Manchester Memoirs, Vol. lix. (191 5), No. 8. 35 



initial cells appear to be present, the possibility of there 

 being a single initial cell in some cases is not excluded. 

 The initial cells of the apical group appear to cut off 

 segments parallel to their bases and also to their free 

 sides. Corresponding to this a distinction is evident 

 immediately below the apex between a central column of 

 the procambial tissue, and around this radiating rows of 

 cells serving to carry the surface with the young leaf- 

 rudiments further away from the apex. This construc- 

 tion can be seen clearly in Photos 3, 4. These radiating 

 rows of cells have nothing to do with the secondar} 

 thickening, as has often been assumed, and are also 

 present in shoots where no secondary meristem will be 

 established (cf. Photo 16); the radial arrangement here 

 expresses the cortical expansion, the special importance 

 of which in the growth of the shoot of Isoetes was referred 

 to in the first of these studies. In the procambial region 

 behind the apex no limit for the stele can at first be 

 recognised, but on following back from the meristematic 

 to the differentiated regions in Photos 3, 4 and 16 it will 

 be evident that both the central group of procambium and 

 the inner portions of the radiating rows are involved in 

 the stele when delimited. The meristematic tissue formed 

 behind the initial group of cells appears to give rise only 

 to the central portion of the cylinder of xylem, while from 

 the inner portion of the radiating rows of cells an outer 

 zone of xylem, a parenchymatous xylem sheath and the 

 primary phloem are differentiated. In the absence of any 

 distinct pericyclic or endodermal sheaths the phloem 

 forms the outer limit of the stele. From what has been 

 said and from a consideration of the photographs it will 

 be evident that a more or less regular and continuous 

 radial seriation may be traced through the outer zone of 

 xylem, the phloem, and on into the cortex. The radial 



