414 Scientific Proceedings, Royal Dublin Society. 



ooutraiy to the normal position of wood in, say, a large bow-shaped bundle 

 in a petiole, the wood is on the lower surface. Its origin is very simple. 

 Without regarding details, the vascular strands of the leaf leave it in three 

 aggregates of bundles arranged in a crescentic form. The horns of the 

 crescent gradually fuse, but some of the vascular tissue before the fusion 

 leaves the periphery to form the central bundle. On entering the stem the 

 central bundle splits a portion going to each side to fuse with the upper and 

 lateral bundles, which have all come together, forming a large lateral bundle 

 on the uuder side of the petiole. As it advances to the fusion it twists on 

 itself and completes the fusion with the wood on the more normal upper 

 side. Tliere is little else of importance in the petiole, save that there is a 

 single sub-epidermal layer of coUenchyma which can be made out with 

 difficulty in the photograph, owing to aberration caused by the use of a low- 

 power objective. 



Plate XXX, fig. 2, shows the centre bundle at a magnification of some 

 45 diameters. It shows the wood (a) on one side only. A complete ring 

 of small-celled phloem is on the upper side of this, {b) being one layer, 

 ((") being the other. Tlie ring surrounds a larger-celled mass of parenchyma 

 with thin walls — in no specimen did there appear bast used in Jannicke's 

 sense. Jannicke makes no mention of the presence of phloem (i.e. sieve 

 tubes and companion cells). The wood consists merely of primary elements, 

 that is tracheae and wood parenchyma. 



Plate XXX, fig. 3, shows us that traces of cambium are still to be 

 found, even in well-matured petioles, between the sylem and that part of the 

 phloem lying adjacent to it. Thus {a) shows two small groups of dormant 

 cambium cells, (6) are the masses of small-celled phloem. The tissue lying 

 between (c) and (c) is the large-celled parenchymatous tissue which the 

 phloem ring surrounds. 



Plate XXX, fig. 4, shows that part of the phloem ring farther from the 

 wood which lies just at («). {b) (b) is the phloem nearest to the wood, (c) (c) is 

 tlie central cortical tissue, {(/) (d) the phloem removed from the wood. This 

 phloem is clearly marked off from the medullary tissue which abuts against 

 it — in many petioles even more clearly than here. (This one was, however, 

 photographed, so as to have all the petiole structure thus studied from the 

 same petiole.) As will be seen then, the outer or medullary side of this 

 phloem shows no trace of that cambium which Jaimicke refers to as forming 

 a definite ring. And this is the typical appearance. In one petiole A, 1374, 

 however, at one corner the divisions had so followed in the phloem 

 parenchyma as to constitute a definite cambium for a very short length 

 indeed of the phloem. In one other petiole A, 1386, this cambium had 



