12 LANG, Morphology of the Stock of Isoetes lacustris. 



of the latter has been circular, corresponding to the spiral 

 arrangement of the leaves in the later stages of growth of 

 the plant. In the section figured in H, however, the 

 primary stele of the stem is seen to be small and lens- 

 shaped with the two-ranked leaf-traces pointing in the 

 plane of the lobes ; this is the stele of the plant in the first 

 season of its growth, when the divergence of the leaves 

 was -J. The section is just above the level at which the stem 

 stele passes into the rhizophoric region of the vascular 

 axis. Fig. 2 I y almost immediately below the preceding, 

 is below the stem stele and cuts across the root-bearing 

 region of the vascular axis, extended in the plane of the 

 groove. The bases of old root-traces are seen attached to 

 this, embedded in a secondary prismatic layer, just as the 

 old leaf-traces stood in relation to the stele of the stem. 

 Jn J the rhizophoric stele is seen cut nearer to its convex 

 lower end and therefore shorter in the plane of the groove. 

 Section K passes just below the rhizophoric stele, while 

 section L is still lower and just above the opening out of 

 the groove on the under side of the stock. In these 

 sections the characteristic arrangement of the root-traces 

 in the cortex is shown. 



If this series of transverse sections is considered as a 

 whole, it will be seen to suggest the existence of two 

 distinct regions of the vascular axis, an upper one be- 

 longing to the shoot and bearing the leaf-traces, and a 

 peculiarly-shaped lower region upon which only root- 

 traces are borne. The junction of the two regions lay 

 between H and / in Fig. 2. Not only can these two 

 regions be distinguished in the axial region of the stock, 

 but, when the distribution of the leaf-traces and root- 

 traces are taken into account, corresponding regions of 

 the cortex can be recognised. The root-bearing cortex 

 forms the lower part of the lobes and extends upwards in 



