296 The Physiology of Plants BOOK in 



certain was that chlorophyll is associated in the plastids with 

 many other pigments, but there was no general agreement 

 as to the nature of these, to their mode of origin, nor to 

 their relation to chlorophyll at all. The pigments described 

 by one author could not be satisfactorily identified with those 

 of another, nor could the immediate source of chlorophyll 

 be identified with accuracy. 



In 1859 Sachs called attention to the fact that plants 

 grown in the dark owe their yellow colour to the non- 

 development of chlorophyll in the cells. A little later he 

 showed that in light of moderate intensity, decomposition 

 and recomposition of the pigment continue without cessa- 

 tion. This observation was confirmed by Famintzin in 

 1867, and by Wiesner in 1876. The latter writer stated 

 that in strong light the most refrangible rays are those 

 which are most concerned in the development of the pig- 

 ment. Reinke, in 1893, attributed the effect to all the 

 visible rays, but especially to those between the spectrum 

 lines B and D, which are the most potent. 



The influence of temperature on the development of the 

 colour was first pointed out by Sachs in 1864. He said 

 that at the minimum temperature for growth the plastids 

 do not turn green. Wiesner, in 1877, said that there exists 

 an optimum temperature for the development of the pig- 

 ment, which, however, is not uniform for all plants. In 

 1880 Elfving showed that when the temperature is too low, 

 the access of light produces a deeper yellow in the plastids, 

 increasing the amount of etiolin, instead of giving rise to 

 chlorophyll. 



As a further factor in the development of the latter, we 

 have the free access of oxygen. Correns showed in 1892 

 that if the partial pressure of this gas is reduced to such 

 a point as almost, but not quite, to inhibit growth, the 

 green colour will not develop. 



The idea of the independent nature of the apparatus de- 



