PDF - CES (IISc)

I08
MORPHOLOGY
of the network was supposed by Lutz 1 to be entirely due to the swelling of
the tissues, or the imbibition of water, causing tension and splitting. A more
exact explanation of the phenomenon is given by Peirce 2 : he found that it
was due to the thickened incurved tip, which, on the addition of moisture,
swells in length, breadth and thickness, causing it to bend slightly upwards
and then curve backwards over the thallus, thus straining the part immediately
behind. These various movements result in the splitting of the frond
while it is young and the cortices are thin and weak.
Peirce made a series of experiments to test the capacity of the tissues
to support tensile strains. In a dry state, a piece of the lichen held a weight
up to I50grms.; when wet it broke with a weight of 3Ogrms.
observed that the thickness of the frond doubled on wetting.
It was also
E. ROOTING BASE IN FRUTICOSE LICHENS
Fruticose and filamentous lichens are distinguished by their mode of
attachment to the substratum : instead of a system of rhizinae or of hairs
spread over a large area, there is usually one definite rooting base by which
the plant maintains its hold on the support.
Intermediate between the foliose and fruticose types of thallus are
several species which are decumbent in habit, but which are attached at one
(or sometimes more) definite points, with but little penetration of the under-
lying substance. One such lichen, Evernia furfuracea, has been classified
now as foliose, and again as fruticose. The earliest stage of the thallus is
in the form of a rosette-like sheath which bears rhizinae on the under
surface, very numerous at the centre of the sheath, but entirely wanting
towards the periphery. A secondary thallus of strap-shaped rather narrow
fronds rises from the sheath and increases by irregular dichotomous branching.
These branches, which are considered by Zopf 3 as adventitious, may
also come into contact with the substratum and produce a few rhizinae at
that point; or if the frond is more closely applied, the irritation thus
produced causes a still greater outgrowth of rhizinae and the formation of
a new base from which other fronds originate. These renewed centres of
growth are not of very frequent occurrence; they were first observed and
described by Lindau 4 in another species, Evernia prunastri, and were aptly
compared by him to the creeping stolons of flowering plants.
Evernia furfuracea grows frequently on dead wood, palings, etc., as well
as on trees. E.prunastri grows invariably on trees, and has a more constantly
upright fruticose' habit; in this species also, a basal sheath is present, and
the attachment is secured by means of rhizoidal hyphae which penetrate
deeply into the periderm of the tree, taking advantage of the openings
1 Lutz 1894.
2 Peirce 1898. Zop f 1903.
* Lindau 1895.