CH. XII] 



THE FERNS 



495 



are 



In this they closely resemble early fossil kinds. Their 

 prothallia live underground, are saprophytic, and 

 penetrated by a mycorhizal Fungus (Fig. 350) . 

 Closely allied are the Marattiales, another 

 small order of large tropical ferns having short, 

 tuberous stems. While frondose like the Tree 

 Ferns, the sporangia on those fronds are of the 

 thick-walled, primitive, eusporangiate type, 

 though the prothallus is green, resembles a 

 Liverwort, and has the antheridia and arche- 

 gonia on the under side. 



ORDER 2. FILICALES: THE FERNS PROPER. 

 These are familiar enough in their frondose 

 stage, but not in the stages of their very 

 interesting life history, which may best be 

 understood from a typical example (Fig. 351). 

 The ordinary Fern plant of our woods has 

 a creeping, perennial, often underground stem, 

 from which rise annually, usually in a rosette, 

 the familiar fronds. On the under sides of 

 the fronds occur many brown spots, the SORI, 

 which are clusters of spore cases, SPORANGIA, 

 often partially covered by a membranous 

 INDUSIUM. The stalk of each sporangium is 

 prolonged into a thick-walled incomplete ring, 

 the ANNULUS, which, when mature, suddenly 

 straightens hygroscopically, ruptures the thin archegonia. A 

 side walls, and throws out the spores. These p^yte projects 

 _sp_o_res, which have roughened brown walls, are to the left. The 

 ^disseminated by wind, and germinate upon 

 damp, shaded ground. Here they produce not 

 the familiar Fern plant but a PROTHALLUS. which 

 recalls the homologous thallus of a Liverwort . 

 The prothallus is a dainty structure, mostly one cell layer thick. 

 It synthesizes its own food, sends rhizoids into the ground. 



FIG. 350. 

 The subterra- 

 nean prothallus 

 of Ophioglos- 

 sum vulgatum; 

 X 10. 



The larger 

 dark structures 

 are antheridia, 

 the smaller 

 (just visible 

 each as four 

 are 

 of 



black threads 

 are mycorhi- 

 zal hyphse. 



B 



