PRESIDENTIAL ADDRESS SECTIOX C. 59 



to which the name phytol is given. Alkalis first saponify these 

 ester side-chains giving salts of the corresponding dicarboxylic 

 acid, chlorophyllin. 



The waxy amoq^hous nature of chlorophyll and its colloidal 

 properties are to some extent bound up with the phytyl radicle. 

 Willstcitter and Stoll have shown that an enzyme occurs in the 

 living leaf, called chlorophyllase, which in the presence of ethyl 

 alcohol replaces the phytyl by ethyl. The product of this ex- 

 change is a substance known as " crystalUne chlorophyll." which 

 is deposited in crystalline form from alcoholic solutions. True 

 chlorophyll does not crj-stallise. 



It is clear from the facts here briefly outlined not only that 

 the optical properties of chlorophyll are bound up with the 

 magnesium atom, but also that in true chlorophyll this atom is 

 combined in such a way as to give the complex a pecuhar mo- 

 bility. This is revealed b}^ its behaviour towards alkalis as well 

 as by its sensitiveness to acids. It appeared to Willstatter and 

 Stoll highly probable, therefore, that chlorophyll plays a chemical 

 as well as a purely physical part in the assimilation of carbon di- 

 oxide, and they set themselves to obtain direct evidence of this 

 by investigating the behaviour of pure chlorophyll towards this 

 gas. 



In alcoholic or ethereal solution they found no evidence that 

 CO2 (anhydrous) is taken up by chlorophyll. Chlorophyll solu- 

 tions absorbed no more than the pure solvents. Colloidal 

 eolufeions of chlorophyll in water, on the other hand, absorb more 

 CO2 than pure water. Tlie limit is reached at two molecules of 

 CO2 to each molecule of chlorophyll with decomposition of the 

 chlorophyll, giving iDliaeophytin and magnesium bicarbonate. Here 

 carbonic acid acts like other weak acids, removing the magnesium. 

 The decomposition is not, however, simple and direct. Willstatter 

 and Stoll succeeded in obtaining clear evidence of an inteiTnediate 

 addition compound which can be completely dissociated into un- 

 altered chlorophyll and carbonic acid. On this basis they 

 propound the theory that in the assimilation of CO. by the 

 chloroplast this addition product is first formed, and that the 

 enzymic factor then decomposes it into free chlorophyll, oxygen, 

 and the formaldehyde residue from which carbohydrate is 

 synthesised. 



The formation of the addition product is apparently inde- 

 pendent of light. Willstatter and Stoll suppose that the 

 absolution of light results in a change in its configuration which 

 makes it susceptible to the enzymic factor. 



This theory provides three stages, each dependent upon one 

 of the three external factors which, according to Blackman, act 

 as independent Hmiting factors — a feature which tells in its 

 favour. But it has yet to be learned what Briggs's second 

 photochemical agent is and what part it plays. Leaving there- 

 fore many other suggestions that Willstatter and Stoll make of a 

 more hypothetical nature, reference may be made to an aspect 

 of the subject which is of special interest to South African 

 botanists. 



