194 THIRD GROUP. 



Eusporangiate, the latter with only homosporous, the former with homosporous and 

 heterosporous forms. Stem and root have an apical cell, which in the stem forms two 

 or three rows of segments, in the root always three rows. The vascular bundles 

 are usually very strongly developed, the central xylem, consisting chiefly of tracheides 

 with scalariform thickenings, being usually surrounded with soft phloem. 



A. Leptosporangiate Pilicineae, or Ferns in the more restricted use of the 

 word. The sporangia are formed from a single epidermal cell, and have a peculiarly 

 shaped, usually tetrahedral archesporium. 



1. HOMOSPOROUS FILICINEAE. The spores are of one kind only, and produce 

 monoecious prothallia which live an independent life. The sporophyte is either an 

 erect unbranched stem, or not erect and then usually more or less dorsiventral and 

 sparingly branched. The leaves, which are exstipulate and when young are rolled 

 inwards (circmate\ produce on their laminae, which are not at all or very little 

 metamorphosed, very numerous sporangia usually grouped together in sori and 

 covered with indusia ; the sporangia arise from single cells of the epidermis, contain 

 a central unicellular archesporium, from which usually sixteen spore-mother-cells are 

 formed, and open by means of an annulus. Stem and roots have an apical cell ; the 

 fundamental tissue tends to form a brown-walled schlerenchyma, which serves 

 chiefly to strengthen the bundle-sheaths. 



Families. 1. Hymenophyllaceae. 



2. Cyatheaceae. 



3. Polypodiaceae. 



4. Gleicheniaceae. 



5. Schizaeaceae. 



6. Osmundaceae. 



2. HETEROSPOROUS FILICINEAE (Rhizocarpae or Hydropteridae). Female 

 macrospores and male microspores are produced in sporangia of two kinds ; the 

 macrospores produce small prothallia which do not separate from the spore, the 

 microspores give rise to the mother-cells of the spermatozoids on very rudimentary 

 prothallia. The sporophyte is a dorsiventral, horizontal, regularly branched stem, 

 with two or more rows of leaves on the dorsal and roots on the ventral face, and with 

 branches on the lateral faces ; Salvinia only has no roots. The sporangia are formed 

 in sporocarps with one or more compartments, which are metamorphosed leaves or 

 segments of leaves ; they originate in single superficial cells of placentas, which bear a 

 sorus in each compartment; the sixteen spore-mother-cells in a sporangium are 

 produced by a central one-celled archesporium, as in the homosporous Ferns. The 

 microspores are many in a sporangium (4x16); the macrosporangium matures only 

 one large spore. The stem grows with a two-sided or three-sided apical cell, the root 

 with a three-sided cell. 



Families. 1. Salviniaceae. 

 2. Marsiliaceae. 



The connection between the heterosporous and the homosporous Filicineae has 

 been touched upon above in the description of the former. 



B. Eusporangiate Filicineae. The sporangia proceed from a group of 

 epidermal cells, and the archesporium is the hypodermal terminal cell of the axile 

 row of cells of the rudimentary sporangium. The antheridia are immersed in the 



