TONICITY TO LIGHT III 



169. Trophic Relations of Light. Light bears a very complex 

 relation to the vegetal organism. It differs from all other trophic 

 factors in the fact that it is not absolutely necessary to the activity 

 and existence of living matter even for extended periods, although 

 it is ultimately of the utmost importance to the plant. 



Light exerts a direct chemical effect upon the substances of 

 which protoplasm is composed : it furnishes energy which is ab- 

 sorbed by chloroplasts and is connected with the synthesis of 

 carbon compounds. It stimulates the formation of chlorophyll, 

 although not necessary to the process, and its chemical action 

 disintegrates this substance. The absence of light constitutes a 

 specific stimulus that calls out the various phenomena of etiolation 

 as a reaction, and lastly the rays of light act as a directive or 

 orienting stimulus to which the plant responds by placing its axes 

 at various angles. 



170. Tonicity to Light. Not all of the rays of the spectrum 

 are concerned in the various influences exerted by light upon 

 living matter, but only rays of certain wave-lengths are active in 

 each. It is not possible therefore to fix upon a minimum, opti- 

 mum and maximum of intensity of light which is common to all of 

 the relations between the plant and light. In fact these points may 

 not be distinguished in some of the forms of action enumerated. 



171. Direct Chemical Influence of Light upon Protoplasm. 

 Sunlight has been found to exert analytic, synthetic, isomerismic, 

 polymerismic, and catalytic effects upon the chemical substances 

 occurring in protoplasm. How far these changes may be induced 

 when the substances are actually a part of living matter can not 

 be stated definitely. In its synthetic effect light may cause the 

 addition of oxygen to certain organic substances, to which action 

 the fatal influence of light upon certain organisms is supposed to 

 be due. Substances indifferent in darkness unite when their 

 molecules are acted upon by the vibrations of radiant energy. 

 On the other hand many compounds are split into two or more 

 constituents under the same conditions. Hydrogen may be re- 

 placed by chlorine, or bromine, in carbohydrates, acids, aldehydes, 



