23 



of the sterilized series aiul ull tlie unstcrilized plants except the con- 

 trols kept free from insects, showed an equally rapid development of 

 ai)hides and spots. After this no exact account was kept of the 

 rapidity of increase of aphides or spots in any ease, but the young 

 leaves, as Avell as those which had matured, and also the stems, were 

 completely peppered with spots, and the plants assumed a sickly yel- 

 lowish ai)pearance. 



It was not thought necessary to make cultures from the spots on the 

 sterilized plants, owing to the facts that the work had been done under 

 thoroughly antiseptic conditions; that the entirely negative charac- 

 ter of the previous culture work was deemed sutiicient; and that the 

 three control plants, upon which no aphid«vs were allowed to develop, 

 remained absolutely free from spot. The evidence of these experi- 

 ments, it is believed, settles bevond anv doubt the fact that the disease 

 in question is not due to l)acteria, but is the work of aphides, thrips, 

 and red spiders. 



CAUSE OF TlIK INCliEASK IN THE SIZE OF THE SPOTS. 



It may be asked why in the absence of parasitic and saprophytic 

 plant organisms the work of aphides, thrips, and red spiders is fol- 

 lowed by such marked pathological changes in the tissues, and wh}' do 

 the spots increase in size after the insect or mite has done its work. 

 This question was investigated to some extent, and it is believed that 

 an explanation is at hand. As previously' pointed out, the aphides do 

 not lacerate the mesophyll cells between which the}' force their sucking 

 apparatus and excrete the supporting tube, but still the cells along the 

 line of insertion show the change first, the chloroplasts not developing 

 and becoming yellower and the cells becoming somewhat enlarged and 

 cedematous. 



It is difficult to determine the relative acidity of the tissues involved, 

 the spots being small, but as nearly as could be determined, the dis- 

 eased cells were apparently less acid than the healthy ones. Another 

 difference between the diseased and the health}- cells was more marked 

 and could be readily determined. After the epidermis of the leaf had 

 been carefully peeled off', the surface of the exposed mesophyll cells 

 was moistened with a 2 per cent solution of gum guaiac in absolute 

 alcohol, and in a moment or two the diseased spots became a deep blue, 

 while the surrounding healthy cells showed but a slight reaction, 

 which could be detected only by the apparent deepening of the green. 

 This test was applied in still another manner. Diseased tissues 

 removed from the leaf were first treated with 95 per cent alcohol and 

 then transferred to a small amount of distilled Avater, in which they 

 were crushed and allowed to stand for half an hour. The solution was 

 then tested with guaiac tincture, and for comparison an equal amount 



