876 



SCIENCE. 



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



plant pathology. Eesults of great economic 

 value were reached, but the cause has still 

 eluded proof or well-founded suspicion. 



And yet after the lapse of a decade inter- 

 est in peach-yellows has increased rather 

 than abated in the minds of those who 

 witnessed and properly appreciated the 

 original researches. It is true that the 

 wish to repeat any of Dr. Smith's compre- 

 hensive investigations would scarcely occur 

 to any one who realized the thoroughness of 

 his methods, but in my experience, at least, 

 every new fact or suggestion which seemed 

 in any way likely to furnish an analogy or 

 other clue to peach-yellows has been care- 

 fully scrutinized. During the past summer 

 it has seemed to me to be possible to meet 

 the peculiar requirements of the facts by 

 means of a hypothetical cause, and as this 

 is of a nature such that observation and 

 demonstration may be expected to prove 

 difiicult and time-consuming, it seems 

 justifiable to place on record the circum- 

 stantial evidence drawn from more general 

 biological considerations, in order that those 

 specially concerned with investigations in 

 this line may have any advantage possible 

 from a suggestion which there is no present 

 opportunity to put to a practical test. 



Briefly stated, the proposition is simply 

 that the ' yellows ' of the peach may be the 

 result of the poisoning of the protoplasm of 

 the living cells by the bite of a small ar- 

 thropod, probably a mite of the family 

 Phytoptidse. The fact that plant cells 

 may be so poisoned by mites as to become 

 yellow and yet retain their vitality for 

 many months, or even for years, was im- 

 pressed upon my attention while observing 

 a palm of the genus Thrinax, which had 

 been infested with the so-called ' red-spider ' 

 (^Tetranyclius) . That this animal was the 

 cause of the injury was inferred from the 

 fact that the formation of yellow spots 

 ceased as soon as it was exterminated from 

 the plant. This was graphically shown in 



a young leaf which while still folded was 

 attacked on the exposed lateral segments, 

 where the yellow spots are still distinct, 

 though all other parts are entirely un- 

 touched and have remained uniformly 

 green. Discoloration spreading thus from 

 the puncture of an insect or a mite is, of 

 course, no new thing, and is often accom- 

 panied by much more serious injuries; 

 moreover, we have all the wonderful phe- 

 nomena of galls as evidence of the power of 

 animal secretions in profoundly modifying 

 vegetable tissues and structures. The in- 

 terest of the spots of the Thrinax lies 

 merely in the suggestion of a progressive 

 and permanent injury of the tissues with- 

 out malformation or other striking symp- 

 toms. It is true that there is a wide gap 

 between progressive change in a spot less 

 than a quarter of an inch across and one 

 which covers a whole peach tree, but the 

 difference may be one of degree and not of 

 kind, and from the physiological standpoint 

 the galls go far to connect the two ex- 

 tremes. This analogy is especially pointed 

 with the galls induced by inoculation, such 

 as those of the plant-lice and mites, which 

 remain outside the tissues of the host-plant, 

 and yet cause protective malformations. 



As the effects of yellows upon the plant 

 tissues could scarcely be thought to be of 

 any advantage to the animal, it may be sup- 

 posed that no special secretion is involved 

 other than the normal salivary fluid, which 

 might be expected to contain digestive fer- 

 ments or enzymes,* some of which are now 

 known to exhibit the phenomenon of in- 

 definite self- propagation when brought into 

 contact with substances on which they are 

 capable of acting. 



* I am informed by Mr. A. F. Woods that he has 

 for some time entertained an opinion of the enzymotic 

 nature of peach-yellows, drawn from a study of the 

 symptoms of the disease, and from the presence of 

 abnormal quantities of oxydizing enzymes. Cf. Cen- 

 trdlblalt fiir Bakteriologie, etc., 2 Abt., V, p. 574. 



