THE FAMILIES, GENERA AND SPECIES OF PTERIDOPHVTA. 457 



Horse-tails (Equisetiim), or Stag''s-horn mosses {Lyco podium). 

 The sporangia may be solitary or in groups (called sori) ; in the 

 latter case they are often protected by a special outgrowth of 

 the leaf, called the indusium. 



When the spores fall upon the soil they germinate under suit- 

 aljle conditions, giving" rise to the proihalli, or sexual plants. 

 The prothallus is a small, green, leaf-like, stemless body, usually 

 developing" root-like organs {rJiicoids). by which it absorbs 

 food-materials from the soil. It bears the sexual organs; the 

 male are called the anthcridia and the female the archegonia. 

 In the antheridia are developed the motile male cells, or 

 aiitlicrocoids, which are equivalent to the spermatozoa of ani- 

 mals, and are therefore often called spci'jiiatocoids. The ovum 

 or female cell is developed at the base of the archegonium. 

 After fertilization, the ovum develops into a new "plant" or 

 asexual generation, being nourished by the prothallus until it 

 can assimilate for itself. 



As a rule the Pteridophyta do not attain any great size, the 

 exceptions being" found in the Tree-ferns and in fossil forms. 

 Short, erect stems are produced by many ferns; others have 

 creeping" stems, rhizomes, or even floating" stems. 



The stems contain well-marked vascular bundles, which are 

 without cambium (as in the Monocotyledons) and are closed ; 

 they are therefore incapable of appreciable increase in thick- 

 ness. The bundles are usually concentric, with the bast round 

 the wood. The wood is made up of scalariform cells. 

 I^eaves large in proportion to the stem ; sporangia 

 collected in clusters on the underside or margins 

 of ordinarv foliage leaves or differentiated pinnae 



Class I. FILICALES. 

 Leaves small in proportion to the stem ; sporangia in 

 terminal cones, or singly on the upper side of leaf- 

 bases or in their axils. 



Leaves not green, in small, sheathing whorls; inter- 

 nodes jointed Class II. EQUISETALES. 



Leaves green, scattered or opposite; nodes not 

 jointed Class III. LYCOPODIALES. 



Class I. FILICALES (Ferns). 



Ours are homosporous plants, with monoecious prothalli. 

 Sporangia usually small, collected (but rarely united) in clusters 

 (called sori) on the underside or margins of ordinary foliage- 

 leaves (these called sporopJiylls), and covered with a membrane 

 (indusium): or. in more highly developed forms, on differen- 

 tiated fertile pinnae, and not covered by an indusium; indusium 

 variously opening with the growth of the sporangia. Spores 

 uniform. Archesporium most frequently unicellular. Peren- 

 nial plants with an erect, creeping or climbing rhizome; often 

 epiphytes. Stem small in comparison with the leaves, branch- 

 ing only seldom and then by lateral shoots. Leaves scattered, 

 large, often deeply divided, usually circi)iatc, i.e.. rolled in- 

 wards before expansion. Sporophylls not confined to definite 

 parts of the shoot, nor limiting its growth. 



