THE SCIENCE OF PLANT PATHOLOGY. 407 



recent demonstration of biologic varieties among the rusts, mildews and 

 fusariums opens a large and important field of research. The agencies 

 operating as disease distributors, the wind, insects, soil, man, water or 

 what not must be known that such distribution be more readily con- 

 trolled. The causes of resistance and susceptibility to certain diseases 

 rest in obscurity, except in a few cases where the responsibility has 

 been fixed upon some particular structure or chemical. The breeding 

 of plants resistant to specific diseases not readily amenable to other 

 means of control must proceed. Such work is now in progress with 

 cotton, melons, tomatoes, tobacco, grains, flax and other plants. The 

 relation existing between many root fungi and bacteria and the roots 

 they inhabit remains to be studied. Aside from parasitism there is 

 also mutualism, a kind of beneficial disease falling to the province of 

 plant pathology. It needs much further study. 



Specific problems also abound, the peach yellows and rosette, the 

 mycoplasm theory of rusts, the grape Brunnisure. Differences of opin- 

 ion now exist or the technique or scientific data are insufficient for an 

 adequate solution of these questions and many other similar ones. 

 Work on timber protection, while not strictly a question of disease, but 

 rather a post-mortem problem, falls to the lot of the pathologist for 

 the want of a more appropriate place. That intensive study of a dis- 

 ease, however thoroughly it may seem to have been studied before, may 

 lead to important development is well illustrated in the case of the 

 familiar pear blight, which, though known for ages and the topic of 

 masterly classic research, has recently, under trained observation and 

 critical interpretation and experimentation, revealed new secrets lead- 

 ing to more masterful and complete control. The large fields of plant 

 pathology, grouped under the term ' physiological disorders/ are still 

 practically unworked; diseases due to false nutrition, absorption or 

 assimilation, or to impaired carbon assimilation owing to improper 

 environment, to crowding or shading or to hereditary inabilities. A 

 start has been made sufficient to show the importance of the results 

 awaiting. 



The recent discovery of the so-called ultramicroscopic organisms or 

 filterable enzymes which has robbed the bacteria of the distinction of 

 being the smallest of living things opens a new field in both plant 

 and animal pathology comparable in kind, though probably not in 

 magnitude, with the creation of bacteriology by Pasteur. It is yet 

 unknown whether we have to do here with organisms or enzymes, and 

 contemplation of the problems awaiting in this realm places us in a 

 position to appreciate more fully than ever before the great controversy 

 of spontaneous generation as fought in the sixties. The announcement 

 in a recent periodical of the discovery of soluble protoplasm empha- 

 sizes the existence of a vast unknown covered by the words protoplasm, 



