A. Work of successive Systematists
Since the time when lichens were first recognized as a separate class—as members of the genus Lichen by Tournefort[1016] or as “Musco-fungi” by Morison[1017],—many schemes of classification have been outlined, and the history of the science of lichenology, as we have seen, is a record of attempts to understand their puzzling structure, and to express that understanding by relating them to each other and to allied classes of plants. The great diversity of opinion in regard to their affinities is directly due to their composite nature.
a. Dillenius and Linnaeus. The first systematists were chiefly impressed by their likeness to mosses, hepatics or algae. Dillenius[1018] in the Historia Muscorum grouped them under the moss genera:—IV. Usnea, V. Coralloides and VI. Lichenoides. Linnaeus[1019] classified them among algae under the general name Lichen, dividing them into eight orders based on thalline characters in all but one instance, the second order being distinguished from the first by bearing scutellae. The British botanists of the latter part of the eighteenth century—Hudson, Lightfoot and others—were content to follow Linnaeus and in general adopted his arrangement.
b. Acharius. Early in the nineteenth century Acharius, the Swedish Lichenologist, worked a revolution in the classification of lichens. He gave first place to the form of the thallus, but he also noted the fundamental differences in fruit-formation: his new system appeared in the Methodus Lichenum[1020] with an introduction explaining the terms he had introduced, many of them in use to this day.
Diagnoses of twenty-three genera are given with their included species. The work was further extended and emended in Lichenographia Universalis[1021] and in the Synopsis Lichenum[1022]. In his final arrangement the family “Lichenes” is divided into four classes, three of which are characterized solely by apothecial characters; the fourth class has no apothecia. They are as follows:
- Class I. Idiothalami with three orders, Homogenei, Heterogenei and Hyperogenei: the apothecia differ in texture and colouration from the thallus: Lecidea, Opegrapha, Gyrophora, etc.
- Class II. Coenothalami, with three orders, Phymaloidei, Discoidei and Cephaloidei. The apothecia are partly formed from the thallus: Lecanora, Parmelia, etc. The Pyrenolichens are also included by him in this class, because “the thallus surrounds and is concrete with the partly or wholly immersed apothecia.”
- Class III. Homothalami with two orders, Scutellati and Peltati. The apothecia are formed from the cortical and medullary tissue of the thallus: Ramalina, Usnea, Collema, etc.
- Class IV. Athalami, with but one sterile genus, Lepraria.
The orders are thus based on the form of the fruit; the genera in the Synopsis number 41. Large genera such as Lecanora with 132 species are divided into sections, many of which have in turn been established as genera, by S. F. Gray in 1821, and later by other systematists.
The Synopsis was the text-book adopted by succeeding botanists for some 40 years with slight alterations in the arrangement of classes, genera, etc.
Wallroth[1023] and Meyer[1024] followed with their studies on the lichen thallus, and Wallroth’s division into “Homoiomerous” and “Heteromerous” was accepted as a useful guide in the maze of forms, representing as it did a great natural distinction.
c. Schaerer. This valiant lichenologist worked continuously during the first half of the nineteenth century, but with very partial use of the microscope. His last publication in 1850, an Enumeration of Swiss Lichens, was the final declaration of the older school that relied on field characters. His classification is as follows: