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leaf in which adventitious buds are forming, it has already been 

 briefly stated that the yellow vesicles, initiating the process of bud- 

 formation, are caused by a scale-insect, Aspidiotus spec. 



That suspicion fell on this Aspidiotus had a very simple reason. 



Although not nearly all the leaves, showing yellow vesicles, 

 carried scale-insects, yet the reverse was generally true and it soon 

 became apparent that the leaves, carrying scale-insects generally also 

 had some yellow vesicles. 



But unexpected difficulties were experienced when it was attempted 

 by means of microtome preparations to obtain certainty and a clearer 

 insight in what had been rendered probable by macroscopical obser- 

 vation. A great difficulty was that the majority of the insects refused 

 to stick to the bits of leaf from which the microtome preparations were 

 going to be made. While a great part already loosed their hold during 

 the treatment preceding the embedding itself, their example was 

 followed by most others when they were put into the melted paraffin. 

 It was supposed that perhaps the reason of this was that the scale- 

 insects, when coming into the fixing solution, withdrew their suction 

 organ from the tissue of the leaf, possibly on account of a pre-mortal 

 reactional movement. After this unfavourable result the leaves carrying 

 the insects were always treated before fixation with an anaesthetic, 

 namely aether. This precaution, however, did not materially improve 

 the results. 



Among the microtome preparations which I obtained in spite of 

 these difficulties, there is not a single one in which a scale-insect 

 may be seen in a sucking position. But always in the places were 

 an insect was on the leaf, in the tissue the changes were found 

 which we described as characteristic for the yellow vesicles. 



Here also hypertrophy, accompanied by disorganisation of the cholo- 

 rophyl, of cells, belonging to the mesophyl; some of these cells have 

 brown walls. Also in the epiderm on which the scale-insect is found, 

 some cells are found, the walls of which are suberized and which 

 besides are sometimes slightly swollen. 



I was more fortunate with hand-made preparations, some of which 

 show the suction apparatus of the scale-insect inside the leaf tissue. 

 From these we see that in the yellow vesicles those cells, the wall 

 of which has become suberized, have been in direct contact with 

 the suction apparatus of the scale-insect and that the other cells, 

 which become hypertrophical, only react to a stimulus, exercised by 

 the wounded cells. On the character of this stimulant action we 

 shall speak presently. 



That here the enlargement of the cell should take place at the 



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