ACTION OF MICRO-ORGANISMS 209 



adds formed during digestion escape into the surrounding 

 liquid and by testing this dialysate for amino acids. 



The underlying principle is that the placenta in pregnancy 

 causes an immunity reaction so that the serum of the pregnant 

 female acquires the power of digesting placental tissue.* 



The above description refers to vegetable parasites, but 

 within recent years chemical reactions in relation to animal 

 parasites have acquired considerable importance. That 

 malaria parasites at certain stages of their existence can be 

 easily destroyed by quinine has been known for a long time, 

 but we must refer to some points in connection with 

 trypanosomes, the parasites that cause sleeping sickness. 



Arsenic is toxic to all animal tissues, but by combining 

 arsenic with an organic carbon group it can be shown to be 

 toxic to animal parasites, but not markedly toxic to higher 

 animals. 



Various organic compounds containing arsenic have been 

 used successfully in the treatment of trypanosomiasisf 

 (sleeping sickness), and in syphilis (infection with spirochceta 

 pallida). 



Another element which kills animal parasites in the human 

 body is antimony. 



BIO-CHEMICAL CHANGES IN THE SOIL 



Returning now to our main theme we find that the final 

 stages in the decomposition of organic matter occur in the 

 soil ; the aim of cultivation is to aid the proper transformation 

 of the materials in the soil. 



The soil consists of a framework of particles of different 

 sizes with water and air filling up the spaces between. In the 

 soil water are dissolved various substances, and it is these 

 substances which are taken up by the roots of plants. 



The inorganic salts are of importance. As water drains 

 through the soil it removes some of the soluble material in 

 solution and some constituents wash away more rapidly than 

 others. The supply of inorganic material is maintained by 

 the disintegration of the mineral substances contained in the 

 soil. In the growth of a plant deficiency or excess of a 

 substance is injurious. Mineral fertilisers are used to keep 

 the inorganic constituents up to the optimum concentration. 



As in the law of mass action, if one constituent is deficient 



* E. Abderhalden, Abwehrfermente des tierischen Organismus. Berlin, 



1913- 



f H. W. Thomas and A. Breinl, L'pool. School of Trop.Med., 1905, 

 Memoir xvi. 

 14 



