210 PTERIDOPHYTA. 



hedric with curved sides, depending upon the way in which the 

 tetrad division has taken place. 



The sporangia are almost always situated on the nerves and 

 gathered into groups, sort, which differ in form in the various- 

 genera. The sori, in many genera, may be covered by a scale-like- 

 structure, the indusium (Figs. 211 B, 212). 



In the majority of cases, each sorus is situated on a small 

 papilla (placenta, or receptacle), which is supplied by a small vascu- 

 lar bundle. Between the sporangia, hairs (paraphyses) are often 

 situated, which spring either from the placenta or from the stalks 

 of the sporangia. 



Systematic Division. The Ferns may be divided into -two- 

 groups, characterized by the structure and development of the- 

 sporangia. The sporangia in the EUSPORANGIAT^E take their origin 

 from a group of epidermal cells, and their walls are formed by 

 several layers of cells. The archesporium is the (not tetrahedric) 

 hypodermal terminal cell of the axial row of cells which give rise- 

 to the sporangium. In the LEPTOSPORANGIAT.E the sporangia are 

 developed from single epidermal cells, and their walls are uni- 

 layered. The archesporium is a central, often tetrahedric cell,, 

 from which sixteen spore-mother-cells are developed. 1 It is diffi- 

 cult to say which form is the oldest (according to Prantl, those- 

 which have the sori on the nerve-endings) ; however, the Eusporan- 

 giatise would seem to have made their appearance long before the 

 others, and also well denned Marattiaceae and Ophioglos&acese- 

 occur in the Kulm and Coal period, before the true Polypodiaceae. 



About 4,000 species of Ferns are now existing, and they are-- 

 found especially in tropical and sub-tropical forests, 

 Family 1. Eusporangiatae. 



Order 1. Ophioglossaceae. The prothallium differs from 

 that of all other Ferns in being subterranean, free from chlorophyll,, 

 pale and tuberous. The stem is extremely short, with short 

 internodes, most frequently unbranched, vertical, and entirely 

 buried in the ground (Fig. 208 st}. In several species (among 

 which are the native ones) one leaf is produced every year, which 

 has taken three to four years for its development. In Botry- 

 chium a closed, sheath-like basal part of each leaf cover's the sub- 

 sequent leaves during their development. In Ophioglossum and 



1 The position of the annulus varies in the different orders ; longitudinal 

 in Polypodiaceae, Hymenophyllaceaa, and Cyatheacese ; transverse in Schizaa- 

 acese, Gleicheniacese : indistinct or apical in Osniundacese, Ophiaglossaceae,, 

 Marattiaceae, Salviniaceae, Marsiliaceae. 



