426 Scientific Proceedings^ Royal Dublin Society. 



parts in contact. But tracheide bridges were formed at the cut end of the 

 bundle, near the apex, grew into the small hole in the mica, grew through it, 

 and out to join the other end of tlie cut bundle. The bridge followed the 

 course of a procambial strand, which first appeared. 



Finally, we may mention the case of galls, quoting as an example the 

 S2Mthegaster gall on the male inflorescence of some oaks, where the gall- 

 bearing axes have their life duration greatly increased, and show increase of 

 the vascular bundle beneath the place of inflection (Kuster 23). 



This series of examples shows us that the further differentiation in the 

 petiole of Felargoniwm zonule is no isolated phenomenon, but merely one 

 example of a general characteristic of plants. Because a tissue remains 

 normally at a certain development stage we cannot regard it as an absolutely 

 permanent stage — as Jost (24, p. 444) says : " Wir werden nicht umhin 

 konnen, anzunchmen, dass die Befahigung zu derartigen Lebensausserungen 

 [speaking of the phenomena of Eegeneration] in jeder protoplasmahaltigen 

 Zelle vorhanden ist und nur fiir gewohnlich durch die Beziehungen der Telle 

 untereinander unterdrtickt wird." Let the proper stimulus act on such cells, 

 and " rediiferentiation " is certain to follow. 



And thus it is naturally the object of the next section of this paper to 

 determine as far as possible what are the causes which have operated to pro- 

 duce the remarkable changes which have been described as appearing in the 

 petiole as a result of the grafting of a sprout upon it. 



IV. — The Possible Causes of the Changes in the Petiole. 



When we endeavour to analyze the causes which produce any pliysiological 

 reaction, we must remember the dictum of PfefEer (25), to the effect that " it 

 is evident that no theory can be correct which ascribes a phenomenon of 

 complex origin to tlie action of a single factor," and so we shall have to refer 

 to many factors. Luckily -we have a masterly analysis of the possible causes 

 in Winkler's paper (5). While we may not agree with him in all points, we 

 must thank him for that summary. To his paper reference must be made for 

 fuller discussion of most of the points. Duly such points as are absolutely 

 necessary for completeness, or such points as those on which one cannot quite 

 agree with Winkler, will be introduced here. 



Wound stimulus. — Undoubtedly the petiole when undergoing the 

 grafting process is subjected to a very severe traumatic stimulus. But 

 its effect would appear to be of no very great direct importance in our 

 petiole, even in spite of the well-known development of so-called wound 

 wood in a wound callus (see Kuster (23j). It does not seem that we need 



