56 I'ltESIDENTIAI. ADDKKSS SKCTIOX C. 



work on the pigments of the chloroplast. The elucidation of the 

 nature and composition of these pigments is a monumental 

 achievement in tlie domain of plant chemistry. The work of 

 previous investigators had demonstrated the existence of both 

 green and yellow pigments, but there was a conspicuous lack of 

 agreement on many important questions, especially as to their 

 number, composition and properties. The success of Willstatter 

 and his school is due to the skill and care with which the pigments 

 were separated and obtained in an adequate state of purity, the 

 large scale of their operations and their thorough and systematic 

 study of the pigments themselves and their decomposition 

 products. 



It is now established that two green pigments, and two only, 

 exist in the chloroplast, one blue green and the otlier a pure 

 green in psolution. Willstatter calls these chlorophyll a and h 

 respectively. Both are present, though in slightly varying 

 proportions, in all the numerous green plant-s he studied. They 

 differ but slightly in composition. Chlorophyll a always pre- 

 ponderates. Besides these two green pigments there are two 

 yellow pigments, carotin (the pigment of carrots) and 

 xanthophyll, which are also closely related to each other. In 

 the Brown Algae there is very little chlorophyll h and along with 

 carotin and xanthophyll a third closely related yellow pigment, 

 fucoxanthin, is present; but the living thallus contains no pigment 

 corresponding to the phycophaein of the older textbooks. 



Willstatter and Stoll were unable to obtain any evidence that 

 the yellow pigments play a part in photosynthesis. Using yellow 

 leaves poor in chlorophyll, in which the yellow pigments pre- 

 ponderate, they observed no appreciable reduction of the rate of 

 assimilation when the more refrangible rays of light, specially 

 absorbed by these pigments, were intercepted by a suitable filter, 

 notwithstanding that the light was the limiting factor. 



Having devised a means of separating and determining the 

 chlorophylls quantitatively, they undertook parallel determina- 

 tions of chlorophyll content and rate of assimilation for a wide 

 range of leaves, in order to find out whether and, if so, how the 

 rate of assimilation depended on the amount of chlorophyll 

 present. They summarise their result in the form of " assimila- 

 tion numbers " which represent the number of grams of GOo 

 assimilated per hour by one gram-molecule of chlorophyll. Most 

 of their assimilation experiments were carried out at 25'^C. in 

 light of about the same intensity as direct sunlight, with an 

 abundant supply of COo. Under these conditions they showed 

 that for normal green leaves the temperature was the limiting 

 factor. 



The assimilation numbers vary from about o to 16, the larger 

 numbers being given by herbaceous plants, like the sunflower, 

 noted for their rapid assimilation and vigorous growth. These 

 variations would be sufficient by themselves to throw doubt on 



t Willstatter and Page, Ann. d. Chem. 404, 1914, p. 237. 



