152 



becomes prominent and reveals the differentiation of 

 spécial proliferating centres. 



As a rule we see thèse grow to real knobs, especially 

 in the direction perpendicular to the surface of the leaf 

 (PI. II, flg. 4). 



While the knobs are still relatively small, brownish 

 grey streaks begin to appear on their tops, which graduaily 

 extend, so that flnally the whole knobbed surface becomes 

 brown. 



For some time such a knob shows nothing particular, 

 except that it becomes larger and thicker. Next on a certain 

 day a small opening is formed in its top through which 

 a small green point projects which will grow out into a 

 leafed sprout (PI. II, fig. 1, fîg. 5 a and a'). 



As well on the lower as on the upper surface of the 

 leaf-tip knobs may form. Mostly they form on the upper 

 surface, though. With some leaves I hâve observed knobs 

 on both sides at the same time. 



The observations on the time, needed bij a leafinorder 

 to form „ripe" knobs, after the yellow vesicles hâve appea- 

 red, hâve led to somewhat diverging results. The shortest 

 period was observed with leaves on the upper branches or 

 in the periphery of the crown, which consequently occupied 

 the most favorable position with regard to light. On thèse 

 good-sized, brown knobs had generally formed half ayear 

 after the appearance of the yellow vesicles. 



Also for the question, how old and how large a knob 

 must be in order to open and give the adventitious bud 

 an opportunity for sprouting, no rule can be fixed. I saw 

 one sprout flve weeks after the knob had first been obser- 

 ved as a spécial élévation, while others were still closed 

 after flve to seven months. 



About the size of the knobs we may state that some 

 knobs, scarcely rising more than a millimètre above the 

 surface of the leaf-tip, opened, while others of double and 



