STRATOSE THALLUS 95 



stems of Betula nana. Apical haptera are very frequent in Cladonia rangi- 

 ferina and CI. sylvatica, induced here also by contact. These two plants, as 

 well as several species of Cetraria, tend, indeed, to become entirely epiphytic 

 on the heaths of the Calbma formations. Haptera similar to those oi Alectoria 

 occur in Usnea, Evernia, Ramalina and Cornicularia {Cetraria). In Evernia 

 prunastri var. stictoceros, a heath form, the fronds become attached to the 

 stems and branches of Erica tetralix by hapteroid strands of slender glutinous 

 hyphae which persist on the frond of the lichen after it is detached as 

 small very dark tubercles surmounted, as Parfitt^ pointed out, by a dark- 

 brown grumous mass of cells. Plug-like haptera may be formed at the base 

 of Cladoniae which attach them to each other and to the substratum. The 

 brightly coloured fronds of Letharia vulpina are attached to each other in 

 somewhat tangled fashion by lateral bridges or by fascicles of hyphae dark- 

 brown at the base but colourless at the apices, exactly like aerial adventitious 

 rhizinae. They grow out from the fronds generally at or near the tips and 

 lay hold of a neighbouring frond by means of mucilage. These haptera are 

 evidently formed in response to friction. Haptera along with other lichen 

 attachments have received considerable attention from Gallj^e^ He finds 

 them arising on various positions of the lichen fronds and has classified 

 them accordingly. 



After the haptera have become attached, they increase in size and strength 

 and supply a strong anchorage for the plant ; the point of contact frequently 

 forms a basis for renewed growth while the part beneath the hapteron may 

 gradually die off. Haptera are more especially characteristic of fruticose 

 lichens, but Sernander considers that the rhizinae of foliose species may 

 function as haptera. They are important organs of tundra and heath 

 formations as they enable the lichens to get a foothold in well-lighted 

 positions, and by their aid the fronds are more able to resist the extreme 

 tearing strains to which they are subjected in high and unsheltered moor- 

 lands. 



F. Strengthening Tissues of Stratose Lichens 



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. 



^ Parfitt in Leighton 1871, p. 470. ^ Gallic 1915. 



