III. LICHEN ALGAE
A. Types of Algae
The algal constituents of the lichen thallus belong to the two classes, Myxophyceae, generally termed blue-green algae, and Chlorophyceae which are coloured bright-green or yellow-green. Most of them are land forms, and, in a free condition, they inhabit moist or shady situations, tree-trunks, walls, etc. They multiply by division or by sporulation within the thallus; zoospores are never formed except in open cultivation. The determination of the genera and species to which the lichen algae severally belong is often uncertain, but their distribution within the lichen kingdom is as follows:
a. Myxophyceae associated with Phycolichens. The blue-green algae are characterized by the colour of their pigments which persists in the gonidial condition giving various tints to the component lichens, and by the gelatinous sheath in which most of them are enclosed. This sheath, both in the lichen gonidia[277] and in free-living forms, imbibes and retains moisture to a remarkable extent and the thallus containing blue-green algae profits by its power of storing moisture. Myxophyceae form the gonidia of the gelatinous lichens as well as of some other non-gelatinous genera. Several families are represented[278]:
Fam. Chroococcaceae. This family includes unicellular algae with thick gelatinous sheaths. They increase normally by division, and colonies arise by the cohesion of the cells. Several genera form gonidia:
1. Chroococcus Naeg. Solitary or forming small colonies of 2-4-8 cells ([Fig. 16]) generally surrounded by firm gelatinous colourless sheaths in definite layers (lamellate). Chroococcus is considered by some lichenologists to form the gonidium of Cora, a genus of Hymenolichens.
2. Microcystis Kütz. Globose or subglobose cells forming large colonies surrounded by a common gelatinous layer (gonidia of Coriscium).
Fig. 16. Examples of Chroococcus. A, Ch. giganteus West; B, Ch. turgidus Naeg.; C and D, Ch. schizodermaticus West × 450 (after West).
Fig. 17. Gloeocapsa magma Kütz. × 450 (after West).
3. Gloeocapsa Kütz. (including Xanthocapsa). Globose cells with a lamellate gelatinous wall, forming colonies enclosed in a common sheath ([Fig. 17]); the inner integument is often coloured red or orange. These two genera form the gonidia in the family Pyrenopsidaceae. Gloeocapsa polydermatica Kütz. has been identified as a lichen gonidium.
Fam. nostocaceae. Filamentous algae unbranched and without base or apex.
Nostoc Vauch. Composed of flexuous trichomes, with intercalary heterocysts (colourless cells) ([Fig. 18]). Dense gelatinous colonies of definite form are built up by cohesion. In some lichens the trichomes retain their chain-like appearance, in others they are more or less broken up and massed together, with disappearance of the gelatinous sheath (as in Peltigera); colour mostly dark blue-green.
Fig. 18. Examples of Nostoc. N. Linckia Born. A, nat. size; B, small portion × 340; C, N. coerulescens Lyngbye, nat. size (after West).
Fig. 19. Example of Scytonema alga. S. mirabile Thur. C, apex of a branch; D, organ of attachment at base of filament. × 440 (after West).
Nostoc occurs in a few or all of the genera of Pyrenidiaceae, Collemaceae, Pannariaceae, Peltigeraceae and Stictaceae, and N. sphaericum Vauch. (N. lichenoides Kütz.) has been determined as the lichen gonidium. When the chains are broken up it has been wrongly classified as another alga, Polycoccus punctiformis.
Fam. Scytonemaceae. Trichomes of single-cell rows, differentiated into base and apex. Pseudo-branching arises at right angles to the main filament.
Scytonema Ag. Pseudo-branches piercing the sheath and passing out as twin filaments ([Fig. 19]); colour, golden-brown. This alga occurs in genera of Pyrenidiaceae, Ephebaceae, Pannariaceae, Heppiaceae, in Petractis a genus of Gyalectaceae, and in Dictyonema one of the Hymenolichens.
Fam. Stigonemaceae. Trichomes of several-cell rows with base and apex; colour, golden-brown.
Stigonema Ag. Stouter than Scytonema, with transverse and vertical division of the cells, and generally copious branching ([Fig. 20]). This alga occurs only in a few genera of Ephebaceae. S. panniforme Kirchn. (Sirosiphon pulvinatus Bréb.) has been determined as forming the gonidium.
Fam. Rivulariaceae. Trichomes with a heterocyst at the base and tapering upwards, enclosed in mucilage ([Fig. 21]).
Fig. 20. Stigonema sp. × 200 (after Comère).
Fig. 21. Examples of Rivularia; A, B, C, R. Biasolettiana Menegh.; D and E, R. minutula Born. and Fl. A and D nat. size; B, C and E × 480 (after West).
Rivularia Thuret. In tufts fixed at the base and forming roundish gelatinous colonies; colour, blue-green. The gonidium of Lichinaceae has been identified as R. nitida Ag.
Algae belonging to one or other of these genera of Myxophyceae also combine with the hyphae of Archilichens to form cephalodia[279] and Krempelhuber[280] has recorded and figured a blue-green alga, probably Gloeocapsa, in Baeomyces paeminosus from the South Sea Islands. They also form the gonidia in a few species and genera of such families as Stictaceae and Peltigeraceae.
b. Chlorophyceae associated With Archilichens. The lichens of this group are by far the most numerous both in genera and species, though fewer algal families are represented.
Fam. Protococcaceae. Consisting of globular single cells, aggregated in loose colonies, dividing variously.
Fig. 22. Pleurococcus vulgaris Menegh. (Protococcus viridis Ag.). chl. chloroplast; p. protoderma stage; pa, palmelloid stage; py, pyrenoid. × 520 (after West).
1. Protococcus viridis Ag. (Pleurococcus vulgaris Menegh., Cystococcushumicola Naeg.). Cells dividing into 2, 4 or 8 daughter-cells and not separating readily; in excessive moisture forming short filaments. The cells contain parietal chloroplasts, and, according to Chodat[281], are without a pyrenoid ([Fig. 22]). This alga, and allied species, forms the familiar green coating of tree-trunks, walls etc., and, in lichenological literature, are quoted as the gonidia of most of the crustaceous foliose and fruticose lichens. Chodat[281], who has recently made comparative artificial cultures of algae, throws doubt on the identity of many such gonidia. He lays great emphasis on the presence or absence of a pyrenoid in algal cells. West, on the contrary, considers the pyrenoid as an inconstant character. Chodat insists that the gonidia that contain pyrenoids belong to another genus, Cystococcus Chod. (non Naeg.), a pyrenoid-containing alga, which, in addition to multiplying by division of the cells, also forms spores and zoospores when cultivated. He further records the results of his cultures of gonidia, and finds that those taken from closely related lichens, such as different species of Cladonia, though they are alike morphologically, yet show constant variations in the culture colonies. These, he holds, are sufficient to indicate difference of race if not of species and he designates the algae, according to the lichen in which they occur, as Cystococcus Cladoniae pyxidatae, C. Cladoniae fimbriatae, etc.
Fig. 23. Cystococcus Cladoniae pyxidatae Chod. from culture × 800 (after Chodat).
Fig. 23 A. A, C, Chlorella vulgaris Beyer. B and C, stages in division × about 800 (after Chodat); E, Chl. faginea Wille × 520 (after Gerneck); F-I Chl. miniata; F, vegetable cell; G-I, formation and escape of gonidia × 1000 (after Chodat).
Meanwhile Paulson and Somerville Hastings[282] by their careful research on the growing thallus have thrown considerable light on the identity of the Protococcaceous lichen gonidium. They selected such well-known lichens as Xanthoria parietina, Cladonia spp. and others, which they collected during the spring months, February to April, the period of most active growth. Many of the gonidia, they found, were in a stage of reproduction, that showed a simultaneous rounding off of the gonidium contents into globose bodies varying in number up to 32. Chodat had figured this method of “sporulation” in his cultures of the lichen gonidium both in Chlorella Beij. and in Cystococcus Chod. ([Fig. 23]). It has now been abundantly proved that this form of increase is of frequent occurrence in the thallus itself. Chlorella has been suggested as probably the alga forming these gonidia and recently West has signified his acquiescence in this view[283].
2. Chlorella Beij. Occurring frequently on damp ground, bark of trees, etc., dividing into numerous daughter-cells, probably reduced zoogonidia ([Fig. 23]).
Chodat distinguishes between Cystococcus and Chlorella in that Cystococcus may form zoospores (though rarely), Chlorella only aplanospores. He found three gonidial species, Chlorella lichina in Cladonia rangiferina, Ch. viscosa and Ch. Cladoniae in other Cladonia spp.
3. Coccobotrys Chod. The cells of this new algal genus are smaller than those of Cystococcus or Protococcus and have no pyrenoid. They were isolated by Chodat from the thallus of Verrucaria nigrescens ([Fig. 24]), and, as they have thick membranes, they adhere in a continuous layer or thallus. Chodat also claims to have isolated a species of Coccobotrys from Dermatocarpon miniatum, a foliose Pyrenolichen.
4. Coccomyxa Schmidle. Cells ellipsoid, also without a pyrenoid. Two species were obtained by Chodat from the thallus of Solorinae and are recorded as Coccomyxa Solorinae croceae and C. Solorinae saccatae.
Coccomyxa subellipsoidea is given[284] as the gonidium of the primitive lichen Botrydina vulgaris ([Fig. 25]). The cells are surrounded by a common gelatinous sheath.
Fig. 24. Coccobotrys Verrucariae Chod. from culture × 800 (after Chodat).
Fig. 25. Coccomyxa subellipsoidea Acton. Actively dividing cells, the dark portions indicating the chloroplasts × 1000 (after Acton).
5. Diplosphaera Bial.[285] D. Chodati was taken from the thallus of Lecanora tartarea and successfully cultivated. It resembles Protococcus, but has smaller cells and grows more rapidly; it is evidently closely allied to that genus, if not merely a form of it.
6. Urococcus Kütz. Cells more or less globose, rather large, and coloured with a red-brown pigment, with the cell-wall thick and lamellate, forming elongate strands of cells ([Fig. 26]). Recorded by Hue[286] in the cephalodium of Lepolichen coccophorus, a Chilian lichen.
Fam. Tetrasporaceae. Cells in groups of 2 or 4 surrounded by a gelatinous sheath.
1. Palmella Lyngb. Cells globose, oblong or ellipsoid, grouped without order in a formless mucilage ([Fig. 27]). Among lichens associated with Palmella are the Epigloeaceae and Chrysothricaceae.
Fig. 26. Urococcus sp. Group of cells much magnified (after Hassall).
Fig. 27. Palmella sp. × 400 (after Comère).
Fig. 28. Gloeocystis sp. × 400 (after Comère).
2. Gloeocystis Naeg. Cells oblong or globose with a lamellate sheath forming small colonies; colour, red-brown ([Fig. 28]). This alga along with Urococcus was found by Hue in the cephalodia of Lepolichen coccophora, but whereas Gloeocystis frequently occupies the cephalodium alone, Urococcus is always accompanied by Scytonema, the normal gonidium of the cephalodium.
Fig. 29. A, Trentepohlia umbrina Born.; B, T. aurea Mart. × 300 (after Kütz.).
Fig. 30. Example of Cladophora. Cl. glomerata Kütz. A, nat. size; B, × 85 (after West).
Fam. Trentepohliaceae. Filamentous and branched, the filaments short and creeping or long and forming tufts and felts or cushions; colour, brownish-yellow or reddish-orange.
Trentepohlia Born. Branching alternate; cells filled with red or orange oil; no pyrenoids ([Fig. 29]). A large number of lichens are associated with this genus: Pyrenulaceae, Arthoniaceae, Graphidaceae, Roccellaceae, Thelotremaceae, Gyalectaceae and Coenogoniaceae, etc., in whole or in part. Two species have been determined, T. umbrina Born., the gonidium of the Graphidaceae, and T. aurea which is associated with the only European Coenogonium, C. ebeneum ([Fig. 3]). Deckenbach[287] claimed that he had proved by cultures that T. umbrina was a growth stage of T. aurea.
Fam. Cladophoraceae. Filamentous, variously and copiously branched, the cells rather large and multinucleate.
Cladophora Kütz. Filaments branching, of one-cell rows, attached at the base; colour, bright or dark green; mostly aquatic and marine ([Fig. 30]). Only one lichen, Racodium rupestre, a member of the Coenogoniaceae, is associated with Cladophora. It is a British lichen, and is always sterile.
Fam. Mycoideaceae. Epiphytic algae consisting of thin discs which are composed of radiating filaments.
1. Mycoidea Cunningh. (Cephaleuros Kunze). In Mycoidea parasitica the filaments of the disc are partly erect and partly decumbent, reddish to green ([Fig. 31]). It forms the gonidium of the parasitic lichen, Strigula complanata, which was studied by Marshall Ward in Ceylon[288]. Zahlbruckner gives Phyllactidium as an alternative gonidium of Strigulaceae.
2. Phycopeltis Millard. Disc a stratum one-cell thick, bearing seta, adnate to the lower surface of the leaf, yellow-green in colour. Phycopeltis ([Fig. 32]) has been identified as the gonidium of Strigula complanata in New Zealand and of Mazosia (Chiodectonaceae), a leaf lichen from tropical America.
Fig. 31. Mycoidea parasitica Cunningh. much magnified (after Marshall Ward).
There is some confusion as to the genera of algae that form the gonidia of these epiphyllous lichens. Phyllactidium given by Zahlbruckner as the gonidium of all the Strigulaceae (except Strigula in part) is classified by de Toni[289] as probably synonymous with Phycopeltis Millard, and as differing from Mycoidea parasitica in the mode of growth.
Fam. Prasiolaceae. Thallus filamentous, often expanded into broad sheets by the fusion of the filaments in one plane.
Prasiola Ag. Thallus filamentous, of one-to many-cell rows, or widely expanded ([Fig. 33]). The gonidium of Mastoidiaceae (Pyrenocarpeae).
Fig. 32. Phycopeltis expansa Jenn. much magnified (after Vaughan Jennings).
Fig. 33. Prasiola parietina Wille × 500 (after West).
B. Changes induced in the Alga
a. Myxophyceae. Though, as a general rule, the alga is less affected by its altered life-conditions than the fungus, yet in many instances it becomes considerably modified in appearance. In species of the genus Pyrenopsis—small gelatinous lichens—the alga is a Gloeocapsa very similar to G. magma. In the open it forms small colonies of blue-green cells surrounded by a gelatinous sheath which is coloured red with gloeocapsin. As a gonidium lying towards or on the outside of the granules composing the thallus, the red sheath of the cells is practically unchanged, so that the resemblance to Gloeocapsa is unmistakable. In the inner parts of the thallus, the colonies are somewhat broken up by the hyphae and the sheaths are not only less evident but much more faintly coloured. In Synalissa, a minute shrubby lichen which has the same algal constituent, the tissue of the thallus is more highly evolved, and in it the red colour can barely be seen and then only towards the outside; at the centre it disappears entirely. The long chaplets of Nostoc cells persist almost unchanged in the thallus of the Collemaceae, but in heteromerous genera such as Pannaria and Peltigera they are broken up, or they are coiled together and packed into restricted areas or zones. The altered alga has been frequently described as Polycoccus punctiformis. A similar modification occurs in many cephalodia, so that the true affinity of the alga, in most instances, can only be ascertained after free cultivation.
Bornet[290] has described in Coccocarpia molybdaea the change that the alga Scytonema undergoes as the thallus develops: in very young fronds the filaments of Scytonema are unchanged and are merely enclosed between layers of hyphae. At a later stage, with increase of the thallus in thickness, the algal filaments are broken up, their covering sheath disappears, and the cells become rounded and isolated. Petractis (Gyalecta) exanthematica has also a Scytonema as gonidium, and equally exact observations have been made by Fünfstück[291] on the way it is transformed by symbiosis: with the exception of a very thin superficial layer, the thallus is immersed in the rock and is permeated by the alga to its lowest limits, 3 to 4 mm. below the surface, Petractis being a homoiomerous lichen. The Scytonema trichomes embedded in the rock become narrower, and the sheath, which in the epilithic part of the thallus is 4µ wide, disappears almost entirely. The green colour of the cells fades and septation is less frequent and less regular. The filaments in that condition are very like oil-hyphae and can only be distinguished as algal by staining reagents such as alkanna. They never seem to be in contact with the fungal elements: there is no visible appearance of parasitism nor even of consortism.
b. Chlorophyceae. As a rule the green-celled gonidium such as Protococcus is not changed in form though the colour may be less vivid, but in certain lichens there do occur modifications in its appearance. In Micarea (Biatorina) prasina, Hedlund[292] noted that the gonidium was a minute alga possessing a gelatinous sheath similar to that of a Gloeocapsa. He isolated the alga, made artificial cultures and found that, in the altered conditions, it gradually increased in size, threw off the gelatinous sheath and developed into normal Protococcus cells, measuring 7 to 10µ in diameter. The gelatinous sheath was thus proved to be merely a biological variation, probably of value to the lichen owing to its capacity to imbibe and retain moisture. Zukal[293] also made cultures of this alga, but wrongly concluded it was a Gloeocystis.
Moebius[294] has described the transformation from algae to lichen gonidia in a species epiphytic on Orchids in Porto Rico. He had observed that most of the leaves were inhabited by a membranaceous alga, Phyllactidium, and that constantly associated with it were small scraps of a lichen thallus containing isolated globose gonidia. The cells of the alga, under the influence of the invading fungus, were, in this case, formed into isolated round bodies which divided into four, each daughter-cell becoming surrounded by a membrane and being capable, in turn, of further division.
Frank[295] followed the change from a free alga to a gonidium in Chroolepus (Trentepohlia) umbrinum, as shown in the hypophloeodal thalli of the Graphideae. The alga itself is frequent on beech bark, where it forms wide-spreading brownish-red incrustations consisting of short chains occasionally branched. The individual cells have thick laminated membranes and vary in width from 20 µ to 37 µ. The free alga constantly tends to penetrate below the cortical layers of the tree on which it grows, and the immersed cells become not only longer and of a thinner texture, but the characteristic red colour so entirely disappears, that the growing penetrating apical cell may be light green or almost colourless. As a lichen gonidium the alga undergoes even more drastic changes: the red oily granules gradually vanish and the cells become chlorophyll-green or, if any retain a bright colour, they are orange or yellow. The branching of the chains is more regular, the cells more elongate and narrower; usually they are about 13 to 21 µ long and 8 µ wide, or even less. Deeper down in the periderm, the chains become disintegrated into separate units. Another notable alteration takes place in the cell-membrane which becomes thin and delicate. It has, however, been observed that if these algal cells reach the surface, owing to peeling of the bark, etc., they resume the appearance of a normal Trentepohlia.
In certain cases where two kinds of algae were supposed to be present in some lichens, it has been proved that one species only is represented, the difference in their form being caused by mechanical pressure of the surrounding hyphae, as in Endocarpon and Staurothele where the hymenial gonidia are cylindrical in form and much smaller than those of the thallus. They were on this account classified by Stahl[296] under a separate algal genus, Stichococcus, but they are now known to be growth forms of Protococcus, the alga that is normally present in the thallus. Similar variations were found by Neubner[297] in the gonidia of the Caliciaceae, but, by culture experiments with the gonidia apart from the hyphae, he succeeded in demonstrating transition forms in all stages between the “Pleurococcus” cells and those of “Stichococcus,” though the characters acquired by the latter are transmitted to following generations. The transformation from spherical to cylindrical algal cells had been also noted by Krabbe[298] in the young podetia of some species of Cladonia, the change in form being due to the continued pressure in one direction of the parallel hyphae.
Isolated algal cells have been observed within the cortex of various lichens. They are carried thither by the hyphae from the gonidial zone in the process of cortical formation, but they soon die off as in that position they are deprived of a sufficiency of air and of moisture. Forssell[299] found Xanthocapsa cells embedded in the hymenium of Omphalaria Heppii. They were similar to those of the thallus, but they were not associated with hyphae and had undergone less change than the thalline algae.
C. Constancy of Algal Constituents
Lichen hyphae of one family or genus, as a rule, combine with the same species of alga, and the continuity of genera and species is maintained. There are, however, related lichens that differ chiefly or only in the characters of the gonidia. Among such closely allied genera or sections of genera may be cited Sticta with bright-green algae and the section Stictina with blue-green; Peltidea similarly related to Peltigera and Nephroma to Nephromium. In the genus Solorina, some of the species possess bright-green, others blue-green algae, while in one, S. crocea[300], there is an upper layer of small bright-green gonidia that project in irregular pyramids into the upper cortex; while below these there stretches a more or less interrupted band of blue-green Nostoc cells. The two layers are usually separated by strands of hyphae, but occasionally they come into close contact, and the hyphal filaments pass from one zone to the other. In this genus cephalodia containing blue-green Nostoc are characteristic of all the “bright-green” species. Harmand[301] has recorded the presence of two different types of gonidia in Lecanora atra f. subgrumosa; one of them, the normal Protococcus alga of the species, the other, pale-blue-green cells of Nostoc affinity.
Forssell[302] states that in Lecanora (Psoroma) hypnorum, the normal bright-green gonidia of some of the squamules may be replaced by Nostoc. In that case they are regarded as cephalodia, though in structure they exactly resemble the squamules of Pannaria pezizoides, and Forssell considers that there is sufficient evidence of the identity of the hyphal constituent in these two lichens, the alga alone being different.
It may be that in Archilichens with a marked capacity to form a second symbiotic union with blue-green algae, a tendency to revert to a primitive condition is evident—a condition which has persisted wholly in Peltigera with its Nostoc zone, but is manifested only by cephalodia formation in the Peltidea section of the genus. In this connection, however, we must bear in mind Forssell’s view that it is the Archilichens that are the more primitive[303].
The alien blue-green algae with their gelatinous sheaths are adapted to the absorption and retention of moisture, and, in this way, they doubtless render important service to the lichens that harbour them in cephalodia.
D. Displacement of Algae within the Thallus
a. Normal displacement. Lindau[304] has contrasted the advancing apical growth of the creeping alga Trentepohlia with the stationary condition of the unicellular species that multiply by repeated division or by sporulation, and thus form more or less dense zones and groups of gonidia in most lichens. The fungus in the latter case pushes its way among the algae and breaks up the compact masses by a shoving movement, thus letting in light and air. The growing hypha usually applies itself closely round an algal cell, and secondary branches arise which in time encircle it in a network of short cells. In the thallus of Variolaria[305] the hyphae from the lower tissues, termed push-hyphae by Nienburg[306], push their way into the algal groups and filaments composed of short cells come to lie closely round the individual gonidia. Continued growth is centrifugal, and the algae are carried outward with the extension of the hyphae ([Fig. 12]). Cell-division is more active at the periphery, that being the area of vigorous growth, and the algal cells are, in consequence, generally smaller in that region than those further back, the latter having entered more or less into a resting condition, or, as is more probable, these smaller cells are aplanospores not fully mature.
b. Local displacement. Specimens of Parmelia physodes were found several times by Bitter, the grey-green surface of which was marbled with whitish lines, caused by the absence of gonidia under these lighter-coloured areas. The thallus was otherwise healthy as was manifested by the freely fruiting condition: no explanation of the phenomenon was forthcoming. Bitter compared the condition with the appearance of lighter areas on the thallus of Parmelia obscurata.
Something of the same nature was observed on the thallus of a Peltigera collected by F. T. Brooks near Cambridge. The marking took the form of a series of concentric circles, starting from several centres. The darker lines were found on examination to contain the normal blue-green algal zone, while the colour had faded from the lighter parts. The cause of the difference in colouration was not apparent.
E. Non-gonidial Organisms associated with Lichen Hyphae
Bonnier[307] made a series of cultures with lichen spores and green cells other than those that form lichen gonidia. In one instance he substituted Protococcus botryoides for the normal gonidia of Parmelia (Xanthoria) parietina; in another of his cultures he replaced Protococcus viridis by the filamentous alga Trentepohlia abietina. In both cases the hyphae attached themselves to the green cells and a certain stage of thallus formation was reached, though growth ceased fairly early. Another experiment made with the large filaments of Vaucheria sessilis met with the same amount of success ([Fig. 34]). The germinating hyphae attached themselves to the alga and grew all round it, but there was no advance to tissue formation.
Cultures were also made with the protonema of mosses. Either spores of mosses and lichens were germinated together, or lichen spores were sown in close proximity to fully formed protonemata. The developing hyphae seized on the moss cells and formed a network of branching anastomosing filaments along the whole length of the protonema without, however, penetrating the cells. If suitable algae were encountered, proper thallus formation commenced, and Bonnier considers that the hyphae receive stimulus and nourishment from the protonema sufficient to tide them over a considerable period, perhaps until the algal symbiont is met. An interesting variation was noted in connection with the cultures of Mnium hornum[308]. If the protonema were of the usual vigorous type, the whole length was encased by the hyphal network; but if it were delicate and slender, the protoplasm collected in the cell that was touched by hyphae and formed a sort of swollen thick-walled bud ([Fig. 35]). This new body persisted when the rest of the filament and the hyphae had disappeared, and, in favourable conditions, grew again to form a moss plant.
Fig. 34. Germinating hyphae of Lecanora subfusca Ach., growing over the alga Vaucheria sessilis DC., much magnified (after Bonnier).
F. Parasitism of Algae on Lichens
A curious instance of undoubted parasitism by an alga, not as in Strigula on one of the higher plants, but on a lichen thallus, is recorded by Forssell[309]. A group of Protococcus-like cells established on the thallus of Peltigera had found their way into the tissue, the underlying cortical cells having degenerated. The blue-green cells of the normal gonidial layer had died off before their advance but no zone was formed by the invading algae; they simply withdrew nourishment and gave seemingly no return. The phenomenon is somewhat isolated and accidental but illustrates the capacity of the alga to absorb food supply from lichen hyphae.
Fig. 35. Pure culture of protonema of Mnium hornum L. with spores and hyphae of Lecidea vernalis Ach. a,a,a, buds forming × 150 (after Bonnier).
An instance of epiphytic growth has also been recorded by Zahlbruckner[310]. He found an alga, Trentepohlia abietina, covering the thallus of a Brazilian lichen, Parmelia isidiophora, and growing so profusely as to obscure the isidiose character towards the centre of the thallus. There was no genetic connection of the alga with the lichen as the former was not that of the lichen gonidium. Lichen thalli are indeed very frequently the habitat of green algae, though their occurrence may be and probably is accidental.