134 STATE HORTICULTUKAL SOCIETY. 



of an acqiiaintaince with the general characteristics of these maladies of culti- 

 vated plants, I cannot readily credit the statement that the yellows is due to 

 any similar parasitic fungus, the published descriptions of the disease being 

 for the most part my information concerning it. Besides this the spores of 

 moulds and the hosts of decay producing fungi being ubiquitously present and 

 germinating wherever favorable conditions permit, it is exceedingly easy to 

 mistake their vegetative threads (mycelia) for the true disease producers. It 

 is, therefore, to my mind, quite improbable that any one or more of the species 

 or classes of the ordinary parasitic fungi having distinct vegetative and repro- 

 ductive systems, cause the peach yellows. 



On the other hand the well demonstrated contagious nature of the disease 

 strongly points to a something capable of growth and multiplication ; no mere 

 chemical poison nor simple exhaustion with vital debility. These latter do not 

 increase and become widely distributed through any process of contamination. 

 An inorganic poison accomplishes its work and disappears, or at least grows 

 less effective through tlie dissipation of its substance. A sickly animal or plant 

 may genetically produce offspring inheriting constitutional weakness and dis- 

 eased individuals may thus be increased in numbers; but it is, in questions of 

 this kind, exceedingly important that we distinguish clearly and certainly, as 

 we may, sucli a propagation of disease from that produced in healthy individu- 

 als by inoculation. It is true the effects of the latter may become inheritable, 

 but if so, because the pathological agent continues to reproduce itself after 

 being once introduced ; and this continued reproduction at once announces its true 

 nature. It cannot be too well understood that the separate cellular elements 

 of the higher plants do not readily propagate themselves. Wounds in plants, 

 except under peculiar circumstances in the cambium, do not even heal in 

 any proper sense. The old injuries are simply overgrown, covered up by new 

 material, without union or any sort of connection of growth. It is absolutely 

 impossible to transfer living cells of any of the higher plants to other living 

 plants of the same kind and cause tliem to grow in their new situation by any 

 process like inoculation with a needle. It certainly is never true that the sim- 

 ple fluids of a diseased plant can injuriously affect a healthy one except perhaps 

 as a local and temporary poison. There is no such thing as multiplication and 

 increase in tliese cases. 



Leaving grafting and budding out of the question, whenever a disease is 

 found to be communicable from an unhealthy to a healthy plant by inocula- 

 tion, we may rest assured that the virus contains some organic, living, parasitic 

 germ, capable of self reproduction and possessing a vitality and life history of 

 its own. It is so in the animal economy. There is no longer the least ground 

 for doubt as to the nature of vaccination as a preventive against small pox. 

 The vaccine virus owes its potency to a discovered living, self propagating organ- 

 ism. Small pox itself grows and prevades the entire body from a small begin- 

 ning, and the microscope reveals the minute agent and its manner of increase. 

 Similarly there is no further room for question as to the nature of the morbific 

 agent in diphtheria. Not only the existence of the actual and active disease 

 producer is known, but its shape, size, color, composition, and many of the 

 conditions required for its development have been studied and established. 

 Some of the other contagious diseases of man and the higher animals are as 

 well or better known, all pointing to the same or similar origin, parasitism. 



Having, for reasons stated, expressed the conviction that tlie yellows is not due 

 to the more highly organized parasitic fungi which send their vegetative system 

 through the internal tissues and produce their fruit or reproductive bodies only 



