PTERIDOPHYTA. 



215 



the formation of the annulus, the bursting of the sporangium and its mode of 

 attachment and development, and in the differences in the formation of the pro- 

 thallium, etc. The principal are : 



Order 2. Hymenophyllaceae. To this order belong the lowest and most 

 Moss-like Ferns ; the leaves, with the exception of the veins, are most frequently 

 formed of only one layer of cells, and consequently stomata are wanting ; the 

 formation of the prothallium also somewhat resembles the Mosses. Sori 

 marginal, on the extremities of tlic vascular bundles, and surrounded by a 

 cupular indusiitni. The sporangia are sessile, with equatorial annulus. Hij- 

 meftophyllum (H. (unlridi/ense, European). Trichomanes ('/'. speciostan, Euro- 

 pean). Species about 200, which live especially on rocks and trees in damp 

 and shady tropical forests. Some have no roots. 



Order 3. Cyatheaceae. Annulus cuntplete and oblique. To this order be- 

 long, principally, the tree-like Ferns witli palm-like habit. The number of 

 species is about 200, they are all tropical and form forests in some regions of 

 Australia. Cibotium and Diclcsonia have marginal sori, with cupular, basal 

 indusium. (The stem of D. anlarctica is covered with aerial roots.) Alsophila 

 (without indusium) ; djatlica with cupular, inferior indusium (Fig. 212 E). 



FIG. 213. Gleichenia : A part of a leaf with sori ; B a single sorus. 



Order 4. Gleicheniaceae. Sporangia with equatorial auuulus, and longi- 

 tudinal dehisceuce, most frequently groups of 3-4 in sori without indusium (Fig. 

 213). Gleichenia: the apical growth of tlie leaves continues for a long time. 



Order 5. Schizaeaceae. Annulus apical. To this order belongs Aiu-iinta, 

 which is so commonly cultivated in conservatories. The two lowest pinme are 

 metamorphosed, having no leaf parenchyma and being covered with sporangia. 

 Schiziea. Mohria. Lygodium, a climber, whose leaves have unlimited growth 

 and attain a length of several metres. About 70 species. Tropical. 



Order 0. Osmundaceae. The sporangia have at the apex a lateral group 

 of strongly thickened cells, which gradually pass over into the ordinary cells. 

 The sporangia open by a longitudinal cleft. Indusium wanting. Osmanda 

 bears the sporangia upon peculiar, branched pinmu, without parenchyma (the 

 uppermost in the leaf). O. reyalis (Royal-Fern) : European. 



Sub-Class 2. Hydropterideae (formerly Rhizocarpre), 



Water Ferns. 



The following further characteristics must be added to those 

 given on page 205 : 



