228 PIGMENTS 



exposure to daylight will not form chlorophyll unless a supply 

 of carbohydrate is available. If an etiolated leaf does not con- 

 tain carbohydrate, then greening will take place if the cut leaf 

 be placed in a solution of sugar. Almost any sugar will do, 

 e.g. sucrose, maltose, glucose, fructose, or raffinose ; success was 

 also obtained by the use of glycerine. The solution used must 

 be neither too weak nor too strong ; a strong solution of suc- 

 rose, for instance, will retard the chlorophyll formation because 

 it will depress oxidative processes. On the other hand, Issat- 

 chenko* finds that etiolated leaves of certain plants, e.g. those 

 of Vida Faba, when detached from the plant and placed in 

 strong sugar solution, even 50 per cent, will form chlorophyll. 

 He considers that light is the all-important factor. 



With regard to the substances which immediately precede 

 chlorophyll, and from which chlorophyll is formed, nothing 

 definite is known. 



The chemical study of chlorophyll dates from the year 

 1819, when Felletier and Caventouf first applied this name 

 to the green leaf pigment without, however, isolating the 

 substance. Since then, numerous workers have attempted 

 to prepare chlorophyll in a pure condition, but the methods 

 employed in most cases were of too drastic a nature for the 

 substance to escape destruction. Previous to 1911, there was 

 no chemical evidence to show that chlorophyll was not a 

 single chemical individual, although Stokes,} Sorby, and 

 others had obtained spectroscopic evidence pointing to the 

 existence of more than one substance ; confirmatory evidence 

 was subsequently ob'tained by Tswett.ll In 1912, however, 

 Willstatter and IslerU" definitely showed that chlorophyll as 

 ordinarily obtained, and to which they had originally assigned 

 the formula C 5& H 72 O 6 N 4 Mg, is in reality a mixture of two 



substances : 



Chlorophyll a CsgH^O^Mg ** 

 and Chlorophyll b C 55 H 70 O 6 N 4 Mg. 



* Issatchenko : " Bull. Jard. Imp. Bot., St. Petersbourg," 1906, 6, 20. 

 f Pelletier and Caventou: " Ann. Chim. Phys.," 1819, 9, 194. 

 I Stokes : " Proc. Roy. Soc.," 1864, 13, 144. 

 Sorby : id., 1872, 21, 442. 



H Tswett: " Ber. deut. bot. Gesells.," 1906, 24, 326; 1907, 25, 137; " Ber. 

 deut. chem. Gesells.," 1908, 41, 1352. 



IT Willstatter and Isler : " Annalen," 1912, 390, 269. 



** For the physical characteristics of these t\vo substances see page 234. 



