154 



THE VARIOUS FORMS OF PLANTS 



R 



A common fern is the polypody (Polypodium vulgare), the habitat 

 of which is damp woods and rocky glens. These ferns are hard to pro- 

 cure entire, as they have 

 an underground stem, from 

 which at intervals the leaves 

 or fronds arise. The leaflets 

 or pinna at certain seasons 

 show a series of little brown 

 dots on the under surface. 

 These structures, called col- 

 lectively the sori (singular 

 sorus), are made up of a 

 number of tiny spore cases. 

 These spore cases, or sporan- 

 gia, hold the asexual spores. 

 These spores under favorable 

 conditions of heat and mois- 

 ture may germinate to form a 

 tiny thread of cells which 

 soon develops into a flat, 

 heart-shaped body not much 

 bigger than a pinhead, called 

 a prothallus. 



Prothallus. The pro- 

 thallus clings to the surface 

 of the ground by means of its 

 rhizoids. A careful examina- 

 tion of the prothallus with a compound microscope reveals the fact that 

 scattered among the rhizoids are some tiny rounded elevations ; immedi- 

 ately above the rhizoids and between 'them and the little groove (see Fig- 

 ure) in the prothallus are other structures ; both the above structures 

 are too minute to find with the naked eye. 



Archegonia. The last named are archegonia; they are found to be 

 very tiny flask-shaped organs almost embedded in the surface of the 

 prothallus. Each archegonium contains a single large egg 

 cell. 



Antheridia. The other structures found among the 

 rhizoids are the antheridia. Each antheridium contains a 

 large number of very minute objects which are able to 

 move about in water by means of lashlike threads of pro- 

 toplasm. Each of these motile cells is called an antherozoid; 

 they have, in fact, the same function as the sperm cells of 

 the flowering plants. Because this part of the plant holds 

 the egg cells and sperm cells, we recognize it as the sexual generation of 

 the fern. 



Rock fern, polypody. Notice the underground 

 stem giving off roots (R) from its under sur- 

 face, and leaves (C) from the upper surface. 

 The compound leaf or frond may bear sori 

 (S) on the underside of the leaflets. 



A sporangium. 



