Doyle — Some Researches in Uxpernnental Morphology. 419 



thougli limited in its position, the development of an interfascicular cambium 

 at all from the cortical cells of a mature petiole, is quite an interesting 

 phenomenon. In a mature petiole, in its normal relations with the plant- 

 bearing the leaf, the dead cambium, and, above all, the cortical cells, are as 

 differentiated as they ever will be. This development of an iuterfascicular 

 cambium especially shows that we must not regard a differentiated tissue as 

 necessarily a permanent tissue. As long 

 as the cells contain living proto- 

 plasm, and are bounded by unthickeued 

 cell-walls, so long will they retain the 

 potentiality of manifold development if 

 the proper stimulus is acting. 



The final stage in the old petiole is 

 easy to understand. Four individuals were 

 grown to quite a large size. The final 

 structure of the petioles was, however, not 

 the same in all four. 



Fig. 1 shows the structure of one of the 

 four. It is a diagrammatic sketch drawn 

 very roughly to scale. (1) is the upper 

 surface, (2) the lower surface. We see 

 thus that the greatest increase has been in 

 the vertical diameter. Thus Plate XXX, 

 fig. 1, is a good-sized petiole, it measures 

 2'7 vertically. Fig. 1 measured 4'2 mms. 

 planes. 



We see, however, that the upper and lower bundles have developed 

 wood to about the same extent .The upper bundles fused perfectly, the lower 

 bundles not so well. The wood occupies about half the area of the section. 

 Now, the total area has been increased more than four times. So the wood 

 developed from the small bundles in Plate XXX, fig. 1, is now at the very 

 least equal in area to tlie complete section itself. The comparatively weak 

 development of the central bundle already referred to is also seen ; [c) and 

 (f) are the jagged ends of the old epidermis which was first split by the 

 growing volume of (a) and then thrown off by the development underneath 

 it of a thick cork layer (rf). The under-surface epidermis was still intact and 

 no cork formed. 



Fig. 2 shows a different arrangement. Here, though the upper bundles 

 have completely fused and developed a large quantity of secondary wood, 

 the lower bundles have remained comparatively tiny. 



Fig. 1.— p. 

 A 13103 6/5- 



Old grafted petiole. 

 3/3/14. Very diagrammatic. 



3"6 mms. horizontally and 

 by 6-9 mms, in tlie same 



