Wallroth was the first to make a comparative study of the different lichen thalli. He distinguished those lichens in which the green cells and the colourless filaments are interspersed equally through the entire thallus as “homoiomerous” (Fig. 2), and those in which there are distinct layers of cortex, gonidia, and medulla, as “heteromerous” (Fig. 1), terms which, though now considered of less importance in classification, still persist and are of service in describing the position of the alga with regard to the general structure. A less evident definition of the different types of thallus has been proposed by Zukal[313] who divides them into “endogenous” and “exogenous.” _a._ ENDOGENOUS THALLUS. The term has been applied to a comparatively small number of homoiomerous lichens in which the alga predominates in the development, and determines the form of the thallus. These algae, members of the Myxophyceae, are extremely gelatinous, and the hyphae grow alongside or within the gelatinous sheath. In the simpler forms the vegetative structure is of the most primitive type: the alga retains its original character almost unchanged, and the ascomycetous fungus grows along with and beside it (Fig. 4). Such are the minutely tufted thalli of _Thermutis_ and _Spilonema_ and the longer strands of _Ephebe_, in which the associated _Scytonema_ or _Stigonema_, filamentous blue-green algae, though excited to excessive growth, scarcely lose their normal appearance, making it difficult at times to recognize the lichenoid character unless the fruits also are present. Equally primitive in most cases is the structure of the thallus associated with _Gloeocapsa_. The resulting lichens, _Pyrenopsis_, _Psorotichia_, etc. are simply gelatinous crusts of the alga with a more or less scanty intermingling of fungal hyphae. In the Collemaceae, the gonidial cells of which are species of _Nostoc_ (Fig. 2), there appears a more developed thallus; but in general, symbiosis in _Collema_ has wrought the minimum of change in the habit of the alga, hence the indecision of the earlier botanists as to the identification and classification of _Nostoc_ and _Collema_. Though in many of the species of the genus _Collema_ no definite tissue is formed, yet, under the influence of symbiosis, the plants become moulded into variously shaped lobes which are specifically constant. In some species there is an advance towards more elaboration of form in the protective tissues of the apothecia, a layer of thin-walled plectenchyma being occasionally formed beneath or around the fruit as in _Collema granuliferum_. In all these lichens, it is only the thallus that can be considered as primitive: the fruit is a more or less open apothecium—more rarely a perithecium—with a fully developed hymenium. Frequently it is provided with a protective thalline margin.