CH. iv] CARBON ASSIMILATION 27 



As the concentration of sugar increases in the cell, further condensation 

 may take place to form starch : 



^(C 6 H 12 6 ) = (C 6 H 10 5 ) a; +^ H 2 0. 



The facts in agreement with these views are : first, in most plants a 

 volume of oxygen is given off approximately equivalent to the volume 

 of carbon dioxide absorbed ; secondly, in some plants starch, in others 

 sugar, is known to be produced during photosynthesis. The detection 

 of formaldehyde, either in the plant or in certain systems containing 

 chlorophyll, as a proof of its formation during photosynthesis, has been 

 shown to be invalid (see p. 36) (Jorgensen and Kidd, 2). 



The value and significance of this reducing reaction is seen when it 

 is realized that, by oxidation of the carbohydrates synthesized, energy is 

 produced to supply the needs of the whole metabolism of the plant 

 (see p. 5). 



In the chemical treatment of the subject of carbon assimilation, some 

 of the chemical properties of chlorophyll will first be considered, and, 

 later, its behaviour under certain conditions : the chemistry, however, of 

 the phenomenon itself is as yet unknown. 



The following account, as far as it concerns chlorophyll, and the 

 accompanying experiments are taken from a resume (Jorgensen and 

 Stiles, 3) of the original w,ork (Willstatter und Stoll, 1) upon which the 

 entire knowledge of the subject is based. 



CHLOROPHYLL. 



Our knowledge of the chemistry of chlorophyll has, within recent 

 years, been set upon a firm experimental basis (Willstatter und Stoll, 1). 

 The results which have been arrived at may broadly be summarized as 

 follows : 



In all plants examined the chloroplastids contain four pigments, of 

 which two (termed respectively chlorophylls a and b) are green, and 

 two are yellow. They occur in about the following proportions in fresh 

 leaves : 



(Chlorophyll a ... C 55 H 72 O 5 N 4 Mg . . . 2 pts per 1000 

 {Chlorophyll b ... C 55 H 70 6 N 4 Mg ... | 



Yellow i Car tin C4 H56 * 



IXanthophyll ...CJBLp, J 



A point of great interest in connexion with chlorophyll is thatTit 

 contains magnesium to the extent of 2'7 % but no other metal is present. 

 Chlorophyll a, when isolated, is a blue-black solid giving a green-blue 



