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]).
Fig. 132.—Collema microphyllum. Transverse section through the thallus; g Nostoc-chains; h hyphæ.
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.
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.
Fig. 134.—Nostoc lichenoides, which is attached by a germinating thread (h) of Collema glaucescens.
