SELAGINELLACE^ ' 



'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 parent- 



Fig. 29. — A, megaspore of IsoHes acustris L. B, prothallium ; «, 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^ microspore, showing stages in formation of antherid and antlierozoids 

 ( X 580), 7', sterile cell ; a— a, stages in formation of antherozoid ( X 580) ; ^, f^ mature antherozoid 

 ( X 700). (After Millardct.) 



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 



