686 SCIENCE PROGRESS 



is produced in large amounts during the initial stages of the 

 germination. By the action of a ferment, which can easily be 

 localised, it is, however, very quickly converted into a nitrogen- 

 free substance — homogentisinic acid. This same substance, by 

 the way, is formed from tyrosine in the human body under 

 certain abnormal conditions and excreted in the urine, giving 

 rise to so-called alkaptonuria. 



After the exhaustion of the nitrogen supply from the seed, 

 the plant depends for its nitrogenous foods on the nitrates and 

 ammonia salts of the soil. Those plant organs seem to be 

 predestined to carry out the protein synthesis in the adult 

 plant, in which carbohydrate formation is also most vigorous ; 

 and it was pointed out by Sachs in 1865 that the leaves perform 

 the largest share of this work. Very little definite knowledge 

 exists with regard to the actual processes. It is generally 

 accepted now that the first substances formed are amino acids, 

 and that by the linking of these together into complex chains 

 the proteins are ultimately produced. Light plays a most 

 important part in the protein formation in leaves. Whilst 

 leaves are unable to make use of free nitrogen, several obser- 

 vations confirm the view that small amounts of ammonia 

 are absorbed. The protein synthesis in leguminous plants, 

 which takes place by the help of bacteria, has already been 

 discussed (p. 679). 



Protein Synthesis in the Animal Organism 



Since Loewi's experiments, which showed that in animals 

 weight, health, and nitrogenous equilibrium can be maintained 

 for a considerable time by feeding them on the crystalline 

 cleavage products of protein hydrolysis, the old view that 

 the albumoses and peptones are absorbed as such has been 

 steadily losing ground. The belief is now general that the 

 largest part of the ingested protein undergoes cleavage into 

 its fundamental amino acids, and that the body exerts a 

 selective power in building up its various proteins from these 

 comparatively simple materials. Evidence is accumulating 

 which confirms this view, and of the many researches in this 

 direction, those of Abderhalden and his collaborators should 

 be specially mentioned.^ 



' This question has been fully discussed by F. G. Hopkins, SCIENCE PROGRESS, 

 vol. i. 1906, 159, and by E. P. Cathcart, ibid. vol. ii. 1907, 76. 



