December 7, 1900.] 



SCIENCE. 



877 



Among the important points demon- 

 strated by Dr. Smith, in addition to the 

 absence of any parasitic organism within 

 the tissues, was the communicability of the 

 disease. This fact the present tlieory 

 amply accommodates, and at the same time 

 does not require insects or mites in large 

 numbers, since the disorder is known to be 

 progressive throughout any tree which has 

 once given evidence of infection. Inocula- 

 tion by budding from diseased trees also 

 communicates the yellows, a fact which 

 would exclude insect injuries as commonly 

 understood, but which is not inconsistent 

 with the present theory, although the iso- 

 lation of the deleterious substance or even 

 direct inoculation by the application of the 

 juices and disorganized tissues has thus far 

 failed.* 



The presence of an infected tree has been 

 believed to have a distinct effect in induc- 

 ing the disease in others, though trees will 

 sometimes remain healthy for a considera- 

 ble period in close proximity or even in 

 contact with those already victims of the 

 malady, and young trees planted in the 

 places of those killed by the yellows are 

 no more liable than others to take the dis- 

 ease. These facts seem peculiarly perverse 

 in connection with a theory of direct infec- 



* Dr. Smith tells me that Phytopii were frequently 

 found by him on yellowed peach trees, and that while 

 the idea that they might cause the disease in the usual 

 manner of parasitic insects suggested itself early in 

 the course of his studies, he rejected it because 

 entirely inadequate to explain the extensive constitu- 

 tional symptoms which led him to compare the yel- 

 lows to hydrophobia and smallpox, other commu- 

 nicable diseases of which the causes have resisted 

 investigation. It is true also that the infection phe- 

 nomena of these diseases appear to include the neces- 

 sity of inoculating or poisoning cells which remain 

 alive, and from which the disease may be propagated 

 by protoplasmic continuity. Bacteria, on the con- 

 trary, are able to pass from one plant cell to another 

 only by breaking down the cell-walls, though the dis- 

 organizing effects of their chemical products may pre- 

 cede them. 



tion by a parasitic organism, but become 

 quite comprehensible if the injury be as- 

 cribed to mites carried about by bees or by 

 birds ; infection would thus be largely ac- 

 cidental, and while the chances would be 

 greater in the neighborhood of diseased 

 trees, no regularity nor continuity in the 

 spread of the disorder need be expected.* 

 Finally, it was demonstrated by Dr. 

 Smith that neither the character, condition 

 or relative fertility of the soil, nor the age, 

 vigor or variety of the tree has any appre- 

 ciable influence in predisposing to attack or . 

 in securing immunity. Such facts are evi- 

 dently ample for the exclusion of any hy- 

 pothesis based on the predication of a 

 constitutional disorder originating inde- 

 pendently in the tissues of the trees. The 

 implications point uniformly in the contrary 

 direction, to a definite cause in the form of 

 a specific noxious substance the application 

 of which is followed by a uniform bio-chem- 

 ical reaction. This inference is not weak- 

 ened by the fact that the reaction is slow, 

 and that the results of infection become ap- 

 parent only in new growth from buds or 

 tissues which have been reached by the dis- 

 ease. Thus the first definite symptom of 

 debility caused by infections which may be 

 supposed to have taken place in the spring 

 appears in the premature ripening of the 

 fruits. It has also been noted by Dr. 

 Smith that the degree of prematureness is 

 extremely variable, a fact which seems ex- 



*The presence of bird's nests may prove to be 

 connected with the not infrequent occurrence of sev- 

 eral apparently simultaneous infections in the same 

 immediate vicinity, though not necessarily on con- 

 tiguous trees. If brought in by small birds it is easy 

 to see that the chances of forming a colony of the in- 

 jurious mites are many times greater on a tree where 

 a nest is placed, and where the birds spend a large 

 part of their time during the breeding season . More- 

 over, the well-known persistence of the migratory 

 birds in returning to the same nesting-place would 

 tend to insure a rapid dissemination of the yellows in 

 the year after a colony of the mites had been estab- 

 lished in the orchard. 



