420 



Scientifie Proceedings, Royal Dublin Society. 



This figure shows the formation of two of the remaining petioles. The 

 fourth was about midway between the two. The upper bundles were fused 

 and well developed. The lower bundles and one or two of the lateral had 

 developed fairly well, but were still completely separated by large masses of 

 medullary tissue. The area was hardly equal to one half of that of the 

 fused upper bundles. In the other cases those lateral bundles which had not 

 been crushed out by the upper mass of wood had remained very small 



indeed. 



(ff) is the upper wood mass. 

 {b) the original cortex which remains 

 unruptured on the lower surface. 



(c) a thick layer periderm derived as 

 appears at the points [e] and (e) from the 

 sub-epidermal collenchyma layer. In this 

 i-espect the periderm development in the 

 petiole corresponds with that in the stem 

 where it also develops from the single 

 sub-epidermal layer of collenchyma. 



(d) a crushed mass of old cortex and 

 phloem. 



Fig. 2. — P. zonale. Second type of old 

 grafted petiole. Diagrammatic. 



To recapitulate the changes : — 



1. The old cambium re-awakened ; being more active in the upper and 

 lower than in the lateral bundles. 



2. New cambium appeared on xylem-free side of central bundle, this 

 eventually resulting in a ring of secondary wood whose development, 

 however, never proceeded very far. 



3. The upper bundles are linked by an interfascicular cambium which 

 proceeds to very great secondary activity. 



4. In the mature petiole a large amount of wood is formed. This is due 

 in three cases mainly to the upper bundle. In one ease only it is almost 

 equally contributed to by the upper and lower bundles. 



5. A thick periderm layer is developed on the upper surface where the 

 wood developed has ruptured the epidermis. The petiole thus has taken on 

 the function of the stem, and, with this, tliose properties characteristic of the 

 stem — indefinite life-duration, development of secondary wood, and periderm- 

 formatiou. 



The results in the two species of Solanum are no less remarkable, 



