24 



of a solution made similarly from healthy tissue of the same leaf was 

 also tested in like manner. The action of the diseased extract on gum 

 guaiac was about twice as strong- as that of the health}^ extract. It 

 seems very probable that this remarkable increase in the oxidizing 

 power of these cells must be the result of some irritating substance, 

 perhaps either an organic acid or some other substance from the mouth 

 parts of the insect which it injects into the tissues at the time it punc- 

 tures them. It may be that the increase of the oxidizing enzym in 

 the cells is an attempt on the part of the latter to destroy the injected 

 irritant. 



As the writer has shown in his paper on the relation of oxidizing 

 enzymsto the destruction of chlorophyll/ the destruction of the chloro- 

 plasts can probably be explained by the increased oxidation taking- 

 place in the cells. The nucleus of the cells involved is not destroyed 

 in the earlier stages of the trouble. The nutrition of the cell, how- 

 ever, is interfered with and the chloroplasts do not develop normall}^. 

 As before pointed out, the rapidity of the increase in size of the spots 

 on the young leaves is coordinate with that of the growth of the leaf. 

 In a mature leaf, however, the size of the spots may increase very 

 slowly, and this must be due to either a gradual diffusion of the 

 irritant injected by the insect or to the direct effect of diseased neigh- 

 boring cells. 



The former supposition is more likely to be correct, as the same 

 kind of a mechanical wound is followed in the case of different insects 

 by markedly different results. For example, thrips, aphides, and the 

 ordinary little green leaf hoppers found on Chenopodiuni alhum produce 

 only a slight translucent spot on the leaves, this developing only after 

 a week or ten daj^s from the time the puncture is made. On the other 

 hand, a little reddish leaf hopper, which is usually found working on 

 the lower surface of these plants, produces in eighteen hours from 

 the time a puncture is made a purple spot 1 mm. in diameter which 

 rapidly increases until it is 4 to 5 mm. in diameter. Evidenth-^ this 

 insect injects some quite rapidly diffusing substance into the tissues. 



The writer has carried out careful colonization experiments with this 

 red leaf hopper and has endeavored to determine what the injected 

 substance is, but so far has not settled this point. In the case of the 

 Bermuda lil}^, a great difference has been noted in the relatively slight 

 injury produced by the common green aphis, A. 'inahaleh, and the 

 greater injur}' produced by the large green aphis belonging to the 

 genus Siphonophora. The same is true of the l)rown or black aphides 

 and the green aphides which attack the violet, the former producing a 

 marked stunting of the plants and the latter doing but little injmy 

 outside of distorting the flowers. The work of the San Jose scale on 



^Centralbl. f. Bakt., 1899, Abt. 2, Bd. V. 



