport. C. B. Experimental Morphology. Part I., pp. 162-5, 

 1897. 



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