92 



MORPHOLOGY 



retain the colour of the cortex and are greyish or whitish-grey, as in Physcia 

 ciliaris or in Physcia hispida (Fig. i lo). 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 i^landica 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. 



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^, an alpine species 

 that spreads over bark or soil, and which is further distinguished by being 



A B 



Fig- 53. Rhizoid of Parmelia exasperata Carroll (P. aspidota Rosend.). A, hyphae growing out 

 from lower cortex x 450. B, tip of rhizoid with gelatinous sheath x 335 (after Rosendahl). 



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^ pointed out, from 

 the outer cortical cells, exactly as do the cilia, and are scattered over the 



1 Darbishire 1897. 2 Schwendener i860. 



