24 LANG, Morphology of the Stock of Isoetes laatstris. 



shoot-apex. As in the case of the stem, there is only a 

 slow addition to the vascular axis of the rhizophore 

 behind the deeply seated growing line. The rhizophoric 

 stele and the inner portion of its cortex undergo no sub- 

 sequent longitudinal extension. The elongation of the 

 rhizophoric stele is only sufficient to allow of the insertion 

 of the new root-traces. It is, however, no more correct to 

 say that the rhizophoric stele is simply due to the union 

 of the bases of the vascular bundles of the roots, than to 

 say that the stem-stele is due to the union of the bases of 

 the leaf-trace bundles. The production of new tissues to 

 the sides and outside of the growing line is quite unlike 

 the cortical growth outside the secondary meristem at 

 other regions. It is clearly related to a plane continued 

 inwards from the split of the groove to the meristem and 

 proceeds as if this represented the plane of fusion of the 

 opposed sides of an apical depression. The evidence for 

 this will be given in a detailed study of the segmentation 

 in a later paper, but the accompanying diagrams (Fig. 6, 

 Diagrams C, D) will serve to make the main idea of the 

 interpretation clear and show how naturally it agrees 

 with the position of the young roots and the course of 

 their traces. In Diagram C the line of the future split 

 inwards from the groove is represented by a dotted line, 

 while in Diagram D this is represented as a deep narrow 

 depression continuing down to the growing line ; the former 

 corresponds to the actual structure as shown in Fig. J B 

 and Fig. 5, while Fig. 6 D is imaginary and explanatory. 

 As in the case of the cortical growth around the apex of 

 the shoot, a distinction has to be made between an outer 

 region of the cortex, increasing in superficial area as the 

 root-bases enlarge, and an inner region of the cortex, 

 increasing in radial depth by tangential divisions. The 

 latter process carries the areas of outer cortex, bearing 



