202 HISTORY OF BOTANY 



from Phaeophyceae, save that in these plants one of the 

 green constituents is replaced by a body he called chloro- 

 fucin and one of the yellow by fucoxanthin. " The four 

 substances," he adds, " are soluble in the same solvents 

 and three of them are extremely easily decomposed by 

 acids and even acid salts, such as bisoxalate of potash, 

 but by proper treatment each may be obtained in a state 

 of very approximate isolation so far at least as coloured 

 substances are concerned." In a further paper pubUshed 

 in the same year he emphasises the value of carbon 

 disulphide in conjunction with alcohol, which enabled 

 him " to disentangle the coloured substances which are 

 mixed together in the green colouring matter of leaves." 



Sachs advanced our knowledge of the subject still 

 further, in 1864, by proving conclusively that starch 

 disappeared from the leaf at night and reappeared by 

 day, and disappeared even by day when the leaf was 

 shaded. He investigated the effect of changes in 

 temperature on greening, and determined the optimal 

 temperature for the process in different plant types. 

 He also asserted that chemical changes took place most 

 vigorously when leaves were exposed to yellow-red rays ; 

 employing potassium bichromate and ammoniacal copper 

 oxide as screens, he cultivated etiolated seedlings under 

 the rays transmitted through these solutions, and com- 

 pared the results with those obtained by the action of 

 the same rays on photographic paper. 



During the succeeding four years nothing very material 

 was added to our knowledge of chlorophyll and its 

 functions, but in 1868 two highly important papers 

 appeared by Becquerel and by Boussingault. Becquerel 

 made the first attempt to determine the energy relations 

 of the leaf, and estimated that about -2^ part of the 

 available solar energy was stored in the leaf in the potential 

 form, an estimate that suffered considerable revision in 

 later years at the hands of such investigators as Timiria- 

 zeff, N. J. C. Miiller, Detlefsen, and Brown and Escombe. 



