18 



SCIENCE. 



[N. S. Vol. XI. No. 262. 



leaf. The light-colored areas were found 

 to correspond with the light-colored patches 

 produced by insect punctures, certain fungi, 

 and especially in that group of diseases 

 known as variegation or albinism. 



All these cases show a greater amount of 

 oxidizing enzymes (oxidases as well as 

 peroxidases) in the light-colored tissues. 

 Mainly upon this evidence I was obliged in 

 the article mentioned to differ from most 

 other writers on the disease in question, in 

 calling the lighter-colored tissues diseased, 

 and the deep green patches, especially along 

 the veins, healthy. It is true that much 

 tobacco is apparently normally as light 

 colored as the light-colored areas in the 

 diseased leaf, but, on the other hand, there 

 is quite as much which is normally as green 

 all over as the green areas in many of the 

 diseased leaves. It is true that some of the 

 green patches, especially where the light- 

 colored areas are unusually light, are ab- 

 normally green. A study of the histology 

 of the diseased leaves has now revealed a 

 histological difference which makes it very 

 clear that the light-colored areas are not 

 normal, and this difference consists in the 

 fact that in badly diseased plants the pali- 

 sade parenchyma of the light-colored areas 

 is not developed at all. All the tissue be- 

 tween the upperand thelower epidermiscon- 

 sists of a spongy or respiratory parenchj^ma 

 rather more closely packed than normal. 

 In moderately diseased plants the palisade 

 parenchyma of the light area is greatly 

 modified. Normally the palisade paren- 

 chyma cells of a healthy plant are from 

 four to six times as long as broad. In 

 a moderately diseased plant, however, the 

 cells are nearly as broad as they are long, 

 or at most not more than twice as long as 

 broad. As a rule, the modified cells of the 

 leaf pass abruptly into the normal cells of 

 the green area. In a badly diseased leaf 

 simply looking across the surface with the 

 naked eye shows depressions where the 



light areas occur, or where the leaf is mostly 

 diseased the dark green patches are raised 

 above the general surface. 



The cells of the diseased area also trans- 

 locate their starch with difficulty, the cells 

 often becoming completely gorged with this 

 material. The examination of the diseased 

 spots early in the morning shows only a 

 small decrease in the starch content of the 

 cells from that present the previous after- 

 noon, while the greeu, healthy tissues either 

 contain no starch or contain only traces of it. 

 It was thought that possibly the increase 

 of oxidizing enzyme might either inhibit 

 the production of diastase by the cell or 

 inhibit the action of diastase upon starch. 

 In order to settle this point strong solutions 

 of tobacco oxidase were prepared, and after 

 heating some of the solution to the boiling 

 point, thus killing the oxidase, comparisons 

 were made by adding 10 milligrams of taka 

 diastase in solution to each of the tubes of 

 juice to be tested. Equal quantities of 

 freshly prepared potato starch paste were 

 then added to each tube and the tubes kept 

 at 45 degrees Cent. It was found that in 

 the solution without oxidase the starch was 

 completely converted into sugar in thirty 

 minutes, while the solutions in which the 

 oxidase was active only carried the change 

 of the starch to the erythrodextrin stage. 

 The action of the diastase of malt added in 

 solution in the same quantity was some- 

 what less rapid than that of taka diastase, 

 but the relative effects were exactly the 

 same : the presence of the oxidase in the 

 solution had a marked inhibitory action 

 upon the activity of the diastase. 



In these tests the proportion of diastase 

 to oxidase was much greater than occurs 

 even in the diseased cells, so it is likely that 

 the inhibitor}' action of the oxidase in the 

 cells is much greater than that shown in 

 the tests outside of the cells. 



It would seem a warrantable conclusion, 

 therefore, that the tardiness in the trans- 



