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 

 II. 



329. The Horsetails. At the end of Part II. 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 arche- 

 gonium, 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 fibro- vascular 

 bundles, and the epidermis is especially characterized by 

 the excessive amount of silica contained in it, some of 

 the species being used for scouring or polishing by reason 

 of this property. 



