SELA GIN ELLA CE^ 



49 



The lonsf-continued increase in thickness which distin2;uishes this o;enus 

 alone among Vascular Cryptogams — except possibly Botr^xhium (Filices) 

 — is dependent on an internal layer of meristem which surrounds the 

 axial 'vascular' cylinder, and continually produces new layers of paren- 



Fig. 29. — A, mega'^pore of Isoctes aciistris L. B, prothallium ; a, archegone (x about 50). 



(After Hofmeister.) 



chyme on the outside. This takes place especially in either two or 

 three directions, so that a corresponding number of projecting masses of 

 tissue are formed, which slowly die oif on the outside ; and between 

 them lie the same number of furrows meeting on the ventral side of the 

 stem, which has hence the appearance of a laterally elongated plate or 



^ A 



Fig. 30. — /. lacicstris. A — D, microspore, showing stages in formation of antherid and antherozoids 

 (X580). 7', sterile cell ; a — d, stages in formation of antherozoid (x 580) ; e,/, mature antherozoid 

 (X700). (After Millardet.) 



disc. In these furrows are produced a large number of rows of roots in 

 acropetal succession. In the stem is a single cauline axial bundle com- 

 posed of short reticulate and spiral tracheides, surrounded by a rudi- 

 mentary phloem without sieve-tubes. From this axial bundle there 



E 



