ISOETE^E. 



387 



than three or four species occurring within the limits of the United 

 States. Many are cultivated as ornaments. 



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

 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 nrchegonia (Fig. 279). The 

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

 A to C), which, as in Selaginella, 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, Longitndin 1 section of a prothallium of Tsoetes lacustris, four weeks 

 after sowing the spore ; ar. an arcbegoninm. 2, a por.ion of the apex of a prothal- 

 lium cut through longi udinally, with two archegonia, ai\ ar, still in process of devel- 

 opment ; g, ff, the germ-cells of the archegonia. 3, longitudinal section of an arche- 

 goninm 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 Selaginella, 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- 



