258 FILICES. 



these will show the characteristic form and arrangement 

 of the vascular bundles of Ferns, which are closed, each 

 bundle, consisting chiefly of transversely-barred (scalari- 

 forni) vessels, is surrounded by a firm, dark-coloured 

 sheath of narrow thick- walled cells. These bundles, u, v, 

 or w-shaped in section, surround a cellular pith which 

 often decays away, so that old stems usually become 

 hollow. From the apparent mode of growth of Fern- 

 stems, by successive additions to the extremity only, 

 they have been termed Acrogens (point-growers). 



The Common Brake-Fern (Pteris aquilind) is collected 

 in hilly districts for litter, and one or two species are used 

 in medicine. 



2. Natural Order Equisetaceae. The Horsetail 

 Family. 



Type Field Horsetail (Equisetitm arvense). 



Herb with a creeping rhizome, and erect jointed-fertile 

 and barren stems bearing toothed sheath-like leaves 

 at the joints. 



The fertile or fruiting stem is unbranched, 6 to 10 

 inches high, and withers in spring almost before the 

 barren fronds appear. It bears a terminal, cone-like 

 catkin, consisting of numerous closely packed peltate 

 scales, upon the under margins of which are the 

 sporanges, containing microscopic spores of one kind, 

 embraced by elastic, hygroscopic filaments, which result 

 from the spiral splitting up of an outer coat of the spores. 



The barren summer fronds give off numerous slender, 

 jointed branches, in whorls of 10 or 12. In some British 

 species, the fruiting and barren stems are nearly or quite 

 alike, and often both unbranched. 



The reproductive process in Horsetails is essentially 

 the same as in most Ferns. From the germinating spore 

 is developed a lobed, usually unisexual prothallium, the 

 antheridia and archegonia * being upon different pro- 

 thallia. The development of the asexual generation 

 the Horse-tail is determined by the fertilization of the 

 germ-cell of the archegonium by the antherozoids liberated 

 by the antheridia. In the contrast and general relation 

 between the sexual and asexual generations; of the cycle 



