Tuatara: Volume 11, Issue 1, March 1963
Some Facts About Lichens
Some Facts About Lichens
Lichens must surely be the most neglected members of the Plant Kingdom. Yet they may be found all over the world, from the Antarctic continent to the tropics, in habitats ranging from spray-washed rocks by the sea, to boulders at the edge of the snowline on mountains; from rain forests to deserts. In harsh, inhospitable environments they may be the only vegetation, and they are almost invariably the first colonisers of exposed rock surfaces. The only places where lichens are unlikely to be found is in or near cities, as they are very sensitive to atmospheric pollution by smoke or fumes.
These remarkable organisms are not really one plant but two — an association between an alga and a fungus; an association so successful that a new entity results, capable of surviving under conditions in which either partner alone would perish. Although lichens were studied and named by early botanists, their dual nature was not at first realised. When the Swiss botanist Schwendener in 1869 suggested that they consisted of an alga parasitised by a fungus, his contemporaries were scornful, regarding his theory as ‘purely imaginary, “the baseless fabric of a vision”,’ but subsequent studies proved Schwendener to be correct. Perhaps the dual nature of lichens is the main reason they have been little studied — the mycologists leave them for the algologists and vice versa. Probably their extremely slow growth rate and the difficulty of cultivating them has further discouraged botanists. However, it seems that lichens are slowly emerging from their obscurity. Considerable work has been done in the last fifty years on taxonomy of lichens, which opens up the field for ecological and physiological studies.
The algae present in lichens are all forms that are found free-living in more favourable habitats. They are either green algae (Chlorophyceae), or blue-green algae (Cyanophyceae). They seem little changed by their association with a fungus to form a lichen thallus, except that all reproduction is suppressed except simple division of the cells.page 42
The fungi present in lichens, however, are never found free-living, though when separated experimentally from their algal partners they are capable of limited growth in culture. The vast majority of lichen fungi are Ascomycetes, but a few tropical genera are Basidiomycetes.
Morphologically, lichens are of three main types: crustose, foliose and fruticose. Crustose lichens are those that grow flat on the substratum, forming a ‘crust,’ or may even be partially embedded in it. They are very firmly attached to the substrate and can only be removed in small fragments. Foliose lichens, as the name implies, are leaf-like, attached to the substratum either by a single umbilicus, or loosely attached at many points by rhizines. Fruticose lichens (L. frutex = bush or shrub) are more or less shrub-like, erect or pendulous, usually firmly attached to the substrate by a hold-fast. The substrate may be almost any fixed surface — rock, wood, bark, soil, bone, even leather or glass. Lichens are so slow-growing (often as little as 1 mm. per year or less) that they will not grow on a changing surface, such as an unconsolidated earth bank, or on stones on screes or in river beds.
Lichens vary in the complexity of their organisation. The simplest crustose types are relatively undifferentiated associations of fungal hyphae with algal cells. The hyphae may actually penetrate the algal cells, but more commonly are merely closely wrapped around them. Foliose and fruticose lichens show considerable differentiation of the thallus, the algal cells typically being in a restricted layer within the weft of fungal hyphae.
In a typical foliose lichen the uppermost layer is the upper cortex, composed of hyphae, the outermost threads often compacted to form a pseudo-epidermis. Below this is the algal layer, in which the algal cells lie in a fairly loose weft of hyphae. The name gonidia applied to the algal cells is a relic of the days when the dual nature of lichens was not realised, and the algal cells were believed to be some type of reproductive cells. Below the algal layer is the medulla, composed of very loosely woven hyphae, and a firmer hyphal layer, the lower cortex, from which extend the rhizines which serve to attach the lichen to the substrate. On the undersurface are sometimes found circular depressions, cyphellae, or breaks in the lower cortex, pseudocyphellae, commonly interpreted as aerating organs.
Fruticose lichens also show differentiation of the thallus into layers — an outer cortex, an algal layer, usually a medulla, and sometimes a central chondroid cylinder of strong parallel hyphae. The large genus Cladonia has a primary thallus, page 43 usually of small thalline scales or squamules, from which arise erect branches of the secondary thallus, termed podetia, each commonly expanding into a trumpet-shaped scyphus.
On the surface of some lichens are found small nodular structures, cephalodia, which contain an alga (usually blue-green) different from that present in the main thallus.
Vegetative reproduction is undoubtedly the most important method of propagation in lichens. Any portion of the thallus that breaks off can potentially become established as a new thallus, provided the fragment contains both the alga and the fungus. Many lichens become very brittle when dry and are easily broken. Some have small coral-like outgrowths, isidia, which are particularly prone to break off. Specialised structures for vegetative reproduction are the soredia, minute outgrowths of the thallus consisting of one to several algal cells, surrounded by a few hyphal threads. These may be produced singly, or in restricted areas, soralia. Frequently the soredia are so abundant that they give a powdery appearance to the surface of the thallus, from where they are readily blown by the wind.
Some lichens are claimed to produce asexual spores of the fungus, usually in pycnidia, but their importance in reproduction is not known. Similar structures have been interpreted as male organs, spermatia borne in spermagonia.
Sexual reproduction of the fungal component commonly occurs, but has never been recorded in the algal component. In the Ascolichens, asci and ascospores are borne either in flask-shaped perithecia, or in convex or saucer-shaped apothecia. The apothecia may be sessile on the thallus or raised on a stalk. If the edge of the apothecium contains algal elements as well as fungal, it is termed a thalline margin, if it is wholly fungal it is a proper margin. The asci develop among a thick palisade of paraphyses which extend above the asci and whose tips are frequently joined to form a protective layer, the epithecium. Within each ascus, usually eight ascospores develop. These may be single-celled but are frequently multicellular. A fruiting body of a lichen usually persists several years, with asci maturing in succession, so at any time there are likely to be present asci and ascospores in all stages of development. The ascospores may be forcibly ejected, or may accumulate in a powdery mass or mazedium.
The importance of sexual reproduction in nature is not known. In experimental work, the synthesis of lichens has been attempted, using pure cultures of the algal and fungal components, the latter usually derived from ascospores. Early stages have been observed, where hyphae become wrapped around the algal cells, but the growth of a proper lichen thallus page 44 has never been achieved. In nature it is conceivable that the ascopores released from the lichen fruiting body may germinate and come in contact with a free-living alga that it can parasitise and a new thallus thus be initiated.
The Basidiolichens (not known in New Zealand) produce basidia and basidiospores on the undersurface.
The physiology of lichens has been barely studied. There has been much debate on the nature of the association between alga and fungus, for it is a most delicately balanced physiological equilibrium. It has been claimed that it is a parasitic relationship, the fungus parasitising the alga, but as it is evident that the alga derives some benefit from the association, it would seem more accurate to describe the relationship as symbiotic. Apparently the alga supplies carbohydrates, the products of photosynthesis, to the fungus, and may also contribute other substances to the association, such as proteins and growth substances. The fungus' contribution to the association is largely protective. The hyphal cover protects the algal cells against rapid dessication and excessive radiation in direct sunlight. The fungal hyphae penetrating the substrate absorb nutrients which may benefit the alga, and serve to anchor the lichen securely.
The lichens are noted for the production of strange substances, found nowhere else in the plant kingdom, known as lichenic acids. These complex insoluble substances may make up almost a quarter of the dry weight of the thallus. Their significance is not known, they may be merely excretory substances, though some have antibiotic properties.
- (K) potassium hydroxide — 25-50% aqueous solution.
- (C) chloride of lime — saturated aqueous solution.
- (I) iodine solution — 2% aqueous solution KI plus sufficient iodine to give a deep yellow colour.
- (P) p-phenylene-diamine — fresh saturated alcoholic solution.
The reagents are commonly represented as K, C, I, P, followed by (+) or (−) indicating whether a colour change does or does not occur when a drop is placed on the thallus.
The significance of lichens lies mainly in their role as pioneer colonisers of rock surfaces. These pioneers are mainly crustose lichens, which, once established in a minute crack in the rock surface, and supplied with moisture, can gradually disintegrate the hardest rocks. The lichen can obtain water from the atmosphere and nutrients from dust falling on the thallus, so does not depend entirely on the substrate. A lichen thallus page 45 may be richer in some elements, particularly potassium and phosphorus, than the rock on which it grows. Below such a lichen an ‘organo-mineral dust horizon’ gradually develops, which is the first step towards soil formation. In this shallow layer of decomposed rock and organic matter mosses may become established and constitute the next phase in the succession.
Lichens have a few minor commercial uses, but their extremely slow growth and the problems of supply makes their commercial exploitation difficult. The common pH indicator, litmus, is obtained from species of Roccella, the main supplies coming from Madagascar. Lichens have traditionally been used for dyeing cloth, but commercially have been completely superseded by synthetic dyes. Lichen extracts have also been used as perfume stabilisers. The antibiotic properties of lichen substances are of interest, inhibiting such pathogens as the organisms of tuberculosis, tinea and ringworm.
Some are used in traditional ‘folk-medicines’ but none are in clinical use, as other antibiotics are equally effective and more readily available. In northern regions lichens are important as food for reindeer and caribou and have in times of famine been used as human food.
It has been claimed that lichens may aid in dating the exposure of rocks in glacial moraines or in rock slides. If the rate of growth of a lichen is known, by measuring the largest plants on such rocks, an assessment may be made of the time since those rocks were first exposed. ‘Lichenometry’ may also be useful in assessing the rate of retreat of the snowline up mountains, and in Europe is believed to be relatively accurate up to about 1,000 years.
This article is a brief introduction to a group of plants that is little understood but which offers a fascinating field of study. The nature of the strange association of two unrelated organisms that results in the formation of a new entity of regular form and function, poses many intriguing problems in physiology. The success of lichens as pioneer colonisers of bare rock and other inhospitable surfaces gives them considerable ecological importance.
Brightman. F. H. 1959. Neglected plants-lichens. New Biology No. 29.
Lamb, I. M. 1959. Lichens. Scientific American 201:4:144-156.
Quispel, A. 1959. Lichens. Encyclopaedia of Plant Physiology XI. 577-604. Springer-Verlag, Berlin.
Smith. G. M. 1955. Cryptogamic Botany I. 516-526. McGraw Hill, N.Y.