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ai all. they (lower ven rarely. Personally I only observed it with 

 one of the plants from the cooler hothouse. This latter plant produced 

 one single ? inflorescence, which enabled me to check the accuracy 

 of the determination. 



The first external change, noticed with a leave which will form 

 adventitious buds, is that on the lip extremely small yellow dots 

 appear, winch are seen best when light is falling through the leaf. 

 They remind us in this respect of the oil dots in the leaves (if the 

 Rutaceae or Hypericum, hut as a rule they are bigger and less 

 densely spread than these. 



With the bigger ones a hand-magnifier will show that where the 

 dots are, the epiderro of the upper or lower side or of both together 

 is slightly bulged, so that we have to do with small vesicles. 



It will be shown presently that these vesicles are caused by the 

 sting of a scale-insect Aspidiotus spec, and as such are not restricted 

 at all to the tip of the leaf. Normally, however, it is only the tip 

 which can form adventitious buds. The remaining part of the leafblade 

 can only form adventitious buds when the organic relation with the 

 lip has been disturbed in some way or other. But even then they 

 arise apically in this pari. Hence only the vesicles which have 

 originated on the apical part of a leaf, form the introduction to the 

 process of bud-formation. 



For the sake of simplicity we shall in what follows, only mention 

 the tip of the leaf, since the statements referring to the tip also 

 apply to the other cases. 



After some time also the region, surrounding the vesicles, become-; 

 discoloured; as a rule the tip of the affected leaf soon becomes 

 distinctly yellow, although in some cases it long keeps a more or 

 less greenish tint. 



At the same time with this discoloration the tip of the leaf becomes 

 thicker. This thickening is at first not easy to observe microscopi- 

 cally ; gradually, however, it becomes stronger and at last generally 

 advances so far that the tip becomes stiff and difficult to bend. 



Of the yellow vesicles nothing can then be seen any longer. 



The extent of this region of discoloration and thickening varies 

 much m a basal direction; along the edge it generally extends farther 

 basipelally than in the middle; always, however, the phenomenon 

 is restricted to the apical part of the leaf. A new stage sets in, 

 when the surface of the thickened leaf-tip which until now had 

 remained smooth, on account of the swelling being even, becomes 

 uneven: as well on the lower as on the upper surface this may as a 

 rule be observed; on the upper surface it is generally more pronounced. 



