36 BULLETIN 1038, V. S. DEPARTMENT OF AGEICULTUEE. 



any sucli comparison. Diphtheria toxin, for instance, if injected into 

 a susceptible host may cause all essential signs of the disease, but it 

 takes living bacteria to produce more of the toxin so that the disease 

 may be transmitted down the line through a successive series of in- 

 dividuals. 



On the other hand, the " Contagium vivum fiuidum " theory of 

 Beijerinck (17) in some modification is at least worthy of serious 

 consideration as a working hypothesis. It is not impossible that de- 

 composition processes may be propagated by methods entirely unlike 

 reproduction by division as known in living organisms and so as to 

 imitate self-reproduction. If some such course of events could be 

 demonstrated it would perhaps explain the known facts, including 

 the gradual progress of the disease from the point of entry, equally 

 as well as the organism theory. However, though it is well to keep 

 an open mind on all questions still in the realm of theory, no such 

 course of events is yet known to chemistry. On the other hand, most 

 of the known facts concerning the so-called filterable virus diseases, 

 so it seems to the writer, conform to the known results of invasion by 

 parasitic organisms. 



SUMMARY. 



As a class, the chloroses due to soil or atmospheric conditions are 

 rather general effects which are more or less comparable to starva- 

 tion, overfeeding, or direct poisoning. There are not the profound 

 or strictly localized derangements in both metabolic and anatomical 

 development, and it is a question in regard to many of these phe- 

 nomena whether, in the restricted sense, they should be regarded as 

 diseases at all. 



In the specific chlorotic diseases of an infectious nature funda- 

 mental derangements in both physiological and structural develop- 

 ment are simultaneously brought about. Although all effects of 

 parasite upon host when reduced to their ultimate reactions are 

 probably to be explained in terms of physics and chemistry, it is 

 in the regulatory effect and in the extreme complexit}^ that these 

 and many other infectious diseases differ from the direct effects of 

 nonliving substances. 



The histological and cytological evidence suggests that pecan 

 rosette in its specific sequence of signs and -in the complexity of 

 the structural and physiological derangements bears far more simi- 

 larity to the known infectious chloroses than to those caused 

 by soil or climatic conditions. Whether in this particular disease 

 the factors responsible for alterations in the normal structure and 

 metabolism must be introduced into the plant from without, or 

 whether they originate within the plant itself, is a question yet to 

 be answered ; but whatever the ultimate solution of the problem the 

 cause will undoubtedly not be found in any simple soil or water 

 relation. 



