Such structures are almost wholly confined to the larger foliose and fruticose lichens and are all of the same simple type; they are fungal in origin and very rarely are gonidia associated with them.
Fig. 52. Usnea florida Web. Ciliate apothecia (S. H., Photo.).
a. Cilia. In a few widely separated lichens stoutish cilia are borne, mostly on the margins of the thallus lobes, or on the margins of the apothecia ([Fig. 52]). They arise from the cortical cells or hyphae, several of which grow out in a compact strand which tapers gradually to a point. Cilia vary in length up to about 1 cm. or even longer. In some lichens they retain the colour of the cortex and are greyish or whitish-grey, as in Physcia ciliaris or in Physcia hispida ([Fig. 110]). They provide a yellow fringe to the apothecia of Physcia chrysophthalma and a green fringe to those of Usnea florida. They are dark-brown or almost black in Parmelia perlata var. ciliata and in P. cetrata, etc. as also in Gyrophora cylindrica. The fronds of Cetraria islandica and other species of the genus are bordered with short spinulose brown hairs whose main function seems to be the bearing of “pycnidia” though in many cases they are barren ([Fig. 128]).
Superficial cilia are more rarely formed than marginal ones, but they are characteristic of one not uncommon British species, Parmelia proboscidea (P. pilosella Hue). Scattered over the surface of that lichen are numerous crowded groups of isidia which, frequently, are prolonged upwards as dark-brown or blackish cilia. Nearly every isidium bears a small brown spot on the apex at an early stage of growth. Similar cilia are sparsely scattered over the thallus, but their base is always a rather stouter grey structure, which suggests an isidial origin. Cilia also occur on the margin of the lobes.
As lichens are a favourite food of snails, insects, etc., it is considered that these structures are protective in function, and that they impede, if they do not entirely prevent, the larger marauders in their work of destruction.
Fig. 53. Rhizoid of Parmelia exasperata Carroll (P. aspidota Rosend.). A, hyphae growing out from lower cortex × 450. B, tip of rhizoid with gelatinous sheath × 335 (after Rosendahl).
b. Rhizinae. Lichen rootlets are mainly for the purpose of attachment and have little significance as organs of absorption. They have been noted in only one crustaceous lichen, Varicellaria microsticta[363], an alpine species that spreads over bark or soil, and which is further distinguished by being provided with a lower cortex of plectenchyma. In foliose lichens they are frequently abundant, though by no means universal, and attach the spreading fronds to the support. They originate, as Schwendener[364] pointed out, from the outer cortical cells, exactly as do the cilia, and are scattered over the under surface or are confined to special areas. Rosendahl[365] has described their development in the brown species of Parmeliae: the under cortex in these lichens is formed of a cellular plectenchyma with thickish walls; the rootlets arise by the outgrowth of several neighbouring cells from some slight elevation near the edge of the thallus. Branching and interlacing of these growing rhizinal hyphae follow, the outermost frequently spreading outwards at right angles to the axis, and forming a cellular cortex. The apex of the rhizoid is generally an enlarged tuft of loose hyphae involved in mucilage ([Fig. 53]), a provision for securing firmer cohesion to the support; or the tips spread out as a kind of sucker. Not unfrequently neighbouring “rootlets” are connected by mucilage at the tips, or by outgrowths of their hyphae, and a rather large hold-fast sheath is formed.
