ISOETE^. 



387 



than three or four species occurring witliin tlie limits of tlie United 

 Statei'. Many are cultivated as ornaments. 



Order Isoetese, the Qiiil 1 worts. The prothallia of the Isoeteae are dioe- 

 cious, and resemble closely those of Se'ac/mella. 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 (b, Fig. 280, 

 A to 0), which, as in Selag'nella, 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. 380, A to 



Fig, 279.— 1, Longitudin 1 section of a prothallium of /fioefes ^ttci^sins, four weeks 

 after powing the spore ; ar^ an archegonium. 2, a por.ion of the apex of a prothal- 

 lium cut through longi udioaily, with two archegonia, ar, ar, still in process of devel- 

 opment ; fir, ^, the germ-cells of the archeeonia. 3, longitudinal section of an arche- 

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

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

 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 Hufmeister. 



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

 ends (Fig. 380,/). 



Tue germ-cell, after fertil'zation, undergoes transverse division 

 (Fig. 379, 4 and 5), as in Selag nella, 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- 



