151 



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 tip has been disturbed in some way or other. 

 But even then they arise apically in this part. Hence 

 only the vesicles which hâve originated on the apical part 

 of a leaf, form tho 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 région, surrounding the vesicles, 

 becomes discoloured ; as a rule the tip of the affected 

 leaf soon becomes distinctly yellow, al though 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 flrst not easy to 

 observe macroscopically ; gradually, however, it becomes 

 stronger and at last generally advances so far that the tip 

 becomes stiff and difflcult to bend. 



Of the yellow vesicles nothing can then be seen any 

 longer. 



The extent of this région of discoloration and thickening 

 varies much in a basai direction ; along the edge it gene- 

 rally extends farther basipetally than in the middle ; al- 

 ways, 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. 



During the first weeks generally no striking changes are 

 observed until after about a month a varying number of 

 local élévations, yellow like the leaf-tip that produces them, 



Recueil des trav. bot. Néerl. Vol. IV. 1907. 10 



