878 



SCIENCE. 



[N. S. Vol. XII. No. 310. 



plainable by possible differences in the dates 

 of infection and in the distances through 

 which the disease must propagate itself be- 

 fore it reaches the growing fruit, the normal 

 development of which is inhibited, although 

 the tissues previously formed remain appar- 

 ently uninjured. The suggestive phenom- 

 ena of the palm are also capable of a similar 

 explanation, the susceptibility to spotting 

 being apparently confined to the young 

 leaves ; otherwise it is difficult to under- 

 stand why the entire area of the older leaves 

 had not turned yellow long since. The 

 same is likewise true of the leaf-spot or 

 stigmonose of carnations, investigated by 

 Mr. A. F. Woods.* 



It can scarcely be expected that many 

 cases completely parallel to the yellows 

 will be found in nature, since parasites 

 which produce such disastrous effects upon 

 their hosts must be unable to maintain ex- 

 tended existence. It is accordingly not to 

 be supposed that the peach is the natural 

 or exclusive habitat of an insect or mite 

 which is able to produce such a disease as 

 peach-yellows. In the biology of galls the 

 localization of the irritant or pathological 

 effect is essential to the establishment of a 

 successful and permanent symbiotic rela- 

 tion, although the possibilities of extensive 

 change through animal irritants are amply 

 shown in the general debility evident in 

 some plants when parasitized by gall- 

 forming insects. These considerations are 

 of special interest in connection with a fact 

 which is of use at least as an analogy, and 

 which may possibly furnish a direct clue to 

 the mystei'y, since on other plants both 

 galls and hexenbesens have been connected 

 with species of the same group of mites. 

 In a disease of plum nursery-stock re- 



* Bulletin No. 19, Division of Vegetable Physiology 

 and Pathology, U. S. Department of Agriculture, 

 1900. This disease -was formerly ascribed to bacteria, 

 but is here shown to be due to the punctures of 

 aphides. 



ported by Mr. M. B. Waite, one of the 

 prominent sj'mptoms of peach-yellows, the 

 fasciculate branching or hexenbesen forma- 

 tion, was found to be caused by a minute 

 parasitic mite (Phytoptus ?) . In this instance 

 the terminal bud is killed, while the lateral 

 buds in the neighborhood are pushed into 

 premature growth. That this latter de- 

 velopment is not caused simply by the 

 death of the terminal bud, but is stimu- 

 lated by the presence of a noxious com- 

 pound, is probable, though the disorder is 

 not progressive in the plum, and the re- 

 moval of the malformed branches permits 

 the resumption of normal growth. The 

 suspicion that the active cause may be sim- 

 ilar, if not the same as that of the yellows, 

 is considerably strengthened by the fact 

 that the yellows, while recorded as occurring 

 in almonds, apricots, nectarines and Japa- 

 nese plums, is not known to affect other 

 sorts of plums. With the supposition of 

 such identity of cause we arrive at the 

 proposition that the mite elaborates in its 

 salivary or other glands an enzyme or other 

 active compound to which the tissues of the 

 peach and closely related fruits are pecu- 

 liarly susceptible, and which produces in 

 them a permanent and ultimately fatal 

 debility accompanied by definite constitu- 

 tional symptoms. And that this suscepti- 

 bility depends on some delicate relation of 

 structure or composition is shown by the 

 fact that the Japanese plums are affected, 

 while the European and American culti- 

 vated species appear to stand on the plane 

 of at least partial immunity, being able to 

 resist and recover from the infection. It 

 would thus be possible to accommodate all 

 the related facts in the construction of a 

 complete analogy with the known limita- 

 tions of many other diseases to groups of 

 related varieties and species.* 



*The force of this analogy is rather strengthened, al- 

 though its terms may need to be altered, by a fact ver- 

 bally communicated by Mr. Waite, that a similar dis- 



