[Illustration: FIG. 44.--_A_, a common lichen (_Parmelia_), of the natural size. _ap._ spore fruit. _B_, section through one of the spore fruits, × 5. _C_, section through the body of a gelatinous lichen (_Collema_), showing the _Nostoc_ individuals surrounded by the fungus filaments, × 300. _D_, a spermagonium of _Collema_, × 25. _E_, a single _Nostoc_ thread. _F_, spore sacs and paraphyses of _Usnea_, × 300. _G_, _Protococcus_ cells and fungus filaments of _Usnea_.] If a thin vertical section of the plant is made and sufficiently magnified, it is found to be made up of somewhat irregular, thick-walled, colorless filaments, divided by cross-walls as in the other sac-fungi. In the central parts of the plant these are rather loose, but toward the outside become very closely interwoven and often grown together, so as to form a tough rind. Among the filaments of the outer portion are numerous small green cells, that closer examination shows to be individuals of _Protococcus_, or some similar green algæ, upon which the lichen is parasitic. These are sufficiently abundant to form a green line just inside the rind if the section is examined with a simple lens (Fig. 44, _B_). The spore fruits of the lichens resemble in all essential respects those of the cup fungi, and the spore sacs (Fig. 44, _F_) are much the same, usually, though not always, containing eight spores, which are sometimes two-celled. The sterile filaments between the spore sacs usually have thickened ends, which are dark-colored, and give the color to the inner surface of the spore fruit. In Figure 45, _H_, is shown one of the so-called "_Soredia_,"[7] a group of the algæ, upon which the lichen is parasitic, surrounded by some of the filaments, the whole separating spontaneously from the plant and giving rise to a new one. [7] Sing. _soredium_. Owing to the toughness of the filaments, the finer structure of the lichens is often difficult to study, and free use of caustic potash is necessary to soften and make them manageable. [Illustration: FIG. 45.--Forms of lichens. _A_, a branch with lichens growing upon it, one-half natural size. _B_, _Usnea_, natural size. _ap._ spore fruit. _C_, _Sticta_, one-half natural size. _D_, _Peltigera_, one-half natural size. _ap._ spore fruit. _E_, a single spore fruit, × 2. _F_, _Cladonia_, natural size. _G_, a piece of bark from a beech, with a crustaceous lichen (_Graphis_) growing upon it, × 2. _ap._ spore fruit. _H_, _Soredium_ of a lichen, × 300.]