THE SPREAD OF PLANT DISEASES. 121 



only of human and animal diseases, but also of plant diseases. The 

 plant world has its own consumption and leprosy, its cholera and 

 black plague, its typhoid and malaria, its grippe, diphtheria, an- 

 thrax, and glanders — diseases due to other organisms and 

 totally different, of course, in their symptoms from those I have 

 named, but no less widespread and destructive in their course, and 

 no less amenable to hygienic measures, than the terrible human 

 and animal scourges with which we are all more or less familiar. 



These plant parasites are plants or animals which have in some 

 way lodged on or gained an entrance into the plant, or " host 

 plant," as we usually designate it, and which there multiply to its 

 detriment. Many of these organisms are exceedingly small, so as 

 to be readily overlooked, and all of them are well provided with 

 means of reproduction. In case of the parasitic fungi the repro- 

 ductive bodies developed on a single host plant often amount to 

 hundreds of thousands, and in case of bacteria even to hundreds 

 of millions. These bodies are, of course, very small, requiring 

 good microscopes and other facilities for their study, but if the 

 conditions are favorable a single one of them may be all that is 

 necessary to start disease in a healthy plant. 



Why, then, are not all our plants destroyed outright ? This 

 leads to another question, viz., what constitutes a parasite ? In a 

 crude way, and for the sake of convenience, we often divide the 

 lower organisms into two classes — saprophytes and parasites ; 

 the former obtaining their nourishment from dead and decaying 

 substances, the latter nourished at the expense of other living 

 organisms. There are, however, all grades of saprophytism and of 

 parasitism. Any saprophyte may become a parasite if it can gain 

 an entrance into the plant, and can there find conditions adapted 

 to its growth. There is a constant struggle for existence in all 

 animate nature, and one organism or another prospers according 

 to environmental conditions. Sometimes the advantage is wholly 

 with the host plant, at other times largely with the parasite. 

 Many saprophytes become only slowly adapted to parasitic ways 

 of living. Many parasites are very sensitive to light, to heat, to 

 dry air, and perish in a short time if exposed to these conditions. 

 Many parasites will thrive only under unusual conditions of heat 

 and moisture. Many of the higher plants have devised special 

 means for their own protection, such as the physical obstacle of a 

 thick and hard covering not easily penetrated, a waxy covering 



