SECTIONAU TRANSACTIONS.— K, 395 



suberised with pits in the suberised membrane. In some cases these walls, 

 especially the inner tangential ones, consist of cellulose. 



The subcrisation and lignification of the rarlial walls renders the endodermis 

 relatively impervious to the passage of water through the walls alone. Thus tlic 

 transpiration current must liow largely througii the eiidodermal cell, and it is 

 probable that the protoplasm exerts some control over the rate of flow. 



3. Presidential Address by Prof. H. H. Dixon, F.R.S., on 



'///(' Transport of Organic Substances in Plants. (See p. 1U3.) 



4. Joint Discussion willi Section B on Photosyntticsis. 



(a) Dr. .F. F. Blackman, F.R.S. — The Biochemical Problems of 

 Chloroplastic Photosynthesis. 



1. The active system of the living cell and its catalytic components. The 

 chlorophyll pigment component : its quantity and surface : Willstatter's chloro- 

 phyll numbers. The protoplasmic component; its possible enzymatic nature. 

 Cases of lack of co-ordination between the two components. 



2. The chemical reactions of reduction and condensation. The spontaneous 

 and catalysed metamorphosis of COo to substances of high anabolic potential. 



3. The energetics of these reactions : the problem of evaluation of energy 

 absorbed by chlorophyll in relation to chemical and physical work done : com- 

 parative efficiency of incident energy of different wave-lengths. 



4. Factors controlling the magnitude of photosynthesis ; quantity of incident 

 energy and of incident CO2. Relation to accumulation of products and to 

 toxic effects. The temperature coefficient of the reaction. 



5. The case of the chemosynthetic micro-organisms carrying out reduction of 

 CO2, in absence of ' light '-energy, by the oxidation of nitrogen, sulphur, or 

 hydrogen. The energy-balance of these processes. 



{b) Professor E. C. C. Baly, C.B.E., V.'R.^.— Photosynthesis. 



In every chemical reaction it is necessary to supply energy to the molecules 

 in order to cause them to react. In some cases this energy change is effected 

 by the solvent, but in other cases the increment of energy required is far too 

 large to be realised in this manner. This latter condition always obtains in 

 highly endothermic reactions, and it is often necessary to supply the energy in 

 the form of light, when the resulting process is called a photochemical one. 



The most interesting of all reactions is the photochemical conversion of 

 carbonic acid into formaldehyde, since this not only marks the first step in the 

 formation of sugars, starches, and celluloses, but it also plays a fundamental 

 role in the synthesis of the nitrogen products of plant life. This reaction, 

 indeed, forms the true key industry of all life. 



In order to enable this reaction directly to take place, considerable energy 

 is required so as to bring the carbonic acid molecule into the reactive form, 

 and this is secured when the molecule is exposed to light of very short wave- 

 length. It has been found, however, that in the presence of a coloured sub- 

 stance of basic properties the reaction takes place in visible light. There is 

 little doubt that in the plant the reaction takes place in three stages : 



1. Chlorophyll A+H^COs + light = Chlorophyll B-|- formaldehyde. 



2. Chlorophyll B -f Carotin = Chlorophyll A + Xanthophyll. 



3. Xanthophyll-f light = Carotin+ Oxygen. 



The formaldehyde when freshly synthesised is endowed with an extra- 

 ordinary reactivity, which is best expressed by the formula CHOH. In the 

 presence of potassium nitrite, which is known to exist in the growing leaf, 

 combination at once occurs to give formhydroxamic acid. This substance reacts 

 with more of the formaldehyde to give on the one hand a-amino acids, and on 

 the other, nitrogen bases, such as glyoxaline, pyridine, pyrrole, quinoline, and 

 indole. These substances are also produced in highly reactive forms, so that 

 combination at once ensues to give substituted amino acids, which then condense 

 to give proteins. The nitrogen bases wliTch do not condense with the amino acids 

 undergo further condensation to give alkaloids. 



