Manchester Memoirs, Vol. lix. (191 5), No. 3. 23 



the case of the apical growth of a shoot with normal 

 longitudinal growth {Fig. 6 Diagram A) the thickness of 

 the cortex increases behind the growing point until the 

 definitive primary diameter of the stem is attained. The 

 growth in length in the region of elongation provides the 

 surface for the insertion of the full-sized functional leaves 

 and carries the apical region forward. The slow growth 

 of the apical region itself, depending on the multiplication 

 of the meristematic cells, affords space around the actual 

 growing point for the appearance of new leaf-rudiments. 

 In the case of the shoot of Isoetes (Fig. 6, Diagram B) on 

 the other hand, the elongation of the central region is 

 quite insufficient to allow of an extension of surface for 

 the insertion of the large leaf-bases. The slight increase 

 in length of the central region of the shoot, comprising 

 the stele and the inner region of the cortex, depends on 

 the addition of new elements by transformation of meri- 

 stematic cells and not on a region of elongation. The 

 surface bearing the leaves must, however, increase in area 

 with the enlargement of the leaf-bases to their full size, 

 and this is permitted by the cortical rows of cells increas- 

 ing in radial depth and so carrying the leaf-bearing 

 surface further from the axis. These processes result in 

 the deep-seated position of the apex of the stem in Isoetes 

 and in the progressive carriage of the leaves out from the 

 apex. This outward carriage continues and removes the 

 old leaf-bases still further from the axis to make room 

 for a new set of functional leaves (cf. Fig. 5). 



Whatever the morphological explanation of the origin 

 of the lower growing region of the stock may be, the 

 mechanism of the progressive carriage outwards of the 

 young roots and the further removal of the older roots to 

 make room for new functional ones is essentially similar 

 to that of the outward carriage of the leaves from the 



