THE HORSETAILS. 187 



328. From the account here given of the mode of 

 reproduction in the Ferns, it will be evident that the 

 Gymnosperms occupy an intermediate position between 

 them and the Angiosperms. 



For a description of other, common Ferns differing in 

 detail from the Polypody, the student is referred to Part 

 IL, page 169. 



329. The Horsetails. At page 181. Part IT., will 

 be found a description of the common Horsetail, with an 

 illustration of the fertile stem, or rather branch, because 

 both the pale spore-bearing branch and the later green 

 shoots with whorled branches are sent up from an under- 

 ground stem or rhizome. The spores, upon germination, 

 give rise to prothallia bearing antheridia and archegonia 

 precisely as in the Ferns. The prothallium is usually 

 small, flat, and irregularly branched or lobed, developing 

 the antheridia at the projecting ends of the lobes, and the 

 archegonia in the angles between them ; or, in other cases, 

 the prothallia may be dioecious. Fertilization of the germ- 

 cell, which occupies a cavity at the base of the archegonium, 

 takes place exactly as in the Ferns, and, as a result of 

 fertilization, the germ-cell developes into a spore-bearing 

 plant similar to the original one. Here, therefore, we 

 have again exhibited an alternation of generations. 



Other species of Equisetum of common occurrence, 

 instead of producing a special fertile branch, develope 

 sporangia at the extremities of the ordinary leafy stems. 



330. These plants, like the Ferns, exhibit noro-vascular 

 Bundles, and the epidermis is especially characterized by 

 the excessive amount of silica contained in it, some of *he 

 species being used for scouring or polishing by rea?<?- -* 

 this property. 



