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Lichens were henceforth regarded as a distinct genus or section of plants. Micheli[56], an Italian botanist, Keeper of the Grand Duke’s Gardens in Florence, realized the desirability of still further delimitation, and he broke up Tournefort’s large comprehensive genera into numerical Orders. In the genus Lichen, he found occasion for 38 of these Orders, determined mainly by the character of the thallus, and the position on it of apothecia and soredia. He enumerates the species, many of them new discoveries, though not all of them recognizable now. His great work on Plants is enriched by a series of beautiful figures. It was published in 1729 and marks the beginning of a new Period—a new outlook on botanical science. Micheli regarded the apothecia of lichens as “floral receptacles,” and the soredia as the seed, because he had himself followed the development of lichen fronds from soredia.

The next writer of distinction is the afore-mentioned Dillen or Dillenius. He was a native of Darmstadt and began his scientific career in the University of Giessen. His first published work[57] was an account of plants that were to be found near Giessen in the different months of the year. Mosses and lichens he has assigned to December and January. Sherard induced him to come to England in 1721, and at first engaged his services in arranging the large collections of plants which he, Sherard, had brought from Smyrna or acquired from other sources.

Three years after his arrival Dillenius had prepared the third edition of Ray’s Synopsis for the press, but without putting his name on the title-page[58]. Sherard explained, in a letter to Dr Richardson of Bierly in Yorkshire, that “our people can’t agree about an editor, they are unwilling a foreigner should put his name to it.” Dillenius, who was quite aware of the prejudice against aliens, himself writes also to Dr Richardson: “there being some apprehension (me being a foreigner) of making natives uneasy if I should publicate it in my name.” Lichens were already engaging his attention, and descriptions of 91 species were added to Ray’s work. So well did this edition meet the requirements of the age, that the Synopsis remained the text-book of British Botany until the publication of Hudson’s Flora Anglica in 1762.

William Sherard died in 1728. He left his books and plates to the University of Oxford with a sum of money to endow a Professorship of Botany. In his will he had nominated Dr Dillenius for the post. The great German botanist was accordingly appointed and became the first Sherardian Professor of Botany, though he did not remove to Oxford till 1734. The following years were devoted by him to the preparation of Historia Muscorum, which was finally published in 1741. It includes an account of the then known liverworts, mosses and lichens. The latter—still considered by Dillenius as belonging to mosses—were grouped under three genera, Usnea, Coralloides and Lichenoides. The descriptions and figures are excellent, and his notes on occasional lichen characteristics and on localities are full of interest. His lichen herbarium, which still exists at Oxford, mounted with the utmost care and neatness, has been critically examined by Nylander and Crombie[59] and many of the species identified.

Dillenius was ignorant of, or rejected, Micheli’s method of classification, adopting instead the form of the thallus as a guide to relationship. He also differed from him in his views as to propagation, regarding the soredia as the pollen of the lichen, and the apothecia as the seed-vessels, or even in certain cases as young plants.

Shortly after the publication of Dillenius’ Historia, appeared Haller’s[60] Systematic and Descriptive list of plants indigenous to Switzerland. The lichens are described as without visible leaves or stamens but with “corpuscula” instead of flowers and leaves. He arranged his lichen species, 160 in all, under seven different Orders: 1. “Lichenes Corniculati and Pyxidati”; 2. “L. Coralloidei”; 3. “L. Fruticosi”; 4. “L. Pulmonarii”; 5. “L. Crustacei” (with flower-shields); 6. “L. Scutellis” (with shields but with little or no thallus); and 7. “L. Crustacei” (without shields).

This period extends till near the end of the eighteenth century, and thus includes within its scope the foundation of the binomial system of naming plants established by Linnaeus[61]. The renowned Swedish botanist rather scorned lichens as “rustici pauperrimi,” happily translated by Schneider[62] as the “poor trash of vegetation,” but he named and listed about 80 species. He divided his solitary genus Lichen into sections: 1. “Leprosi tuberculati”; 2. “Leprosi scutellati”; 3. “Imbricati”; 4. “Foliacei”; 5. “Coriacei”; 6. “Scyphiferi”; 7. “Filamentosi.” By this ordered sequence Linnaeus showed his appreciation of development, beginning, as he does, with the leprose crustaceous thallus and continuing up to the most highly organized filamentous forms. He and his followers still included the genus Lichen among Algae.

A voluminous History of Plants had been published in 1751 by Sir John Hill[63], the first superintendent to be appointed to the Royal Gardens, Kew. In the History lichens are included under the Class “Mosses,” and are divided into several vaguely limited “genera”—Usnea, tree mosses, consisting of filaments only; Platysma, flat branched tree mosses, such as lung-wort; Cladonia, the orchil and coralline mosses, such as Cladonia furcata; Pyxidium, the cup-mosses; and Placodium, the crustaceous, friable or gelatinous forms. A number of plants are somewhat obscurely described under each genus. Not only were these new Lichen genera suggested by him, but among his plants are such binomials as Usnea compressa, Platysma corniculatum, Cladonia furcata and Cladonia tophacea; other lichens are trinomial or are indicated, in the way then customary, by a whole sentence. Hill’s studies embraced a wide variety of subjects; he had flashes of insight, but not enough concentration to make an effective application of his ideas. In his Flora Britannica[64], which was compiled after the publication of Linnaeus’s Species Plantarum, he abandoned his own arrangement in favour of the one introduced by Linnaeus and accepted again the single genus Lichen.

Sir William Watson[65], a London apothecary and physician of scientific repute at this period, proposed a rearrangement and some alteration of Linnaeus’s sections. He had failed to grasp the principle of development, but he gives a good general account of the various groups. Watson was the progenitor of those who decry the makers and multipliers of species. So in regard to Micheli, who had increased the number to “298,” he writes: “it is to be regretted, that so indefatigable an author, one whose genius particularly led him to scrutinize the minuter subjects of the science, should have been so solicitous to increase the number of species under all his genera: an error this, which tends to great confusion and embarrassment, and must retard the progress and real improvement of the botanic science.” Linnaeus however in redressing the balance earned his full approbation: “He has so far retrenched the genus (Lichen) that in his general enumeration of plants he recounts only 80 species belonging to it.”

Linnaeus’s binomial system was almost at once adopted by the whole botanical world and the discovery and tabulation of lichens as well as of other plants proceeded apace. Scopoli’s[66] Flora Carniolica, for instance, published in 1760, still adhered to the old descriptive method of nomenclature, but a second edition, issued twelve years later, is based on the new system: it includes 54 lichen species.

About this time Adanson[67] proposed a new classification of plants, dividing them into families, and these again into sections and genera. He transferred the lichens to the Family “Fungi,” and one of his sections contains a number of lichen genera, the names of these being culled from previous workers, Dillenius, Hill, etc. A few new ones are added by himself, and one of them, Graphis, still ranks as a good genus.

In England, Hudson[68], who was an apothecary and became sub-librarian of the British Museum, followed Linnaeus both in the first and later editions of the Flora Anglica. He records 102 lichen species. Withering[69] was also engaged, about this time, in compiling his Arrangement of Plants. He translated Linnaeus’s term “Algae” into the English word “Thongs,” the lichens being designated as “Cupthongs.” In later editions, he simply classifies lichens as such. Lightfoot[70], whose descriptive and economic notes are full of interest, records 103 lichens in the Flora Scotica, and Dickson[71] shortly after published a number of species from Scotland, some of them hitherto undescribed. Dickson was a nurseryman who settled in London, and his avocations kept him in touch with plant-lovers and with travellers in many lands.