Manchester Memoiis, Vol. lix. (19 15), No. 8. 



39 



bases of the leaf-traces, is a zone in which the radial 

 arrangement of the cells can be traced out to and beyond 

 the phloem. This is the zone destined to give rise to the 

 outer xylem and the parenchymatous xylem-sheath. 



If these sections are followed in the reverse order, 

 from Photo 8 to Photo 5, they will be seen to agree with 

 the information supplied by the longitudinal sections as 

 to the regions of the procambium and their behaviour in 

 the differentiation of the stele. Though the outer zone of 

 xylem is not specially prominent, this specimen would, 

 by itself, justify the distinction of regions in the stek 

 already suggested. These regions are, however, still more 

 clearly shown in Photos 9-13, in which the differentiation 

 of a larger stele from another plant is followed from the 

 procambial region downwards. After the description of 

 the preceding plant attention need only be briefly called 

 to the features exhibited by this series of photographs. 

 The differentiation may here be traced from above down- 

 wards. The plane of section in this series was slightly 

 oblique, sloping downwards from right to left as the 

 photographs are placed on the plate. 



The section in Photo 9 is just below the level of the 

 apex and above any definitely limited stele. It shows 

 the central group of cells of the procambium (a), and 

 around this the radiating arrangement of the rows of 

 cells (/3), which carry out the young leaves and will give 

 rise to the cortex and the outer regions of the stele. In 

 Photo 10 there is still no bonification of the stele. The 

 outer limit of the stele is, however, clearly marked on the 

 left side of the photograph by the differentiation of the 

 phloem which is seen to be continuous with that of the 

 leaf-traces, as they enter the stele. The phloem is ren- 

 dered conspicuous in this section by the deep staining of 

 its walls with hematoxylin. Within the phloem comes 



