V. H. Blackman 323 



^that in the matter of the determination of the nature of a disease the 

 plant pathologist is in a more difficult position than the ordinary medical 

 man. The doctor attending a human patient is usually able — as the result 

 of a few questions and the observation of symptoms — to state in a few 

 minutes the nature of the disease; it is only rarely necessary to await 

 the discovery of the causal organism before diagnosing the case. This is 

 of course due to the fact that the symptoms of different diseases are, 

 usually, sharply marked in the higher animals. Many diseases of man 

 were quite sharply characterised before the germ theory of disease arose, 

 and to this day the germs of many infectious diseases of man are quite 

 unknown. In plants on the other hand the symptoms are so generalised — 

 that is, common to so many different diseases— that a disease can rarely 

 be determined from symptoms alone. A search has to be made for the 

 causal organism, and when it is found it may have to be grown in arti- 

 ficial culture so that the reproductive organs can be obtained and the 

 nature of the organism established. Days or weeks may thus elapse 

 before the causal organism is identified and the nature of the disease 

 determined. 



As however the diseased condition which arises is due to the inter- 

 action of the physiological processes of the host and parasite the symp- 

 toms of each disease must be different, just as the structure of the gall 

 is different for each combination of host and insect. The trouble is that 

 at present our methods of analysis are not fine enough, so that these 

 symptoms escape observation. We must however look forward to the 

 time when better developed methods of micro-analysis will enable us 

 to classify plant diseases by their symptoms. I may be allowed this 

 digression to indicate the importance of the physiological aspect of plant 

 pathology. 



To return to the question of training. What is more important than 

 a knowledge of a large number of diseases is the acquaintance with the 

 sources of information, i.e., books and scientific journals, so that the 

 student may find his way about the subject and have confidence in his 

 power to '"get up" any branch of his subject or the knowledge of any 

 special disease. 



Since in so many cases insects play a large part in the dissemination 

 of plant diseases some acquaintance with entomology, or at least with 

 the habits of insects which attack plants, is of great advantage. It is 

 clear, however, that it is practically impossible to combine in the same 

 individual a competent knowledge of mycology and a competent know- 

 ledge of entomology. 



Ann. Biol, vi 22 



