III. POLARILOCULARES. A phylum including all types of struc ture but with a distinctive and characteristic spore—ellipsoid and mostly one-septate, with the median septum becoming so thick that the spore loculi are often relegated to minute spaces at the tips, hence the name polarilocular. A delicate canal passes through the thickened septum and forms a connection between the polar cells.
Basidiolichens are few in number and now present no problems. It is mainly with Ascolichens that workers have been concerned. Before the true nature of lichen plants was understood, many attempts had been made to classify them in relation to each other and to other members of the plant king dom—to mosses, hepatics or algae. Tournefort (1700) placed them all in one genus Lichen, and was followed by Linnaeus (1753).
of their number and variety increased, and Acharius (1803) gave diagnoses of 23 genera with their included species. Nylander (1854) issued what he considered a final state ment on lichen families and genera and of their relationships. His arrangement began with those nearest akin to algae, gelatinous blue-green forms, and wound up with those he considered to be most like fungi—the Pyrenocarpineae. Later students have worked on this basis and now a system of classification has been achieved that largely satisfies modern views. The arrangement of lichens in a natural order has presented great difficulties: it is by following the lines of development as outlined above that a way through the maze of forms—like and unlike—has been reached. The four series of Ascolichens, for instance, are marked by fruiting characters. These are subdivided into families (58 in number) largely on the structure of the thallus. The genera in these families are distinguished by minor differences of thalline though mainly of fruiting characters.
Lichens are widely distributed : members of nearly all the different families are to be found in every quarter of the globe. Winds or other agencies carry the spores of thalline particles immense distances, and these grow to full stature when they alight on a favourable substratum. It is impossible at the present stage of faulty co-ordination of knowledge to reckon their numbers, but many thousands have been recorded, and new fam ilies, genera and species are constantly being discovered. Some lichens flourish best in temperate zones, others in tropical regions, a few are restricted to polar areas, the same species appearing both in the Arctic and Antarctic. They grow best where they can
secure light : they are abundant on the tundra or on rocks and walls with a sunny exposure ; but a few are shade-plants and grow even in caves. Some can withstand the heat and scanty rainfall of the desert and others advance to the limits of perpetual snow. A fairly large number are cosmopolitan; a lesser number are en demic in larger or smaller areas.
Though self-supporting, lichens exhibit a consider able choice of habitat and form more or less constant associations of lichens only or with other plants. They are the pioneers of vegetation and soil-formation. By their delicate filaments they cling to the rock surfaces which they gradually penetrate and dis integrate. By mechanical action due to alternate wetting and drying of the gelatinous hyphae a sucker-like detachment of mi nute rock particles is constantly taking place (Fry 1924; 1926) ; by chemical action the acids discharged by the hyphae (carbonic, oxalic or lichen acids) dissolve the hardest rocks and even old window glass. The detached particles and the humus of cast-off portions of the thallus, together with blown dust, form a nidus for other vegetation—mosses and flowering plants—and mixed associations arise. The chief ecological factors are the types of substratum : the associations or communities are therefore natu rally divided into arboreal and lignicolous, 2, terricolous, 3,
saxicolous and 4, localised communities such as maritime lichens. Within these great groups there are minor associations influenced by the kind of bark, the nature of the soil (sand, clay or humus) the character of the rock (siliceous or calcareous) and also by conditions of temperature, moisture and exposure. A very dis tinct association is that of nitrophilous lichens: it constantly occurs on any kind of habitat in places frequented by birds and small mammals, and near to farm-yards or on road-sides where the dust is mixed with nitrogenous animal matter. As in other plant communities there is a struggle for place and light. Crus taceous species are invaded and ousted by those of thicker or squamulose thallus or by the larger foliose species. Some mis chance may in time dispossess them all and colonization begins afresh. Leaf lichens so abundant in the tropics also form dis tinctive associations. Lichens are rare or absent in the neighbour hood of large towns or industrial areas owing to the impure and smoke-laden atmosphere.