4. _Parmelia parietina_, gonidia and asci. 5. _Parmelia parietina_, asci and paraphyses; 5 _a_, spores. 6. _Calicium clavellum._ 7. _Calicium clavellum_, stalked apothecia. 8. _Calicium clavellum_, apothecium. 9. _Polypodium vulgare_, frond. 10. _Polypodium vulgare_, lobe of frond. 11. _Polypodium vulgare_, group of capsules (thecæ). 12. _Polypodium vulgare_, capsule and spores. 13. _Polypodium vulgare_, spore germinating. 14. _Polypodium vulgare_, prothallium. 15. _Polypodium vulgare_, portion of prothallium, with archegonia. 16. _Aspidium filix mas_, frond. 17. _Aspidium filix mas_, pinnules with sori. 18. _Aspidium filix mas_, single pinnule. 19. _Scolopendrium vulgare_, frond. 20. _Scolopendrium vulgare_, portion of frond. 21. _Cladonia coccifera._ 22. _Cladonia cornuta._ 23. _Cladonia pyxidata._ 24. _Cladonia rangiferina._ 25. _Cladonia rangiferina_, ends of podetium. 26. _Graphis scripta._ 27. _Graphis scripta_, lirellæ. 28. _Graphis scripta_, asci and spores. 29. _Graphis scripta_, spore. 30. _Opegrapha betulina._ 31. _Opegrapha betulina_, lirellæ. 32. _Opegrapha betulina_, lirella. 33. _Opegrapha betulina_, ascus with spores. 34. Scalariform ducts of Brake (_Pteris_). [Illustration: Plate II. W Bagg sculp _London: John Van Voorst._] CHAPTER V. FERNS, OR FIL´ICES. The general appearance of plants belonging to the class of Ferns is so well known that it need scarcely be described, especially since the introduction of the glass plant-cases, by means of which the air can be kept so damp that ferns are now grown in the very heart of our cities. Their bright green and finely cleft leaves (PI. II. figs. 9 & 16) or fronds (_frons_, a leaf) as the leaf-like organs of the lower plants are called, arising in tufts from the stems, give them the elegant appearance we are called upon to admire, whenever they are met with. The brownish spots or stripes seen upon the back or under surface of the fronds, and consisting of the fructification, form also a simple character by which they may generally be distinguished; although in a few of them the fructification is placed upon a distinct stalk. The stem or rhi´zome (ῥἱξωμα, a root) of a fern is mostly situated just beneath or at the surface of the ground, and is commonly mistaken for the real root, which is buried in the earth. It is brownish outside, and covered with scurfy scales or _ramen´ta_ (_ramentum_, a shaving). These scales are interesting microscopic objects, from the distinctness with which they exhibit the cellular network.