The study of Archilichens as of Phycolichens is complicated by the many different kinds of fungi and algae that have entered into combination; but the two principal types of algae are the single-celled Protococcus group and the filamentous Trentepohlia: as before only the broad lines of thalline development will be traced.
The elementary forms in the different series are of the simplest type—a somewhat fortuitous association of alga and fungus, which in time bears the lichen fructification. It has been stated that the greatest advance of all took place with the formation of a cortex over the primitive granule, followed by a restricted area of growth outward or upward which resulted finally in the foliose and fruticose thalli. Guidance in following the course of evolution is afforded by the character of the fructification, which generally shows some great similarity of type throughout the different phyla, and remains fairly constant during the many changes of thalline evolution. Development starting from one or many origins advances point by point in a series of parallel lines.
a. Thallus of Pyrenocarpineae. In this series there are two families of algae that function as gonidia: Protococcaceae, consisting of single cells, and Trentepohliaceae, filamentous. Phyllactidium (Cephaleuros) appears in a single genus, Strigula, a tropical epiphytic lichen.
Associated with these types of algae are a large number of genera and species of an elementary character, without any differentiation of tissue. In many instances the thallus is partly or wholly embedded in the substratum.
Squamulose or foliose forms make their appearance in Dermatocarpaceae: in Normandina the delicate shell-like squamules are non-corticate, but in other genera, Endocarpon, Placidiopsis, etc., the squamules are corticate and of firmer texture, while in Dermatocarpon, foliose fronds of considerable size are formed. The perithecial fruits are embedded in the upper surface.
In only one extremely rare lichen, Pyrenothamnia Spraguei (N. America), is there fruticose development: the thallus, round and stalk-like at the base, branches above into broader more leaf-like expansions.
b. Thallus of Coniocarpineae. At the base of this series are genera and species that are extremely elementary as regards thalline formation, with others that are saprophytic and parasitic. The simplest type of thallus occurs in Caliciaceae, a spreading mycelium with associated algae (Protococcaceae) collected in small scattered granules, resembling somewhat a collection of loose soredia. The species grow mostly on old wood, trunks of trees, etc. In Calicium (Chaenotheca) chrysocephalum as described by Neubner[1004] the first thallus formation begins with these scattered minute granules; gradually they increase in size and number till a thick granular coating of the substratum arises, but no cortex is formed and there is no differentiation of tissue.
The genus Cyphelium (Cypheliaceae) is considered by Reinke to be more highly developed, inasmuch as the thalline granules, though non-corticate, are more extended horizontally, and, in vertical section, show a distinct differentiation into gonidial zone and medulla. The sessile fruit also takes origin from the thallus, and is surrounded by a thalline amphithecium, or rather it remains embedded in the thalline granule. A closely allied tropical genus Pyrgillus has reached a somewhat similar stage of development, but with a more coherent homogeneous thallus, while in Tylophoron, also tropical or subtropical, the fruit is raised above the crustaceous thallus but is thickly surrounded by a thalline margin. The alga of that genus is Trentepohlia, a rare constituent of Coniocarpineae.
A much more advanced formation appears in the remaining family Sphaerophoraceae. In Calycidium, a monotypic New Zealand genus, the thallus consists of minute squamules, dorsiventral in structure but with a tendency to vertical growth, the upper surface is corticate and the mazaedial apothecia—always open—are situated on the margins. Tholurna dissimilis, (Scandinavian) still more highly developed, has two kinds of rather small fronds corticate on both surfaces, the one horizontal in growth, crenulate in outline, and sterile, the other vertical, about 2 mm. in height, hollow and terminating in a papilla in which is seated the apothecium.
Two other monotypic subtropical genera form a connecting link with the more highly evolved forms. In the first, Acroscyphus sphaerophoroides, the fronds are somewhat similar to the fertile ones of Tholurna, but they possess a solid central strand and the apical mazaedium is less enveloped by the thallus. The other, Pleurocybe madagascarea, has narrow flattish branching fronds about 3 cm. in height, hollow in the centre and corticate with marginal or surface fruits.