A note by Lightfoot[995], one of our old-time botanists who gave lichens a considerable place in his Flora, foreshadows the theory of evolution by gradual advance, and his views offer a suggestive commentary on the subject under discussion. He was debating the systematic position of the maritime lichen genus Lichina, considered then a kind of Fucus, and had observed its similarity with true lichens. “The cavity,” he writes, “at the top of the fructification (in Lichina) is a proof how nearly this species of Fucus is related to the scutellated lichens. Nature disdains to be limited to the systematic rules of human invention. She never makes any sudden starts from one class or genus to another, but is regularly progressive in all her works, uniting the various links in the chain of beings by insensible connexions.”
III. THE THALLUS
A. General Outline of Development
a. Preliminary Considerations. The evolution of lichens, as such, has reference mainly to the thallus. Certain developments of the fructification are evident, but the changes in the reproductive organs have not kept pace with those of the vegetative structures: the highest type of fruit, for instance, the apothecium with a thalline margin, occurs in genera and species with a very primitive vegetative structure as well as in those that have attained higher development.
Lichens are polyphyletic as regards their algal, as well as their fungal, ancestors, so that it is impossible to indicate a straight line of progression, but there is a general process of thalline development which appears once and again in the different phyla. That process, from simpler to more complicated forms, follows on two lines: on the one there is the endeavour to increase the assimilating surface, on the other the tendency to free the plant from the substratum. In both, the aim has been the same, to secure more favourable conditions for assimilation and aeration. Changes in structure have been already described[996], and it is only needful to indicate here the main lines of evolution.
b. Course of Evolution in Hymenolichens. There is but little trace of development in these lichens. The fungus has retained more or less the form of the ancestral Thelephora which has a wide-spreading superficial basidiosporous hymenium. Three genera have been recognized, the differences between them being due to the position within the thallus, and the form of the Scytonema that constitutes the gonidium. The highest stage of development and of outward form is reached in Cora, in which the gonidial zone is central in the tissue and is bounded above and below by strata of hyphae.
c. Course of Evolution in Ascolichens. It is in the association with Ascomycetes that evolution and adaptation have had full scope. In that sub-class there are four constantly recurring and well-marked stages of thalline development. (1) The earliest, most primitive stage, is the crustaceous: at first an accretion of separate granules which may finally be united into a continuous crust with a protective covering of thick-walled amorphous hyphae forming a “decomposed” cortex. The extension of a granule by growth in one direction upwards and outwards gives detachment from the substratum, and originates (2) the squamule which is, however, often of primitive structure and attached to the support, like the granule, by the medullary hyphae. Further growth of the squamule results in (3) the foliose thallus with all the adaptations of structure peculiar to that form. In all of these, the principal area of growth is round the free edges of the thallus. A greater change takes place in the advance to (4) the fruticose type in which the more active growing tissue is restricted to the apex, and in which the frond or filament adheres at one point only to the support, a new series of strengthening and other structures being evolved at the same time.
The lichen fungi associate, as has been already stated, with two different types of algae: those combined with the Myxophyceae have been designated Phycolichenes, those with Chlorophyceae as Archilichenes. The latter predominate, not only in the number of lichens, but also in the more varied advance of the thallus, although, in many instances, genera and species of both series may be closely related.
B. Comparative Antiquity of Algal Symbionts
One of the first questions of inheritance concerns the comparative antiquity of the two gonidial series: with which kind of alga did the fungus first form the symbiotic relationship? No assistance in solving the problem is afforded by the type of fructification. The fungus in Archilichens is frequently one of the more primitive Pyrenomycetes, though more often a Discomycete, while in Phycolichens Pyrenomycetes are very rare. There is, as already stated, no correlation of advance between the fruit and the thallus, as the most highly evolved apothecia with well-formed thalline margins are constantly combined with thalli of low type.