SELA GIN ELL A CE^ 



49 



The long-continued increase in thickness which distinguishes this genus 

 alone among Vascular Cryptogams except possibly Botrychium (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, megaspore of Isoctes acustris L. , 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 off 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 



FIG. 30. /. lacustris. A D, microsppre, showing stages in formation of antherid and antherozoids 

 (x 580). ?', sterile cell ; a d, stages in formation of antherozoid (x 580) ; e,f. mature antherozoid 

 ( x 700). (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 



