no SYMBIOSIS 



wholesome, energising, regulating and restraining role played 

 by symbiotic food. Just as in the case of plant nutrition, 

 treated of in the second chapter, so as regards animal nutrition, 

 we have learnt that it is the simple materials which chiefly 

 count. Though we use proteins for instance, it is their con- 

 stituents, the amino-acids, which are really wanted. These 

 are the indispensable " building-stones " in animal nutrition 

 and they are for the most part manufactured by the plant. They 

 are produced not only for the support of the plant's own off- 

 spring, but also for the purpose of " export," i.e., the support of 

 the animal as the biological partner of the plant. 



From a paper on The Bio-chemical Analysis of Nutrition, 

 by C. L. Alsberg (U. S. Bureau of Chemistry) in the Scientific 

 American, Supplement, 24th March, 1917, we gather the follow- 

 ing : In Liebig's time proteins were regarded as that element of 

 the food which supplied the material for growth, tissue main- 

 tenance and repair, as well as for most of the energy. It was 

 however soon demonstrated that while proteins did and could 

 furnish energy, under ordinary conditions this was supplied in 

 the main by sugar and other carbo-hydrates and by catabolised 

 fats [i.e., the materials chiefly drawn from cross-feeding]. For 

 long it was held that one protein was of about as much dietary 

 value as another, which, however, was found to be an erroneous 

 notion. Then a startling discovery published in 1901, by Loewi 

 tended to show that it was not absolutely necessary to life that 

 protein be an element of the diet at all. What is really indis- 

 pensable is a suitable mixture of " building stones," i.e., amino- 

 acids ordinary organic acids in which one or two hydrogen 

 atoms have been replaced by the amino group NH 2 . The 

 proteins are combinations of a number of these amino-acids 

 with one another. It should be theoretically possible, says 

 Dr. Alsberg, to supply the " so-called " protein needs of animals 

 by wholly artificial substances, such as the seventeen or eighteen 

 pure crystalline amino-acids that we knew of. [This I venture 

 to interpret as meaning that normally the animal can well be fed 

 by the surplus " building stones " of the plant.] 



On the matter of the " conversion " of proteins by means of 

 digestion, it will be best to quote Dr. Alsberg in extenso : This 

 is what he states : 



Whatever may be the ultimate practical significance of the observa- 

 tions that animals can supply themselves with most or all of their nitrogen 



