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A TEXTBOOK OF BOTANY 



[Ch. 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, 

 all of one kind, occur regularly in sporangia, which lie on 

 the under side of peltate sporophylls of such form that they 

 fit together in ovate cones or strobili. The spores bear each 



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, which help to lift 

 them from the 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. These prothallia (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. 



