XIV 



ISOETACE^' 



557 



rate the sporophylls of successive years, bears a single very large 

 sporangium, situated upon the inner surface of the expanded 

 base. 



According to Goebel (3) the young sporangium consists of 

 an elongated elevation composed of cells which have divided by 

 periclinal walls; but both Bower (15) and Smith (i) state that 

 it can be traced back to a small group of strictly superficial cells 

 which later undergo periclinal divisions. 



Fig. 322. — Isoetes echinospora. A, section of young sporophyll, X325; /, ligule; the 

 sporangial cells have the nuclei shown. B, section of part of a young macro- 

 sporangium, X325; the sporogenous cells have the nuclei shown. C, cross-section 

 . of the base of a young sporophyll, with microsporangium, X25; v, the velum; vb, 

 vascular bundle; the trabeculae are left unshaded. (After Wilson-Smith). 



The very complete account of the development of the spo- 

 rangium of /. echinospora made by Wilson-Smith (i) differs 

 in some important details from that of Goebel. The first peri- 

 clinal division, while it may separate a definite parietal layer, 

 does not, as a rule, do this; but there are further periclinal 

 divisions in the superficial layer of cells which add to the spo- 

 rogenous tissue, much as is the case in Equisetum and Ophio- 

 glossiim. There is not, therefore, the early and definite segre- 

 gation of the archesporium described by Goebel, nor do the 

 archesporial cells remain independent, as Goebel states is the 

 case in /. lacttstris. 



Wilson-Smith finds a complete absence of the regular 



