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III. THE AGRICULTURAL PROBLEM. 



By E. J. RUSSELL, D.Sc, F.R.S., 

 Director of the Rothamsted Experimental Station. 



In attempting to integrate mycological research with agriculture the 

 great need of the present time is to study the growing plant in the field 

 in health and in disease, both states being regarded as the resultant of 

 the prevailing conditions. The problem falls naturally into two parts 

 which may be labelled the scientific and the practical, the broad dis- 

 tinction being that the practical problem is one for which on grounds of 

 expediency a working solution has rapidly to be found, while the scientific 

 problem though of equal or greater fundamental importance is subject 

 to no such necessity and can be studied at leisure. 



The scientific problem is largely one of response. Within certain 

 limits agricultural plants are fairly plastic: by altering easily controlled 

 external conditions wheat plants in one and the same field may be 

 induced to send up 8 or 10 stout stalks bearing ears well set with corn; 

 or on the other hand, only one or two stalks carrying fewer grains. They 

 may be made to form large broad leaves distinctly susceptible to the 

 attacks of rust, or somewhat smaller leaves less susceptible. This is 

 well shown on the Broadbalk wheat field at Rothamsted where there are 

 a number of plots over which the conditions are uniform excepting only 

 in regard to one factor, the nutritive treatment, which varies from 

 plot to plot. The experiment is continued year after year without modi- 

 fication so as to ensure as high a degree of accuracy as is possible. The 

 differences in the characters of the various crops are very clear and 

 sharply correlated with the differences in treatment: the withholding 

 of potash, for instance, is accompanied by marked susceptibility to 

 adverse conditions and to rust attacks. Similarly a field of mangolds 

 (Barnfield) is divided up chess-board fashion into a number of plots on 

 which notable differences in size, vigour, habit of growth and suscepti- 

 bility to disease are induced by alterations in the soil conditions. 



In the mycological problem three sets of factors are involved : the 

 crop, the parasite, and the external conditions. External conditions can 

 again be subdivided into soil and climatic conditions. At Rothamsted 



