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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).