[Illustration: FIG. 131.--Transverse section through the thallus of _Sticta fuliginosa_ (× 500): _r-r_ rhizoid-strands, which arise from the under side; _g-g_ gonidial layer; _m_ medullary layer; _o_ upper, _u_ lower cortex.] Two cellular forms are therefore to be found in each Lichen: 1. The cells which belong to the Fungus. These are generally septate, branched _hyphæ_ without any trace of chlorophyll. In the thallus of the majority of Lichens there may be found a medullary layer (Fig. 131 _m_) of loosely-woven hyphæ, between which there are large air chambers; and an _external layer_ (cortex) (Fig. 131 _o_, _u_) formed of closely-woven hyphæ without any intercellular spaces. In some Lichens (Collemaceæ) the hyphæ wind about in the thallus, being equally distributed throughout, without forming any decided strata. These Lichens moreover become gelatinous when exposed to moisture (Fig. 132), on account of the swelling of the walls of the Algæ. The hyphæ contain protoplasm with drops of oil, but never starch; their walls easily swell when exposed to damp after having been dried, and in some (_e.g._ _Cetraria islandica_) they become gelatinous when cooked. Certain strata of hyphæ become blue on treatment with iodine alone, from which it is inferred that the wall is allied, in its chemical nature, to starch. 2. The enclosed Algæ, termed “gonidia.” Some belonging to the Cyanophyceæ, Protococcoideæ, (especially _Pleurococcus_) and Chroococcaceæ, are spherical and are found isolated, or in irregular _groups_ of cells (Fig. 131 _g_); some belonging to _Nostoc_ (Fig. 132 _g_), Lyngbyaceæ, etc., are placed in cell-rows. Each Lichen, as a rule, has only one definite Algal-form for its gonidium. The gonidia either lie together in a certain stratum between the cortex and the medullary layer (Fig. 131 _g_), or are scattered irregularly throughout the entire thallus (Fig. 132). The thallus is in the first instance termed “heteromerous,” in the second instance, “homoiomerous.” The Fungal-hyphæ embrace the gonidia and apply themselves closely to, or even penetrate them, and hence it has been difficult to decide whether the one cellular form does or does not develop from the other (Figs. 134, 135). This theory regarding the symbiosis of Fungi and Algæ to form a Lichen is termed the Schwendenerian theory, after the first scientist who advanced it with any weight. It had been already indicated by De Bary, and further arguments in its support have at a later time been adduced by Bornet, Stahl, Treub, Frank, Bonnier, Alfr. Möller and others. [Illustration: FIG. 132.--_Collema microphyllum._ Transverse section through the thallus; _g Nostoc_-chains; _h_ hyphæ.] [Illustration: FIG. 133.--_Ephebe pubescens._ The apex of a branch of the thallus with two lateral branches (_s_): _h_ its hyphæ; _g_ the apical gonidium of the main branch.]