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A TEXTBOOK OF THEORETICAL BOTANY 



its embryo while still attached to the sporophyte. In Selaginella, therefore, 

 we find for the first time that complete dependence of the gametophyte on 

 the sporophyte which marks the Flowering Plants. 



Isoetales 



This is a monotypic order, that is, it contains only one genus, Isoetes, 

 the characters of which are therefore those of the order as a whole. As in 

 some other related orders, we can trace the ancestry of this plant back to 

 Palaeozoic times, and insignificant as it appears to-day, it is probably the 

 direct descendent of the tall, columnar tree, SigiUaria, of the Carboniferous 

 Period. 



Isoetes lacustris (Quillwort) 



Isoetes is a genus of small aquatic or occasionally terrestrial plants, with 

 about sixty species distributed principally in the Northern Hemisphere. Our 



Fig. 6oi. — Isoetes lacustris showing habit of 

 growth. Half natural size. 



type species is not uncommon in mountain lakes in this country, especially 

 in very soft and pure waters (Fig. 6oi). The stem is a blackish corm about 

 the size of a small Hazel nut, bearing a crop of thin, adventitious roots below 



