18 SCIENCE BULLETIN, No. 9. 



The minute quantities used are quite inadequate to supply plant-food in 

 the generally accepted sense of the term. For example, Aso, in some experi- 

 ments with peas, found that the growth of the crop was stimulated, and the 

 yield increased by O'OOl gramme sodium fluoride per 2 to 3 kilos of soil. 



Another Japanese investigator found 940 grammes of the same salt per 

 hectare to benefit barley and certain grasses. 



In the cases also where these substances act as plant-poisons, the propor- 

 tions are exceedingly minute. Similarly we know that iron-salts are neces- 

 sary for the production of chlorophyll, and that in the absence of iron in 

 the soil or culture medium the chlorophyll cells do not develop, and yet 

 chlorophyll itself contains no iron. 



There is some action of which we are ignorant in all these cases, for an 

 explanation of which we must wait for the plant physiologist. 



Recent work by Willstatter, Marchlewski, and others, has established the 

 fact that a great similarity exists between some of the products of the green- 

 colouring matter of plants and the haemoglobin or red-colouring matter 

 of the blood of animals and human beings. It has been shown that 

 chlorophyll is a magnesium compound, and contains no iron, which latter is 

 an essential constituent of the red-colouring matter of the blood. It would 

 appear as if the peculiar property of chlorophyll to absorb and split up 

 carbonic acid is due to the presence of magnesium in the chlorophyll mole- 

 cule, whereas its replacement by iron effects the absorption of oxygen. We 

 know of similar instances in which the introduction into an otherwise inert 

 organic molecule of metallic or elementary atoms results in remarkable 

 physiological activity. Ehrlich's celebrated specific against syphilis (a definite 

 amido-benzol compound containing arsenic) is one of the best known instances 

 in point. Wassermann has used a selenium derivative of eosin successfully 

 in the cure of cancer in mice. 



A number of similar compounds are at present under trial, particularly in 

 the case of cancer. 



The remarkable effects produced by the entrance of such elementary atoms 

 into the molecule is a fact of the highest significance, not only in the study 

 of disease in men and animals, but in plant physiology also. 



The above short review of the work which is being done in the solution 

 of a certain class of soil problems shows that the action of fertilisers is not 

 confined to supplying the crop with food, but that it is far more complex, and 

 that fertilisers influence the physical structure of the soil, and also its 

 biological and chemical condition in a great variety of ways ; further, that 

 we have to take into account a large number of factors which afiecfc the 

 fertility of the soil and which are quite independent of its supply of plant- 

 food. 



We have seen that fertilisers may exert an influence on the toxic matters 

 produced in the soil, the texture and the moisture-condition of the soil, on the 

 development of bacteria or fungi, on the oxidising power of the soil, and that 



