ISOETEJE. 



387 



than six or seven species occurring witliin the limits of tlie United 

 States. Many are cultivated as ornaments. 



Order Isoeteee, the Quill worts. The prothallia of the Isoetese are dioe- 

 cious, and resemble closely those of Selaginella. The macrospores give 

 rise to small prothallia, which project through the triangular slit in the 

 spore- wall, and bear several or many sunken archegonia (Fig. 279). The 

 microspores, in their germination, first cut off a small cell («, Fig. 280, 

 A to P), which, as in Sdaginella, represents the prothallium ; the re- 

 mainder of the spore contents becomes divided into four cells (the 

 primordial cells), and these give rise to the sperm-cells (Fig. 280, A to 



Fig. 279.— 1, Longitudinal section of a prothallium of isoefes Zacwsins, four weeks 

 after sowing the spore ; ar, an archegonium. 2, a poriion of the apex of a prothal- 

 lium cut through longi'udinally, with two archegonia, ar, ar, still in process of cleveU 

 opment ; g, g, the germ-cells of the archegonia. 3, longitudinal section of an ai'che- 

 gonium ready for fertilization. 4, longitudinal section of a fertilized archegonium, 

 showing the gerra-cell transversely divided. 5, a section similar to the last ; in the 

 lower cell of the embryo-rudiment preparation for division has been made by the ap- 

 pearance of two nuclei. 1 x 40 ; 2 and 3 X 300 ; 4 and 5 X 400.— After Hofmeister. 



C). The spermatozoida are elongated and provided with cilia at both 

 ends (Fig. 280,/). 



The germ-cell, after fertilization, undergoes transverse division 

 (Fig. 279, 4 and 5), as in SelagMeUa, and its subsequent development 

 is essentially the same. 



The adult plant of the asexual generation consists of a very short, 

 thick, tuber-like stem, which bears numerous long, narrow, grass-like 

 leaves, which are sheathing at the base. There are also numerous 

 roots. The sporangia are produced in grooves on the inner side of the 

 bases of the leaves : those attached to the outer leaves contain macro- 



