GROUP III. PIERIDOPHYTA : FILICINJ1 ; EUSPORANGIATJE. 381 



CLASS V'. FILICIBLE. 



The mutual relations of the orders forming this class are clearly 

 expressed in the following table : 



Eusporangiatce. Leptosporangia tee. 



Homosporece. Ophioglossacea?. Osmundaceee. 



Marattiaqeoe. SchizaBacese. 



Gleicheniacea3. 

 Polypodiacese. 

 Cyatheaceae. 

 Hymeriophyllaceae. 



Heterosporece. Isoetaceae. Salviniacese. 



Marsileacese. 



A. HOMOSPOROUS EUSPOBANGIAT^E. 



Order 1. Ophioglossaceae. This order includes the three genera Ophio- 

 glossum, Botrychium, and Helminthostachys. 



SPOROPHYTE. The stem is a subterranean rhizome (except in epiphytic 

 Ophioglossums), which does not branch at all in Ophioglossum, and but little 

 in Botrychium and Helminthostachys ; it is usually short and erect, but in 

 Helminthostachys it is elongated, dorsiventral, and creeping. The rather thick 

 and fleshy roots are unbranched in Ophioglossum, but they give rise to ad- 

 ventitious buds ; they are branched in Botrychium and Helminthostachys, and 

 produce no buds. The leaves are developed close together at the apex of the 

 rhizome, and are not circinate, or only slightly so, in vernation ; their growth 

 is so slow that a leaf does not appear above ground until the fifth year after its 

 first development ; generally, only a single leaf appears above ground each year, 

 when more are developed some of them are sterile. The sporophylls are re- 

 markable for their peculiar branching (see p. 51) ; they are petiolate, and the 

 petiole branches into two, the one bearing a sterile and the other a fertile lamina 

 (Fig. 255), the fertile branch being situated on the ventral surface of the sterile ; 

 the sterile lamina is leafy, whilst the fertile lamina consists of little more than 

 the sporangia. In Ophioglossum the sterile lamina is entire, and the fertile 

 lamina is spicate with two lateral rows of sporangia ; in Botrychium the sterile 

 lamina is pinnate, and the fertile lamina is bi-pinnate with marginal sporangia ; 

 in Helminthostachys the sterile lamina is digitate pedate, and the fertile lamina 

 is spicate with the sporangia in pedicellate clusters. The sporangia are em- 

 bedded in the tissue of the sporophyll in Ophioglossum, but are free in Botry- 

 chium and Helminthostachys : they are not arranged in sori ; they are globose, 

 have no annulus, but dehisce into two equal valves by a transverse (Ophio 

 glossum, Botrychium) or vertical (Helminthostachys) slit ; the wall of the 

 sporangium consists of several layers of cells ; the spores are numerous and 

 tetrahedral ; the archesporium is the hypodermal terminal cell of the axial row 

 of cells in the young sporangium, and is not tetrahedral. 



The sporophyte is characterised histologically by the absence of sclerenchyma, 



