Word Finder - lichens

definition

Any of many symbiotic organisms, being associations of algae and fungi, often found as white or yellow patches on old walls, etc.

definition

Something which gradually spreads across something else, causing damage.

synonyms

• cancer

Still, the reindeer frequents it for its lichens, and on the drier slopes of the moraine deposits there occur four species of lemming, hunted by the Arctic fox (Vulpes lagopus).

Lichens for the herbarium should, whenever possible, be sought for on a slaty or laminated rock, so as to procure them on flat thin pieces of the same, suitable for mounting.

The cortical layer is usually more developed on the side towards the light, while in many lichens this is the only side provided with a cortical layer.

The majority of the lichens, however, possess a stratified thallus in which the gonidia are found as a definite layer or layers embedded in a pseudoparenchymatous mass of fungal hyphae, i.e.

Funf stuck gives ten free living algae which have been identified as the gonidia of lichens.

We find two chief types of fruit bodies in the lichens, the perithecium and apothecium; the first when the fungal element is a member of the Pyrenomycetes division of the Ascomycetes, the second when the fungus belongs to the Discomycetes division.

Cytological details of nuclear behaviour among the lichens are, however, difficult to obtain owing 'to the slow growth of these forms and the often refractory t' nature of the material in the matter of preparation for microscopical ex amination.

As is clear from the above, nearly all the lichens are produced by the association of an ascomycetous fungus with algae.

For some obscure reason the Basidiomycetes do not readily form lichens, so that only a few forms are known in which the fungal element is a member of this family.

Lichens are found growing in various situations such as bare earth, the bark of trees, dead wood, the surface of stones and rocks, where they have little competition to fear from ordinary plants.

As is well known, the lichens are often found in the most exposed and arid situations; in the extreme polar regions these plants are practically the only vegetable forms of life.

The life of many lichens thus consists of alternating periods of activity when moisture is plentiful, and completely suspended animation under conditions of dryness.

Though so little sensitive to drought and extremes of temperature lichens appear to be very easily affected by the presence in the air of noxious substances such as are found in large cities or manufacturing towns.

The growth of lichens is extremely slow and many of them take years before they arrive at a spore-bearing stage.

The chemistry of lichens is very complex, not yet fully investigated and can only be very briefly dealt with here.

Calcium oxalate is a very common substance, especially in crustaceous lichens; fatty oil in the form of drops or as an infiltration in the membrane is also common; it sometimes occurs in special cells and in extreme cases may represent 90% of the dry substance as in Verrucaria calciseda, Biatora immersa.

The classification of lichens is unique in the fact that chemical colour reactions are used by many lichenologists in the discrimination of species, and these reactions are included in the specific diagnoses.

A similar relation between oil formation and the nature of the substratum has been observed in many lichens.

In the arts, as food and as medicine, many lichens have been highly esteemed, though others are not now employed for the same purposes as formerly.

Lichens Used in the Arts.

Amongst other lichens affording red, purple or brown dyes may be mentioned Ramalina scopulorum, Parmelia, saxatilis and P. omphalodes, Umbilicaria pustulate and several species of Gyrophora, Urceolaria scruposa, all of which are more or less employed as domestic dyes.

These pigments primarily depend upon special acids contained in the thalli of lichens, and their presence may readily be detected by means of the reagents already noticed.

During the middle ages, and even in some quarters to a much later period, lichens were extensively used in medicine in various European countries.

Zukal has considered that the lichen acids protect the lichen from the attacks of animals; the experiments of Zopf, however, have cast doubt on this; certainly lichens containing very bitter acids are eaten by mites though some of the acids appear to be poisonous to frogs.

Theoretically the lichens may be classified on the basis of their algal constituent, on the basis of their fungal constituent, or they may be classified as if they were homogeneous organisms. The first of these systems is impracticable owing to the absence of algal reproductive organs and the similarity of the algal cells (gonidia) in a large number of different forms. The second system is the most obvious one, since the fungus is the dominant partner and produces reproductive organs.

There are two main divisions of lichens, Ascolichenes and Basidiolichenes, according to the nature of the fungal element, whether an ascomycete or basidiomycete.

Sometimes various lichens occur abnormally in such unexpected habitats as dried dung of sheep, bleached bones of reindeer and whales, old leather, iron and glass, in districts where the species are abundant.

It is apparent that in many cases lichens are quite indifferent to the substrata on which they occur, whence we infer that the preference of several for certain substrata depends upon the temperature of the locality or that of the special habitat.

Thus in the case of saxicolous lichens the mineralogical character of the rock has of itself little or no influence upon lichen growth, which is influenced more especially and directly by their physical properties, such as their capacity for retaining heat and moisture.

As a rule lichens grow commonly in open exposed habitats, though some are found only or chiefly in shady situations; while, as already observed, scarcely any occur where the atmosphere is impregnated with smoke.

It is to the different characters of the stations they occupy with respect to exposure, moisture, &c., that the variability observed in many types of lichens is to be attributed.

In arctic regions lichens form by far the largest portion of the vegetation, occurring everywhere on the ground and on rocks, and fruiting freely; while terrestrial species of Cladonia and Stereocaulon are seen in the greatest luxuriance and abundance spreading over extensive tracts almost to the entire exclusion of other vegetation.

In intertropical regions lichens attain their maximum development (and beauty) in the foliaceous Stictei and Parmeliei, while they are especially characterized by epiphyllous species, as Strigula, and by many peculiar corticole Thelotremei, Graphidei and Pyrenocarpei.

To give any detailed account, however, of the distribution of the different genera (not to speak of that of individual species) of lichens would necessarily far exceed available limits.

Some of the species look more like lichens than flowering plants.

On the other hand, lichens, previously regarded as autonomous plants, are now known to be dual organisms - fungi symbiotic with algae.

The excluded genera are distributed among the liverworts, lichens and fungi; but notwithstanding the great advance in knowledge since the time of Linnaeus, the difficulty of deciding what limits to assign to the group to be designated Algae still remains.

Peysonellia squamaria, Melobesia lichenoides, Leathesia difformis are forms which are not attached throughout but grow in plates like the foliaceous lichens.

The play of colour is exquisite, the basalt combining every tint of warm red, brown and rich maroon; sea-weeds and lichens paint the cave green and gold; while the lime that has filtered through has crusted the pillars here and there a pure snow-white.

Elsewhere even the leaved lichens are precarious, though the leather lichens flourish.

Lichens and mosses clothe many of the boulders that are scattered over the upper slopes.

There are innumerable kinds of moss and lichens and ferns with leaves 12 ft.

The birch and larch woods of this zone give way to pine forests as the altitude increases; and the pines to mosses, lichens and alpine plants, just below the jagged iron-grey peaks, many of which attain altitudes of 6000 to 8000 ft.

Covered with snow for the greater part of the year, and growing nothing but lichens, mosses and some scanty grass, the South Shetlands are of interest almost solely as a haunt of seals, albatrosses, penguins and other sea-fowl.

About one tenth of the land is covered by forests, which give place, at an altitude of 5000 ft., to lichens and mosses.

The writer is not aware of any evidence for the occurrence of Palaeozoic Lichens.

Concentrations of lead, zinc, cadmium and other heavy metals in lichens and marine bivalves are measured.

Forms of scree dominated by bryophytes and lichens have also been taken into account in site selection.

The wood contains a rich variety of plants and includes a number of notable bryophytes and lichens.

Look for plants such as sea pink, moss campion, scurvy grass, sea mayweed and lichens on the rocky foreshore.