502 A TEXTBOOK OF BOTANY [Cn. XII 



tiated; the vascular bundles being much like those of endog- 

 enous stems (page 135), while there are separate strengthen- 

 ing strands. Commonly the epidermis contains much silica, 



enough to permit the use of the stems for polishing or 

 scouring purposes, whence one of their names. The spores^ 



dlof one kind, occur regularly in sporangia 



the under side of peltate sporophvlls of such form that-the 



fit together in ovate-cones or strobilL __ The 



FIG. 355. The prothallia of Equisetum arvense. 



Left, male prothallus, showing antheridium, X 35, and, next, an antheri- 

 dium, X 80. Below, female prothallus, X 10, and above, an archegonium, 

 X 80. Right, young sporophyte, X 6. 



four slender hygroscopic arms or elaters, wjiLetLJielp to lift 

 them fromThe sporangia, and keep them hooked together in 

 groups as they are disseminated by the wind, This at- 

 tachment to one another involves physiological advantage, 

 since the prothallia produced by the spores are dioecious ; 

 and fertilization, by the free-swimming sperm cells, is there- 

 fore facilitated by their contiguity. Theje_43HilJiallia^Fig. 

 355), somewhat resemble Liverworts, and bear the an- 

 theridia at the tips, and archegonia near the bases, of 

 the lobes. After fertilization the egg cell grows to a new 

 plant, much as in Ferns. While all living forms are 

 homosporous, many of the fossil kinds show well developed 

 heterospory. 



