DISEASES IN PLANTS 119 



the rare event of the man of science who is a 

 genius ; but in pure science itself is the vision. 

 The story of Biffen's discovery will make clear 

 anything that there may seem cryptic in this 

 utterance. About 50 years ago the belief that 

 disease is the maleficent work of evil spirits still 

 lingered in the minds of simple folk. The sophisti- 

 cated had long abandoned the superstition but were 

 none the wiser in their agnosticism. Simultaneously 

 with the work of Pasteur on the nature of disease 

 in animals, de Bary demonstrated that disease in 

 plants is in the great majority of cases due to the 

 intrusion of a parasite into the body of the plant. 

 He proved moreover that in the case of many 

 diseases the intruder is a fungus and that specific 

 diseases are due to the intrusion of specific fungi. 

 Gradually the co-workers and successors of de 

 Bary arrived at a precise appreciation of the nature 

 of these infectious diseases. As a result of these 

 prolonged labours it is now recognised that it 

 takes three to make that quarrel that we call 

 disease: the infecting organism, conditions in the 

 victim which lay it open to or screen it from attack, 

 conditions in the aerial and terrestrial environ- 

 ments which favour either the parasite or the host 

 and so facilitate or discourage attack. The pro- 

 fessional mycologist is accustomed to fix his 

 attention too exclusively on the first of these 

 factors, the active agent of disease. The pro- 

 fessional cultivator gives habitually great weight 



