THE QUARTERLY REVIEW OF BIOLOGY 





be well worth mentioning here. The 

 physical and chemical properties of chloro- 

 phyll were learned gradually during the 

 course of many experiments by a study of 

 its chemical reactions and of its acid and 

 alkaline decomposition products . In fact, 

 practically all of its physical and chemical 

 properties were known before the pure 

 pigment was obtained and so little new 

 was learned from studying the properties 

 of the green pigment when isolated. 



Some of the difficulties of the prepara- 

 tion of pure chlorophyll should be pointed 

 out. In looking over the papers which 

 have been published by Wills tatter and his 

 coworkers one should keep in mind the 

 fact that he was working with a green 

 pigment whose color was easily altered, 

 yet when altered a pronounced color 

 always resulted. The fact that he was 

 working with a colored pigment in which 

 any chemical or physical change could be 

 easily detected probably aided greatly in 

 the preparation of the pure compound. 

 Also, it was present in quite large amounts 

 in dry leaves, for from one kilogram of 

 dried leaves eight grams of pure chloro- 

 phyll may be isolated. The color proper- 

 ties of the pigment as well as its relative 

 abundance are factors which should have 

 made it easy to obtain it in the pure state, 

 but on the other hand the extreme solu- 

 bility and the ease of alterability of the 

 compound made the task a difficult one, 

 which took many years before the feat was 

 finally accomplished. 



I have attempted to give in the preced- 

 ing part of this paper a brief summary of 

 our knowledge concerning the carotinoids. 

 The chief facts of interest here are that 

 the carotinoids of the plant kingdom are 

 generally believed to be the source of many 

 of the carotinoids found in the animal 

 kingdom; this knowledge has been learned 

 only after many years of experimentation. 

 As yet no direct relation has been dis- 



covered between the carotinoids and the 

 vitamines, or in other words the vitamines 

 are probably not carotinoids. 



It would be very difficult to say which 

 of the many views regarding the role of 

 chlorophyll is the most acceptable one. 

 The general trend of events seems to be 

 that we are coming more and more to 

 accept such views as those of Wiesner. 

 This view holds that chlorophyll is 

 formed and decomposed simultaneously. 

 If such is the case then we are lead to 

 wonder regarding the part that pyrrole 

 plays in the whole process, and also which 

 pyrrole, if any, is the most beneficial. 



Can the plant absorb pyrrole as such or 

 does all nitrogen enter it in the commonly 

 accepted way? The relation of nitrogen to 

 the development of chlorophyll is some- 

 thing which has been little stressed in the 

 literature, but can anyone who has worked 

 with nitrogenous fertilizers doubt that 

 there is a direct connection between the 

 green pigment of plants and nitrogen? It 

 seems as if we are coming to learn that the 

 whole process of assimilation is not 

 directly dependent upon chlorophyll, for 

 evidence seems to show that chlorophyll 

 formation is not a limiting factor in plant 

 growth. 



Perhaps the only reason that pyrrole 

 compounds have never been found in the 

 soil is that nobody has ever looked for 

 them there. Since many other organic 

 compounds which are present in plants 

 and in animals too have been found in the 

 soil, is there any reason to doubt that 

 pyrrole, which is known to be present in 

 living things in relatively large amounts, 

 will not also be found there? In other 

 words, anything which is present in 

 living things should also be found in the 

 soil, because of the very nature of the 

 origin of the organic part of the soil, unless 

 chemical changes have resulted which 

 decomposed the substances found in living 





