18 TRANSACTIONS OF THE [Sebs. lxxvi. 



physical conditions which may render the host plants more 

 vulnerable, and thus enable the parasite to attack and 

 cause disease. The weather as a physical factor may pre- 

 dispose plants to certain organic diseases, so that the 

 observers in these early times were quite correct in their 

 observations, but their conclusions were inaccurate or 

 incomplete. 



Bacteriology has mainly owed its development to the 

 work and research carried out in connection with patho- 

 genic forms. The importance of this department of applied 

 botany is too obvious to require more than a passing 

 comment. Although the layman may be accustomed to 

 think of all bacteria as harmful, still the great dependence 

 of the higher forms of plant life on those lower organisms 

 is being made clearer every day. As I have already said, 

 as man becomes more exacting upon the natural resources, 

 so must he in turn endeavour to help Nature by artificial 

 means. As cultivation becomes more intensive, the more 

 must man employ scientific methods to conserve and 

 improve the fertility of the soil, and, in this connection, 

 valuable service has been rendered to agriculture, forestry, 

 and horticulture by the botanist and chemist. The stud}' 

 of plant chemistry and plant physiology has opened up a 

 wide field of research, in which already great progress has 

 been made, many valuable results achieved, and probably 

 nowhere with greater success than in the study of the soil 

 bacteria. The role played by the nitrogen-fixing soil 

 bacteria is becoming better understood every day. Suc- 

 cessful experiments have been carried out in artificially 

 inoculating the soil with these important organisms. In 

 other words, the soil may be sown with these useful 

 organisms, and upon the success of the development of this 

 invisible soil flora depends the success of crops of higher 

 plants. As we find almost always in Nature, these useful 

 soil organisms have their enemies in other soil micro- 

 organisms, and it has been found that by partially sterilising 

 the soil, crops are improved, the improvement being due 

 to the removal of those organisms inimical to the useful 

 ones. In other words, methods of plant protection may 

 be applied to protect these invisible plants, and therebj^ 

 improve the quality of the soil and sustain its fertility. 



