[Illustration: 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. [Illustration: 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).]