THE PTERIDOPHYTA : LYCOPSIDA, ETC. 



595 



shows no definite apical cell.* It rarely branches, but when this does occur 

 it is as a dichotomy, in the plane of the stem grooves, and scarcely amounts 

 to more than the formation of two short stumps. 



If a longitudinal section of the stem is cut in the median plane of the stem 

 lobes the vascular stele appears cigar-shaped, tapering above and below. 

 It is doubtful whether any portion of it exists which is entirely distinct from 

 the leaf traces, that is to say, is truly cauline, as in the Ferns, and indeed the 

 whole stem structure in Isoetes seems subordinated to the relatively large 

 leaves. 



If, however, a longitudinal section is cut at right angles to the last, i.e., 



Fig. 604. — Isoetes lacustris. Longitudinal section 

 through the stem apex showing leaf rudi- 

 ments with prominent ligules and the com- 

 pressed stele giving off leaf and root traces. 



in the plane of the stem grooves, the lower end of the stele is seen to expand 

 on both sides into two remarkable, upwardly curved horns, forming a structure 

 with an outline like that of a gardener's edging tool (Figs. 605 and 606). It 

 is from the two flat sides of this curved portion that the roots arise. They are 

 endogenous and are produced in definite parallel rows, in acropetal succession 

 throughout the year. 



The actual cleft at the base of the stem shelters a linear meristem, which 

 helps to build up the stem lobes and grows very slowly downwards. It is 

 sunk in the tissues, being covered in by the fusion of the lobar parenchyma on 

 each side. We have here a rare example of what has been called a caudex 

 descendens. It has been compared to a rhizophore, probably correctly, 



* In an allied, terrestrial species, /. hystrix, there is probably a pair of four-sided apical 

 cells, as in Lycopodium. 



