ISOETEJE. 



387 



than six or seven species occurring within the limits of the United 

 States. Many are cultivated as ornaments. 



Order Isoeteae, the Quillworts. The prothallia of the Isoeteae are dioa- 

 cious, and resemble closely those of Se/aginella. 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 (v, Fig. 280, 

 A to C), which, as in Selaginetta, 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 



Pig. 279. 1, Longitudinal section of a prothallium of Isoetes laoustris, four weeks 

 after sowing the spore ; nr, an archegonium. 2, a portion of the apex of a prothal- 

 lium cut through longitudinally, with two archegonia, ar< ar, still in process of devel- 

 opment ; ff, y, the germ-cells of the archegonia. 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 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 spermatozoids 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 Selaginetta, 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- 



