FIELD CLUB RAMBLES.

Otago Witness, Issue 3250, 28 June 1916, Page 69

FIELD CLUB RAMBLES.

By C. M. Thomson, F.L.S,

During the winter months, when both animal and vegetabe life are more or less quiescent, the botanist who takes the whole field of plant life for bis study can turn with much advantage to the cryptogam! c floca—that is, to the flowering plants—and find leisure for an examination of their .structure. Ferns and clnb. mosses can be studied without the aid of a microscope, and many of the lower orders can be identified, as it were, by head-mark ; but the details of structure require considerable magnijieation to make then; out, and even the best pocket-lens is not good enough for this. I referred hut week to the symbiotic relationship existing between nmny floweriug plants and certain subterranean fungi, as shown in the fibrous heaps on red pine roots and elsewhere. This week I wish to refer to another interesting case of symbiosis—that, namely, of the algra and fungi, which, grow together and produce the plant associations known as lichens. (As to the pronunciation of this word most people, I think, call them likens: some pronounce them 1 itch ens: there is no definite rule governing ■ the sound of the word.) Fifty years ago lichens were considered to Lc a definite family of plains, as distinctive as the algae and fungi, but with affinities with both those groups; but the researches of Rchwendener, Bonnet, and succeeding investigators showed that they were really communities formed by groups or filaments of mounded green cells belonging to certain families of alg&j, in close association with the hyphro of certain fungi. The following experiment,' made in a mountain valley in the Tyrol, and which can bo readily demonstrated anywhere under similar conditions, shows how lichens arise. A sheet or thin mass of sheets of white filter-paper, which was •kept moist, was exposed to the southerly wind ; in tho course of a few horn's numerous dust-like particles were found to have adhered to the paper. Amongst them were groups of alg;e (nostoc and others), consisting either of single mi mite green cells or group- of cells, grains of pollen from various flowers, spores of all sorts of mosses and fungi, and organio fragments of various kinds. These were deposited on the depressions of the paper, just a-a tney would be caught on the damp surface of a rock, or the bark of a tree, or any piece, of old wood-work which is full of grooves, cavities, and cracks, and where they would develop in presence of the moisture vdVob would accumulate and remain in such places. As they grew and increased, wherever" the iryphaj of a fungus came in contact with these little groups of algse it encircled and ultimately enmeshed them, and each of these ultimately produced the con- J federaey called a lichen. This natural process has been artificially copied in I numerous cases. On a thin layer of some i nostoc-like alga the spores ' of certain species of fungus have been sown, and in each case a perfectly definite form of ' lichen is developed. There seems little doubt then that all lichens arc sue 1 ? symbiotic asfociations. The fungus is the dominant partner; its hyphre take up nutriment from the external environment-. They possess the property of condensing aqueous vapour and also harce the power of excreting substances which briny tho solid substratum on which they grow into solution. There is no question as to the latter kind of action, for where Mchous prow on limestone or marble thc v etch the surface, eating it awny by degrees till they make it roach or corrugated. Probably they excrete carbonic acid, nod so atta k and dissolve the carbonate of lime: so that what they ,do on limestone they do in varying degrees on all kin ' of rocks and stone.''. The hyphec nlso attach the lichens to the body on which

they grow, and in the majority of cases I hey determine the form and lolonr of the lieheu-tiiailus as a whole. On the other hand the alga, who-o cells contain chlorophyll, produces organic matter, under the influence of sunlight, from the carbon dioxide and other material: in the air and from water. By this mean: it multiplies the number oj it.- cells and increase in volume, while, at the same time, it fields to the associated fungus so much as is necessary to enable the latter to keep pace with it in growth. The number of kinds of alga' which enler into a partnership of this kind is much mure limited than that of the fungi, and il is probable that one species of alga may unite with the hypjue of different ichen ■ a.ga Tr-s v< ■i£l;y oi Is •:■ s wV-ici l may he found on a very limited area favour.! this view. On a piece oi wood or stone no bigger than one's hand it is sometimes possible to find half a doz.en different fungi, so that apparently one bed of nostoc may furnish the alga on which distinct fungus spores mav produce distinct lichen groups. J instance nostoc | because it is such a common form, occurring as purple, brown, or greenUh gelatinous colonies on damp rock-face. All lichen colonies do not owe their origin to fungus sports being blown on to or brought in contact with alga groups. In many cases already completed social colonies are carried by the wind, very considerable distance-. ' The thallus 'of a lichen is of; en ruptured, and the little miniatine social groups, which are termed ''soredia," come to the surface. A single soredium may be only visible on the surface of the thallus as a bright dot. but viewed ail together iey have the appearance of a mass of powder or mealy dust, and in dry weather this is easily' blown away. If it lights on a suitable substratum it quickly developes a lichen thallus. The alga? which enter into this symbolic relationship are limited to a few families, two of them (including nostoc) belonging to what may be termed the blue-green alga?, and three to a section of the green algae On the other hand, the envelop'ng fungi belong to a much greater range of forms, so that in old systematic arrangements the lichens appeared to pessess an extraordinary large number of spe ies differentiated into thousands of forrn-a Representatives of these are f< und everywhere from the seashore to the highest mountain peaks, and from the equator to the Arctic and Antarctic zones. In the technical classification of aTea) they are grouped into four families, and these are based on the characters of the fungal constituent. Anyone knowing the characters of the various families of fungi can easily follow the corresponding grouping of the lichens, but it is barely possible to give an unt.ochnic.il description of them. It is, therefore, simpler to give a popular classification based on their external form. In this way we can divide, them into five main croups, as follows: 1. The crustaceous lichens lie ensconced and embedded in the depressions of weathered surfaces of stone, or between the cell-walls of dead fragments of wood and bark, so that often their presence is revealed only by the colour of the substratum or by the fructifications which are produced. 2. The foliaceous lichens are more or less leaf-like in form, as their name implies. They usually have a number of lobes radiating irregularly and branching repeatedly, and they are very- loosely joined to the surface on which they growby root-like fringes. 3. The fruticose lichens have the thallus rising from the ground in the form of a shrub with numerous branches. _ 4. This fourth group includes the Beard lichens, represented lv such a form as Usriea barbata, the Old Man's Beard, which hangs in tufts and festoons from the branches of trees. T do not know whether our common species is the same as the European form : but it is very like it in externa] appearance, and produces the same mass of pale, copiously-branched filaments. 5. The gelatinous lichen- constitute the fifth j.:vnvv>. These when moistene ! look like dark olive-green or almost black ljimps of wrinkled and wavy je! y. or as if composed of variously-divided bands and strips packed together into little cushions. In these the alga] cells arc arranged in rows like strings of beads, and. are interwoven with the hyphal filaments of the fungus throughout the entire thickness of the thallus : or pise thev form regular ribbcn-.sh.Tped double rows," interwoven with a few hypha?. In the crustaceous. foliose, and fruticose lichens the alga] cells'constitute a. sort of disorderly heap, and are crowded together in the middle stratum of the thallus, where they j are embedded between an upper and a lower layer of deu-ely felted 'hyplue. Tito full study- of lichens to enable one j to correctly classify them necessitate the possession of a good microscope, and the ! power to use it; but the plants are easily i collected and noted, and a local collection j would be both valuable and interesting.