412 Scientific Proceedings, Royal Duhlin Society. 



Just where the petiole leaves the stem we see a swelling like a very large 

 pulvinus. This was very well marked in all the Solanums, and to a less 

 extent in the Pelargoniums. It is the place where the wood of the petiole is 

 being gathered together preparatory to fusion with the vascular ring of the 

 stem. I shall have occasion to refer very briefly to it later. 



Plate XXYIII., fig. 5, is a photograph on January 7th, 1914, of the 

 petiole of Solanum Balhesii, A, 13119, 24/5. The cortical splits with cork 

 development are clearly shown. But it shows how thoroughly the petiole 

 in certain cases becomes included in the branching system of ttie plant. 



Plate XXIX, fig. 2, is a photograph taken on January 8th, 1914, of Pelarg. 

 zonale v. meteor A, 13104, 6/5 taken to show the size of the plant for which 

 the petiole can act, as a supporting and conducting organ. This plant was 

 about 2 feet high — the main stem below being about f inch thick. The 

 grafted petiole can be seen running in between the two supporting sticks. 

 This plant grew well till it was cut down early in March to provide cuttings 

 of the same variety for further investigation. It shows then to what extent 

 the length of the petiole's life is increased. It would seem that a petiole 

 which normally has only a limited length of life may have its life-duration 

 indefinitely prolonged when thus included in the branching system of a 

 plant. 



At the risk of wearying I would like to include Plate XXIX, fig. 3. 

 It is the root-system developed from a petiole of Sancliezia noMlis, of which a 

 leaf had been rooted. On the petiole of this leaf a sprout was then grafted.' 

 The roots are attached to the petiole about 1 inch, along which can be 

 noticed a small bend. Here the V of the wedge-graft can be seen ; farther 

 on, running out of the picture, is the stem developed from the grafted sprout. 

 The approximation in size of stem and petiole is liere very marked. The 

 root-system is comparatively small, as the plant grew weakly. 



Plate XXIX, figs. 2 and 3, were specially included to emphasize under 

 what different conditions the petiole is after the development of the grafted 

 sprout, being the only support for a large plant, and the only conductive 

 passage between a large root development and a large foliage development. 



B. — Microscopic. 



Before going on to the microscopical history of these changes, and indeed 

 to understand them at all, we must first study the structure of the normal 

 petiole. The changes have only been followed in Pelargonium sonale in 

 detail — -in the other cases we must content ourselves with the final form, 



