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STRATOSE THALLUS 95
stems of Betula nana. Apical haptera are very frequent in Cladonia rangiferina
and Cl. 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 Calluna formations. Haptera similar to those of 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 1
pointed out, by a darkbrown
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 darkbrown
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
2
attachments have received considerable attention from . Gallic 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
characteristic of fruticose
gradually die off. Haptera are more especially
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 lands.
high and unsheltered moor-
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.
and the lobes remain
Special provision for support is therefore not required,
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.
1 Parfitt in Leighton 1871, p. 470.
2 Gallic 1915.