438 Scientific Proceedings, Royal Duhlin Society. 



is useless at present trying to settle the exact mechanical " how " of trans- 

 piration ; and tliat the branch of botany — ecology — which could yield us 

 most assistance really makes us more confused. Much researcli by many 

 workers must be carried out before the question of the origin of secondary 

 wood will be definitely settled. For these reasons it is difficult to associate 

 oneself with Winkler's very definitely expressed opinion. 



Summary. 



(«) A petiole, by grafting a sprout on it, can be made to assume the 

 functions of the stem. 



(h) The properties of a stem, viz., long life-duration, indefinitely active 

 cambium, interfascicular cambium linking up bundles, periderm develop- 

 ment, and considerable secondary thickening, all appear in the petiole. 



(c) The causes of this secondary thickening are to be souglit : — 



1. In the removal of correlational influences. 



2. Increased mechanical strain. 



3. Some influence exerted as a result of foliar development. This 



influence is probably bound up with the water economy of the 

 plant — particularly transpiration — but its precise nature must 

 still be determined. 



Postscript. 

 Much work remains to be done. 



1. The nature of the ordinary secondary wood in the petioles and the 

 peculiar wood mentioned in the Solanums must be studied. 



2. It is intended to carry on exactly similar researches with tlie flower 

 stalk of Pelargonium sonale. 



3. It is intended to repeat these experiments in detail witli Phytolacca 

 dioica, only one or two preliminary graftings having been made. The 

 interest of this plant lies in the fact tliat the secondary thickening of the 

 stem — as in most Piiytolaccaceae — is abnormal. It forms a normal ring 

 wliose cambium soon dies. Well outside the phloem of this ring a new 

 cambium is formed which gives rise to a new ring. In this way a series of 

 xylem rings are formed separated by bauds of parenchyma (Kiuch 48). The 

 secondary tliickening in the petiole, and the position of abnormal cambiums, 

 if any, should prove interesting. 



