[Chap. XLIX FERNS, CLUB MOSSES AND EQUISETUMS 665 



and spores develop. Such special spore-bearing leaves have been called 

 sporophylls; and this term is applied to any spore-bearing leaf, whether 

 of fern or of seed plant. Stamens and carpels of ordinary flowers and 

 the scales of pine cones are often referred to as sporophylls. The cinna- 

 mon fern has cinnamon-colored sporophylls. 



Fern sporangia develop at the ends of short stalks. The epideiTnis of 

 the sporangium is composed of two kinds of cells: a single row of cells 

 with walls greatly thickened inwardly ( the annulus ) , and several larger 

 thin-walled cells forming the remainder of the sporangium wall. Upon 

 losing water the cells of the annulus contract, and the maturing sporan- 

 gium splits open at the side. The number of spores in a sporangium 

 ordinarily ranges from 32 to 64, but it may be higher in a few genera: 

 400-500 in Osinwida, the royal ferns; 1500-2000 in Botn/chium, the 

 grape ferns; and 12,500-15,000 in Ophioglossum, the adder's-tongue 

 ferns. Many millions of spores may develop on one fern plant in a 

 single season. The spores develop in tetrads following reduction division 

 of the spore mother cells. 



Familiar ferns include, besides the ones referred to above, the walking 

 fern, Camptosortis; the common polvpodv, Polijpodiwn; the maidenhair 

 fern, Adianttim; the bracken fern, Pteris; the Christmas fern, Polys- 

 tichum; and the sensitive fern, Onoclea. 



The gametophyte. Under appropriate conditions the spores germinate 

 either directly or after a short period of dormancv, and from each spore 

 there develops a small heart-shaped thallus that superficially resembles 

 a very simple liverwort. The first few cells fomi a filament that resembles 

 an alga or a moss protonema; in a few species of feni the filamentous 

 form is permanent. In most species, however, subsequent cell divisions 

 result in a thallose structure. This structure, unnecessarily termed a 

 prothalltis, is the gametophyte of the fern. Thalli of ferns may be found 

 growing naturally on moist rocks, on soil, and on decaying logs near 

 the sporophytes. They grow readily from spores sown on culture media 

 or on the surface of flower pots after the pots have been filled with 

 sphagnum moss and inverted in a pan of water. 



In two genera of ferns (the grape fern, Botrychiurn and the adder's- 

 tongue, Ophioglossum) the gametophyte is quite unique. In Botrychiurn 

 it is entirely subterranean and without chlorophyll; in Ophioglossum it 

 is subterranean witli green parts extending above the soil surface. In 

 both, a fungus lives in the non-green part of the gametophyte and ex- 

 tends into the surrounding humus. 



