162 



THE CELL AND PROTOPLASM 



action with violent material changes which 

 lend themselves for analysis, and partly 

 because in principle oxidation is a simple 

 process, a transfer of hydrogen or electrons 

 through a series of substances. Fortu- 

 nately there are a few vitamins involved in 

 this process, like alloxazines, nicotinic acid, 

 and probably Bi. The function of these 

 substances is to transfer electrons or hydro- 

 gen, and is in no way different from the 

 action of other non-vitaminic catalysts in- 

 volved in this process. 



As far as we can see, therefore, the mode 

 of action of a vitamin is in no way differ- 

 ent from the mode of action of other sub- 

 stances and this point, too, fails to distin- 

 guish the vitamins as a specific group of 

 substances. 



At the beginning of my lecture I defined 

 the Vitamins as substances which make us 

 ill if we don't eat them, because we are un- 

 able to make them ourselves. This last 

 specific quality of vitamins remains — that 

 they cannot be made by our cells. One 

 might thus say that here is the real spec- 

 ificity of vitamins which must have some 

 common mysterious quality in their mole- 

 cules which makes their synthesis impos- 

 sible within our cells. One might also add 

 that all my talk about the unity of Nature 

 is nonsense, for here is a great point of 

 difference between plant and animal: the 

 plant can make vitamins, the animal can't. 

 This point deserves our full attention and 

 we must try to find out whether it is 

 really some basic quality, or merely an ac- 

 cidental factor which makes this synthesis 

 impossible. 



At first thought, we might suspect that 

 there could be no real difficulty about syn- 

 thesizing a vitamin, or else the plants could 

 not do it either. After all, the cabbage is 

 not much wiser or more clever than we are. 

 But let us follow historic developments. 



Some time ago we believed that vitamins 

 were substances synthesized by plants but 

 not by animals. There was a little hitch in 

 this definition. It was known that most 

 animals of our climate, for example rats, 

 did not contract scurvy if fed on a vita- 

 min-C free diet. This observation was dis- 



missed with the hypothesis that these ani- 

 mals did not need vitamin C. But some 

 fifteen years ago M. Pearce showed that 

 rats could synthesize vitamin C. If she 

 bred rats on a C-f ree diet, the tissues of the 

 great-grandchildren contained just as much 

 of this substance as those of their great- 

 grandmother. Today we have good reasons 

 for believing that there is no higher organ- 

 ism which can live without ascorbic acid, 

 vitamin C, and that if most animals of our 

 climate — dogs, cats, rats, fowls, cows, rab- 

 bits, etc. — do not contract scurvy, it is not 

 because they can dispense with vitamin C 

 but because they can make it. For these 

 animals ascorbic acid is not a vitamin at all, 

 and you must be careful what you say to a 

 rabbit, for if you tell him that ascorbic acid 

 is a vitamin he will just laugh at you. 

 For him vitamin C is not a vitamin at all : 

 he can make it. 



It is easy to understand why this should 

 be so ; if the rabbit could not make ascorbic 

 acid it would die from scurvy during the 

 winter when there is no ascorbic acid to 

 be found in Nature, and there would be no 

 rabbit at all. 



However this may be, it is evident that 

 the old sharp borderline between vitamins 

 and not-vitamins has vanished. What is a 

 vitamin for one species is not a vitamin for 

 another, and if we say a substance is a 

 vitamin, we should mention the species for 

 which it is a vitamin. Thus, the inability 

 to synthesize vitamins gives us no reason 

 for separating the vitamins from other sub- 

 stances; moreover, there are many sub- 

 stances which we do not call vitamins which 

 we are also unable to synthesize. 



All the same, because this question of 

 synthesis is most intimately connected with 

 the whole conception of a vitamin, we must 

 try to understand it still better and try to 

 find out why some species are unable to 

 make some of these substances while others 

 can. We will do best not to lose ourselves 

 in generalities. Let us take one example 

 and ask, for instance, why we are unable 

 to make ascorbic acid, while the rabbit can 

 make it. But to be more discreet we will 

 talk not about ourselves but about our ex- 



