April 27th, ipij.] Proceedings. xxiii 



III., The structure and growth of the rhizophoric 

 region of I. lacustris, and the development and arrange- 

 ment of the roots. Part IV., The progressive growth 

 of the young plant of I. lacustris, and the nature of the 

 cortical extension of the stock." 



In Part III., the structure of the rhizophoric lower region of 

 the stock of Isoetes lacustris ; the nature of its meristematic 

 growth ; the way in which the segmentation of the growing line 

 leads to the growth of a root-bearing surface, exposed by the 

 progressive splitting and to the carriage outwards of the roots 

 initiated close to the meristem are described in detail. The 

 organisation of the central vascular axis of the rhizophore behind 

 the meristematic line is shown to correspond remarkably to 

 that of the stem-stele as described in Part II. The arrange- 

 ment of the roots, their exogenous insertion, and the course 

 of the root-traces are compared with the corresponding features 

 of SHgma?'ia. 



In Part IV., the progressive growth and organisation of the 

 young plants of Isoetes lacustris are traced from the stage of an 

 advanced embryo to that at which a small plant exhibits adult 

 characters as regards root- and leaf-arrangement. The symmetry 

 of the plant is only evident when the second leaf and second 

 root are developed. Further roots arise from a meristem 

 established at the base of the vascular axis of the shoot long 

 before any cambial activity has begun. The rhizophore con- 

 tinues from this meristem as a region of progressive growth, 

 bearing roots acropetally. It may correspond strictly to the root- 

 bearing region in Lepidodendreae. The primary root in Isoetes 

 is lateral to the axis of the rhizophore ; the construction of the 

 plant thus appears fundamentally distinct from the Gyn.nospeims 

 and Angiosperms where the first root continues the axis of the 

 plant and behaves as a tap-root. 



The progressive cortical growth of the young plants of Isoetes 

 appears to continue uninterruptedly into that of the adult stock. 

 In the latter it is regarded as generalised in the cortex and not 

 limited to the secondary meristem. 



