PLANO; DISEASES. 139 



this account it was long before they came to be 

 understood and studied, and even after they began to 

 be familiar to botanists it was difficult for people 

 who had never used a microscope to realize that 

 there were plants which could be named, described 

 and recognized, but which were wholly invisible to 

 the naked eye, and that many of these were the sole 

 cause of various disastrous diseases which afflicted 

 the crops of the field and garden. 



The circumstance which more than anything 

 else has hindered the general recognition of the 

 fact that plant diseases are chiefly caused by para- 

 sitic fungi, is that the prevalence of these diseases 

 is largely controlled by the weather. Nearly all 

 plant diseases, such as potato rot, wheat rust, etc., 

 are most prevalent in hot, damp weather. Botanists 

 soon discovered that this was due to the fact that 

 the spores or seed-like bodies by which fangi are 

 propagated, germinate, like ordinary seeds, most 

 readily in such warm, damp weather, and thus 

 spread the disease most rapidly at such times. 

 Most other persons, however, still believed that the 

 weather was the direct and only cause of the dis- 

 ease. Even when it became fully demonstrated 

 that these plant diseases were accompanied by 

 fungi, it was yet thought by many that the plants 

 became first diseased and then attacked by the 

 fungi, or at least that fungi only attacked unhealthy 



