112 PHYSIOLOGY 



the energy of the sun's rays. Most plants will grow in the dark if supplied 

 with sugar and with combined nitrogen either in the form of ammonia or of 

 nitrates. The higher plants are especially dependent on the presence of 

 nitrogen in the latter form, and it is on this account that the nitrifying bac- 

 teria of the soil acquire so great an importance for agriculture. From the 

 carbon dioxide of the atmosphere or from the hexose formed by the assimila- 

 tion of carbon, and from nitrogen, in the form either of ammonia or nitrates, 

 together with inorganic sulphates, the plant cell is able to build up all the 

 various types of protein which are distributed throughout the vegetable 

 kingdom. Our study of the disintegration products of proteins has shown 

 that this class of bodies contains a large number of the most diverse groups, 

 having as a common character the possession of nitrogen in their molecule, 

 generally as an NH 2 or NH group. These disintegration products can be 

 'classified as follows : 



(a) Open chain amino-acids. 



(6) Heterocyclic compounds, including : 



(1) Pyrrol derivatives. 



(2) Pyrimidine derivatives. 



(3) Iminazol derivatives. 



These two last groups co-exist in all the purine compounds. 



(c) Benzene derivatives. 



(d) Indol derivatives. 



The first step in the synthesis of proteins is probably the formation of these 

 constituent groups. Just as in digestion the protein molecule is taken to 

 pieces with the formation of the different amino-acids, so in the synthetic 

 action of protoplasm the reverse process of dehydration occurs, resulting 

 in a coupling up of the different groups, as has been effected by Fischer in the 

 case of the polypeptides. Wherever transport of protein from one part of the 

 organism to another is necessary the protein is carried, not in its original 

 form, but in the hydrolysed condition of amino-acids. Thus the germination 

 of seeds which contain rich stores of protein is accompanied by a liberation 

 of proteolytic ferments within the cells of the seeds, and the breakdown 

 of the reserve protein into its constituent amino-acids. As amino-acids it 

 is transported into the growing tip and leaves of the seedling, analysis of the 

 latter showing a very large percentage of nitrogen in the form of amino-acids. 

 This is especially the case if the synthetic functions of the growing tip are 

 hindered by interference with assimilation, as, e.g. by keeping the plant in the 

 dark. Under these circumstances, asparagine may form as much as 25 

 per cent, of the total dried weight of the seedling. In animals the greater 

 part of the protein of the food is broken down into its constituent amino- 

 acids in the intestine. These are absorbed and probably carried to the 

 different organs of the body, where they are resynthetised, generally in 

 different proportions from those of the original protein, into the protein 

 specific for the organ or tissue. The same process of hydrolysis and 

 subsequent synthesis occurs whenever the transport of protein is neces- 

 sary from one organ to another. We shall later on have to discuss the 



