[Chap. XLIX FERNS, CLUB MOSSES AND EQUISETUMS 673 



growth bursts the spore wall, and part of the gametophyte protrudes. 

 The archegonia develop on the apical segments that extend beyond the 

 spore wall. Fertilization takes place as in ferns. 



Germination of the zygote begins at once. The early stages of the 

 developing embryo occur within the walls of the megaspore. Later the 

 partly formed embryo continues growth on the soil or other substrate. 

 These gametophytes are among the smallest found in pteridophytes, and 

 are somewhat similar in size to those of seed plants. 



Seeds, Occasionally a megaspore of Selaginella germinates within the 

 megasporangium, in contrast to others that fall out of the sporangium 

 and germinate on the soil. Consequently the female gametophyte with 

 its archegonium and egg is inside the megaspore wall within the mega- 

 sporangium. Fertilization may take place, followed by the development 

 of an embryo sporophyte within these structures. These phenomena 

 occur rarely in Selaginella, but when they do occur a seed is the result. 

 The seeds of pine and all other gymnosperms are formed in this same 

 manner. A seed is merely the result of the germination of the megaspore 

 within the megasporangium or ovule, and the formation of the gameto- 

 phyte and of a new sporophyte within the megasporangium. If the 

 spores fall out of the sporangium and germinate on the soil as they ordi- 

 narily do in mosses and ferns, no seeds are formed. 



The quillworts. This group of unique plants is quite unlike any other 

 pteridophvtes. Approximatelv 60 species of quillworts have been de- 

 scribed and named, and all of them belong to the genus Isoetes. They 

 have the general appearance of a small tufted rush and grow on muddy 

 flats or in the water. They are perennial, and native to north temperate 

 regions. The plant has a bulb-like axis consisting of an upper leaf- 

 bearing part and a lower part from which lateral roots grow. The leaves 

 are narrow and grass-like, with a bulbous clasping base, rarely more 

 than a few inches long. Isoetes and the grape fern, Botrychium, are the 

 only modern pteridophvtes in which the stem uniformly has a cambium. 

 Each leaf of Isoetes is a potential sporophyll. The plants are heterospo- 

 rous, and as many as 300 megaspores or 300,000 microspores may be 

 formed within a single sporangium. The spemis are multiciliate, but the 

 other reproductive structures are similar to those of Selaginella. 



The Equisetums 



The equisetums constitute another small group of about 25 living 

 species that superficially bear little resemblance to the ferns. Their life 



