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Squamulose and foliose lichens grow mostly in close relation with the support, and the flat expanding thallus, as in the Parmeliae, is attached at many points to the substance—tree, rock, etc.—over which the plants spread. Special provision for support is therefore not required, and the lobes remain thin and flaccid. Yet, in a number of widely different genera the attachment to the substratum is very slight, and in these we find an adaptation of existing tissues fitted to resist tearing strains, resistance being almost invariably secured by the strengthening of the cortical layers.

a. By development of the Cortex. Such a transformation of tissue is well illustrated in Heppia Guepini. The thallus consists of rigid squamules which are attached at one point only; the cortex of both surfaces is plectenchymatous and very thick and even the medulla is largely cellular.

The much larger but equally rigid coriaceous thallus of Dermatocarpon miniatum (Fig. 56) has also a single central attachment or umbilicus, and both cortices consist of a compact many-layered plectenchyma. The same structure occurs in Umbilicaria pustulata and in some species of Gyrophora, which, having only a single central hold-fast, gain the necessary stiffening through the increase of the cortical layers.

Fig. 56. Dermatocarpon miniatum Th. Fr. (S. H., Photo.).

In the Stictaceae there are a large number of widely-expanded forms, and as the attachment depends mostly on a somewhat short tomentum, strength is obtained here also by the thick plectenchymatous cortex of both surfaces. When areas denuded of tomentum and cortex occur, as in Lobaria pulmonaria, the under surface is not sensibly weakened, since the cortical tissue remains connected in a stout and firm reticulation.

b. By development of Veins or Nerves. Certain ground lichens belonging to the Peltigeraceae have a wide spreading thallus often with very large lobes. The upper cortex is a many-layered plectenchyma, but the under surface is covered only by a loose felt of hyphae which branch out into a more or less dense tomentum. As the firm upper cortex continues to increase by intercalary growth from the branching upwards of hyphae from the meristematic gonidial zone, there occurs an extension of the upper thallus with which the lower cannot keep pace[372]. A little way back from the edge, the result of the stretching is seen in the splitting asunder of the felted hyphae of the under surface, and in the consequent formation of a reticulate series of ridges known as the veins or nerves; they represent the original tomentose covering, and are white, black or brown, according to the colour of the tomentum itself. The naked ellipsoid interstices show the white medulla, and, if the veins are wide, the colourless areas are correspondingly small. Rhizinae are formed on the nerves in several of the species, and anchor the thallus to the support. In Peltigera canina, the under surface is almost wholly colourless, the veins are very prominent (Fig. 55), and are further strengthened by the growth and branching of the parallel hyphae of which they are composed. They serve to strengthen the large and flabby thallus and form a rigid base for the long rhizinae by which the lichen clings to the grass or moss over which it grows.

The most perfect development of strengthening nerves is to be found in Hydrothyria venosa[373], a rather rare water lichen that occurs in the streams of North America. It consists of fan-like lobes of thin structure, the cortex being only about one cell thick. The fronds are about 3 cm. wide and they are contracted below into a stalk which serves to attach the plant to the substratum. Several fronds may grow together in a dense tuft, the expanded upper portion floating freely in the water. Frequently the plants form a dense growth over the rocky beds of the stream.

At the point where the stalk expands into the free erect frond, there arise a series of stout veins which spread upwards and outwards. They are definitely formed structures and not adaptations of pre-existing tissues: certain hyphae arise from the medulla at the contracted base of the frond, take a radial direction and, by increase, become developed into firm strands. The individual hyphae also increase in size, and the swelling of the nerve gives rise to a ridge prominent on both surfaces. They seldom anastomose at first but towards the tips they become smaller and spread out in delicate ramifications which unite at various points. There is no doubt, as Bitter[372] points out, that the nerves function as strengthening tissues and preserve the frond from the strain of the water currents which would, otherwise, tear apart the delicate texture.