1893.] President's Address. 393 



exercised by the host in modifying the structure of the parasite, and 

 conversely the effect of the parasite upon the host, and the mechanism 

 of its attack. Subsequent work on the disease of lilies is specially 

 valuable, on account of the recognition and separation of the ferment 

 by which the passage of fungi through the tissues of their host is 

 effected. Upon the recrudescence of the question of the relation 

 of plants to the free nitrogen of their environment, which we owe to 

 the experimental and structural investigations in connexion with 

 leguminous crops, carried on in this country at Rothamsted, and in 

 Germany by Frank and Hellriegel, we find Professor Marshall Ward 

 attacking the point from his practical standpoint, and to him we are 

 indebted for the discovery that the well-known nitrogen-storing root 

 tubercles of the Leguminosse are a consequence of the attack of a 

 fungus, the mycelium of which penetrates the root hairs, and gives 

 origin eventually to the active " bacteroids " in the tubercle, a 

 fundamental fact in the relation of the plants to the soil. 



The best summary of the general bearing of his several investiga- 

 tions up to date is probably that provided by himself in the Crooniaii 

 Lecture he delivered, in 1890, upon the relations between host and 

 parasite in plant diseases, one of the most suggestive of the many 

 interesting lectures that have been given under the foundation, and 

 it is not amiss to mention here also, as remarkable for its suggestive- 

 ness, his paper of earlier date on " Sexuality in Fungi," in which the 

 progressive apogamy of the group is dealt with, and the parasitic 

 habit laid under contribution for an explanation. 



Of the many valuable pieces of work done by Professor Marshall 

 Ward, not the least remarkable is that on the ginger-beer plant a 

 model of experimental biological investigation in which the remark- 

 able symbiosis of yeast and bacteria is unfolded, and the idea, preg- 

 nant for industries such as brewing, of symbiotic as distinct from 

 metabiotic and antibiotic, fermentation is put forward. 



In the Reports, in conjunction with Professor Frankland, upon 

 bacteria in water, now being presented to the Society, we have Pro- 

 fessor Marshall Ward's most recent work, and, in connexion with it, 

 his probing of the question of the action of light in arresting the 

 development of and killing bacteria to mention only one of the 

 many points raised in the Reports has already brought out striking 

 results, the significance of which, from a sanitary point of view, is 

 sufficiently apparent, and it has led to other investigations by the 

 author into the wide question of the function of colour in the vegetable 

 kingdom, which promise to be fruitful in valuable generalisations. 



In the field of work to which I have referred, one of no ordinary 

 difficulty, Professor Marshall Ward has not merely increased enorm- 

 ously our stock of knowledge of solid fact, but in his dealing 

 with facts under review he has thrown many lights upon general 



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