CHAPTER II

CONSTITUENTS OF THE LICHEN THALLUS

I. LICHEN GONIDIA

[Illustration: Fig. 1. _Physcia aipolia_ Nyl. Vertical section of

thallus. _a_, cortex; _b_, algal layer; _c_, medulla; _d_, lower cortex.

× 100 (partly diagrammatic).]

The thallus or vegetative body of lichens differs from that of other

green plants in the sharp distinction both of form and colour between

the assimilative cells and the colourless tissues, and in the relative

positions these occupy within the thallus: in the greater number of

lichen species the green chlorophyll cells are confined to a narrow zone

or band some way beneath and parallel with the surface (Fig. 1); in a

minority of genera they are distributed through the entire thallus (Fig.

2); but in all cases the tissues remain distinct. The green zone can be

easily demonstrated in any of the larger lichens by scaling off the outer

surface cells, or by making a vertical section through the thallus. The

colourless cells penetrate to some extent among the green cells; they

also form the whole of the cortical and medullary tissues.

[Illustration: Fig. 2. _Collema nigrescens_ Ach. Vertical section of

thallus. _a_, chains of the alga _Nostoc_; _b_, fungal filaments. × 600.]

These two different elements we now know to consist of two distinct

organisms, a fungus and an alga. The green algal cells were at one time

considered to be reproductive bodies, and were called “gonidia,” a term

still in use though its significance has changed.

1. GONIDIA IN RELATION TO THE THALLUS

A. HISTORICAL ACCOUNT OF LICHEN GONIDIA

There have been few subjects of botanical investigation that have roused

so much speculation and such prolonged controversy as the question

of these constituents of the lichen plant. The green cells and the

colourless filaments which together form the vegetative structure are so

markedly dissimilar, that constant attempts have been made to explain

the problem of their origin and function, and thereby to establish

satisfactorily the relationship of lichens to other members of the Plant

Kingdom.

In gelatinous lichens, represented by _Collema_, of which several species

are common in damp places and grow on trees or walls or on the ground,

the chains of green cells interspersed through the thallus have long been

recognized as comparable with the filaments of _Nostoc_, a blue-green

gelatinous alga, conspicuous in wet weather in the same localities as

those inhabited by _Collema_. So among early systematists, we find

Ventenat[152] classifying the few lichens with which he was acquainted

under algae and hazarding the statement that a gelatinous lichen such

as _Collema_ was only a _Nostoc_ changed in form. Some years later

Cassini[153] in an account of _Nostoc_ expressed a somewhat similar view,

though with a difference: he suggested that _Nostoc_ was but a monstrous

form of _Collema_, his argument being that, as the latter bore the fruit,

it was the normal and perfect condition of the plant. A few years later

Agardh[154] claimed to have observed the metamorphosis of _Nostoc_ up

to the fertile stage of a lichen, _Collema limosum_. But long before

this date, Scopoli[155] had demonstrated a green colouring substance

in non-gelatinous lichens by rubbing a crustaceous or leprose thallus

between the fingers; and Persoon[156] made use of this green colour

characteristic of lichen crusts to differentiate these plants from fungi.

Sprengel[157] went a step further in exactly describing the green tissue

as forming a definite layer below the upper cortex of foliaceous lichens.

The first clear description and delimitation of the different elements

composing the lichen thallus was, however, given by Wallroth[158]. He

drew attention to the great similarity between the colourless filaments

of the lichen and the hyphae of fungi. The green globose cells in the

chlorophyllaceous lichens he interpreted as brood-cells or gonidia,

regarding them as organs of reproduction collected into a “stratum

gonimon.” To the same author we owe the terms “homoiomerous” and

“heteromerous,” which he coined to describe the arrangement of these

green cells in the tissue of the thallus. In the former case the gonidia

are distributed equally through the structure; in the latter they are

confined to a distinct zone.

Wallroth’s terminology and his views of the function of the gonidia

were accepted as the true explanation for many years, the opinion that

they were solely reproductive bodies being entirely in accordance with

the well-known part played by soredia in the propagation of lichens—and

soredia always include one or more green cells.

B. GONIDIA CONTRASTED WITH ALGAE

In describing the gonidia of the Graphideae Wallroth[159] had pointed

out their affinity with the filaments of _Chroolepus_ (_Trentepohlia_)

_umbrina_. He considered these and other green algae when growing loose

on the trunks of trees to be but “unfortunate brood-cells” which had

become free and, though capable of growth and increase, were unable to

form again a lichen plant.

Further observations on gonidia were made by E. Fries[160]: he found

that the green cells escaped from the lichen matrix and produced new

individuals; and also that the whole thallus in moist localities might

become dissolved into the alga known as _Protococcus viridis_; but, he

continues, “though these _Protococcus_ cells multiplied exceedingly,

they never could rise again to the perfect lichen.” Kützing[161], in a

later account of _Protococcus viridis_, also recognized its affinity with

lichens; he stated that he could testify from observation that, according

to the amount of moisture present, it would develop, either in excessive

moisture to a filamentous alga, or in drier conditions “to lichens such

as _Lecanora subfusca_ or _Xanthoria parietina_.”

[Illustration: Fig. 3. _Coenogonium ebeneum_ A. L. Sm. Tip of lichen

filament, the alga overgrown by dark fungal hyphae × 600.]

A British botanist, G. H. K. Thwaites[162], at one time superintendent

of the botanical garden at Peradeniya in Ceylon, published a notable

paper on lichen gonidia in which he pointed out that as in _Collema_ the

green constituents of the thallus resembled the chains of _Nostoc_, so in

the non-gelatinous lichens, the green globose cells were comparable or

identical with _Pleurococcus_, and Thwaites further observed that they

increased by division within the lichen thallus. He insisted too that

in no instance were gonidia reproductive organs: they were essential

component parts of the vegetative body and necessary to the life of the

plant. In a further paper on _Chroolepus ebeneus_ Ag., a plant consisting

of slender dark-coloured felted filaments, he described these filaments

as being composed of a central strand which closely resembled the alga

_Chroolepus_, and of a surrounding sheath of dark-coloured cells (Fig.

3): “occasionally,” he writes, “the internal filament protrudes beyond

the investing sheath, and may then be seen to consist of oblong cells

containing the peculiar reddish oily-looking endochrome of _Chroolepus_.”

Thwaites placed this puzzling plant in a new genus, _Cystocoleus_, at the

same time pointing out its affinity with the lichen genus _Coenogonium_.

The plant is now known as _Coenogonium ebeneum_. Thwaites was on the

threshold of the discovery as to the true nature of the relationship

between the central filament and the investing sheath, but he failed to

take the next forward step.

Very shortly after, Von Flotow[163] published his views on some other

lichen gonidia. He had come to the conclusion that the various species

of the alga, _Gloeocapsa_, so frequently found in damp places, among

mosses and lichens, were merely growth stages of the gonidia of _Ephebe

pubescens_, and bore the same relation to _Ephebe_ as did _Lepra viridis_

(_Protococcus_) to _Parmelia_. The gonidium of _Ephebe_ is the gelatinous

filamentous blue-green alga _Stigonema_ (Fig. 4), and the separate cells

are not unlike those of _Gloeocapsa_. Flotow had also demonstrated

that the same type of gonidium was enclosed in the cephalodia of

_Stereocaulon_. Sachs[164], too, gave evidence as to the close connection

between _Nostoc_ and _Collema_. He had observed numerous small clumps of

the alga growing in proximity to equally abundant thalli of _Collema_,

with every stage of development represented from one to the other. He

found cases where the gelatinous coils of _Nostoc_ chains were penetrated

by fine colourless filaments “as if invaded by a parasitic fungus.” Later

these threads were seen to be attached to some cell of the _Nostoc_

trichome. Sachs concluded, however, from very careful examination at the

time, that the colourless filaments were produced by the green cells.

As growth proceeded, the coloured _Nostoc_ chains became massed towards

the upper surface, while the colourless filaments tended to occupy the

lower part of the thallus. He calculated that during the summer season

the metamorphosis from _Nostoc_ to a fertile _Collema_ thallus took from

three to four months. He judged that in favourable conditions the change

would inevitably take place, though if there should be too great moisture

no _Collema_ would be formed. His study of _Cladonia_ was less successful

as he mistook some colonies of _Gloeocapsa_ for a growth condition of

_Cladonia_ gonidia, an error corrected later by Itzigsohn[165].

[Illustration: Fig. 4. _Ephebe pubescens_ Nyl. Tip of lichen filament ×

600.]

But before this date Itzigsohn[166] had published a paper setting forth

his views on thallus formation, which marked a distinct advance. He did

not hazard any theory as to the origin of gonidia, but he had observed

spermatia growing, much as did the cells of _Oscillaria_: by increase

in length, and, by subsequent branching, filaments were formed which

surrounded the green cells; these latter had meanwhile multiplied by

repeated division till finally a complete thallus was built up, the

filamentous tissue being derived from the spermatia, while the green

layer came from the original gonidium. In contrasting the development

with that of _Collema_, he represents _Nostoc_ as a sterile product of a

lichen and, like the gonidia of other lichens, only able to form a lichen

thallus when it encounters the fructifying spermatia.

Braxton Hicks[167], a London doctor, some time later, made experiments

with _Chroococcus_ algae which grew in plenty on the bark of trees, and

followed their development into a lichen thallus. He further claimed to

have observed a _Chlorococcus_, which was associated with a _Cladonia_,

divide and form a _Palmella_ stage.

C. CULTURE EXPERIMENTS WITH THE LICHEN THALLUS

It had been repeatedly stated that the gonidia might become independent

of the thallus, but absolute proof was wanting until Speerschneider[168],

who had turned his attention to the subject, made direct culture

experiments and was able to follow the liberation of the green cells.

He took a thinnish section of the thallus of _Hagenia_ (_Physcia_)

_ciliaris_, and, by keeping it moist, he was able to observe that

the gonidial cells increased by division; the moist condition at the

same time caused the colourless filaments to die away. This method of

investigation was to lead to further results. It was resorted to by

Famintzin and Baranetzky[169] who made cultures of gonidia extracted

from three different lichens, _Physcia_ (_Xanthoria_) _parietina_,

_Evernia furfuracea_ and _Cladonia_ sp. They were able to observe the

growth and division of the green cells and, in addition, the formation

of zoospores. They recognized the development as entirely identical

with that of the unicellular green alga, _Cystococcus humicola_ Naeg.

Baranetzky[170] continued the experiments and made cultures of the

blue-green gonidia of _Peltigera canina_ and of _Collema pulposum_. In

both instances he succeeded in isolating them from the thallus and in

growing them in moist air as separate organisms. He adds that “many forms

reckoned as algae, may be considered as vegetating lichen gonidia such

as _Cystococcus_, _Polycoccus_, _Nostoc_, etc.” Meanwhile Itzigsohn[171]

had further demonstrated by similar culture experiments that the gonidia

of _Peltigera canina_ corresponded with the algae known as _Gloeocapsa

monococca_ Kütz., and as _Polycoccus punctiformis_ Kütz.

D. THEORIES AS TO THE ORIGIN OF GONIDIA

Though the relationship between the gonidia within the thallus and

free-living algal organisms seemed to be proved beyond dispute, the

manner in which gonidia first originated had not yet been discovered.

Bayrhoffer[172] attacked this problem in a study of foliose and other

lichens. According to his observations, certain colourless cells or

filaments, belonging to the “gonimic” layer, grew in a downward direction

and formed at their tips a faintly yellowish-green cell; it gradually

enlarged and was at length thrown off as a free globose gonidium, which

represented the female cell. Other filaments from the “lower fibrous

layer” of the thallus at the same time grew upwards and from them were

given off somewhat similar gonidia which functioned as male cells. His

observations and deductions were fanciful, but it must be remembered

that the attachment between hypha and alga in lichens is in many cases

so close as to appear genetic, and also it often happens that as the

gonidium multiplies it becomes free from the hypha.

In his _Mémoire sur les Lichens_, Tulasne[173] described the colourless

filaments as being fungal in appearance. The green cells he recognized

as organs of nutrition, and once and again in his paper he states that

they arose directly by a sort of budding process from the medullary or

cortical filaments, either laterally or at the apex. This apparently

reasonable view of their origin was confirmed by other writers on the

subject: by Speerschneider[174] in his account of the anatomy of _Usnea

barbata_, by de Bary[175], and by Schwendener[176] in their earlier

writings. But even while de Bary accepted the hyphal origin of the

gonidia, he noted[177] that, accompanying _Opegrapha atra_ and other

Graphideae, on the bark were to be found free _Chroolepus_ cells similar

to the gonidia in the lichen thallus. He added that gonidia of certain

other lichens in no way differed from _Protococcus_ cells; and as for

the gelatinous lichens he declared that “either they were the perfect

fruiting form of Nostocaceae and Chroococcaceae—hitherto looked on

as algae—or that these same Nostocaceae and Chroococcaceae are algae

which take the form of _Collema_, _Ephebe_, etc., when attacked by an

ascomycetous fungus.”

All these investigators, and other lichenologists such as Nylander[178],

still regarded the free-living organisms identified by them as similar to

the green cells of the thallus, as only lichen gonidia escaped from the

matrix and vegetating in an independent condition.

The old controversy has in recent years been unexpectedly reopened

by Elfving[179] who has sought again to prove the genetic origin of

the green cells. His method has been to examine a large series of

lichens by making sections of the growing areas, and he claims to have

observed in every case the hyphal origin of the gonidia: not only of

_Cystococcus_ but also of _Trentepohlia_, _Stigonema_ and _Nostoc_.

In the case of _Cystococcus_, the gonidium, he says, arises by the

swelling of the terminal cell of the hypha to a globose form, and by the

gradual transformation of the contents to a chlorophyll-green colour,

with power of assimilation. In the case of filamentous gonidia such

as _Trentepohlia_, the hyphal cells destined to become gonidia are

intercalary. In _Peltigera_ the cells of the meristematic plectenchyma

become transformed to blue-green _Nostoc_ cells.

A study was also made by him of the formation of cephalodia[180], the

gonidia of which differ from those of the “host” thallus. In _Peltigera

aphthosa_ he claims to have traced the development of these bodies to

the branching and mingling of the external hairs which, in the end, form

a ball of interwoven hyphae. The central cells of the ball are then

gradually differentiated into _Nostoc_ cells, which increase to form

the familiar chains. Elfving allows that the gonidia mainly increase by

division within the thallus, and that they also may escape and live as

free organisms. His views are unsupported by direct culture experiments

which are the real proof of the composite nature of the thallus.

E. MICROGONIDIA

Another attempt to establish a genetic origin for lichen gonidia was made

by Minks[181]. He had found in his examination of _Leptogium myochroum_

that the protoplasmic contents of the hyphae broke up into a regular

series of globular corpuscles which had a greenish appearance. These

minute bodies, called by him microgonidia, were, he states, at first few

in number, but gradually they increased and were eventually set free by

the mucilaginous degeneration of the cell wall. As free thalline gonidia,

they increased in size and rapidly multiplied by division. Minks was

at first enthusiastically supported by Müller[182] who had found from

his own observations that microgonidia might be present in any of the

lichen hyphae and in any part of the thallus, even in the germinating

tube of the lichen spore, and was in that case most easily seen when the

spores germinated within the ascus. He argued that as spores originated

within the ascus, so microgonidia were developed within the hyphae.

Minks’s theories were however not generally accepted and were at last

wholly discredited by Zukal[183] who was able to prove that the greenish

bodies were contracted portions of protoplasm in hyphae that suffered

from a lowered supply of moisture, the green colour not being due to any

colouring substance, but to light effect on the proteins—an outcome of

special conditions in the vegetative life of the plant. Darbishire[184]

criticized Minks’s whole work with great care and he has arrived at the

conclusion that the microgonidium may be dismissed as a totally mistaken

conception.

F. COMPOSITE NATURE OF THALLUS

Schwendener[185] meanwhile was engaged on his study of lichen anatomy.

Though at first he adhered to the then accepted view of the genetic

connection between hyphae and gonidia, his continued examination of the

vegetative development led him to publish a short paper[186] in which

he announced his opinion that the various blue-green and green gonidia

were really algae and that the complete lichen in all cases represented

a fungus living parasitically on an alga: in _Ephebe_, for example, the

alga was a form of _Stigonema_, in the Collemaceae it was a species of

_Nostoc_. In those lichens enclosing bright green cells, the gonidia were

identical with _Cystococcus humicola_, while in _Graphideae_ the brightly

coloured filamentous cells were those of _Chroolepus_ (_Trentepohlia_).

This statement he repeated in an appendix to the larger work on

lichens[187] and again in the following year[188] when he described more

fully the different gonidial algae and the changes produced in their

structure and habit by the action of the parasite: “though eventually the

alga is destroyed,” he writes, “it is at first excited to more vigorous

growth by contact with the fungus, and in the course of generations may

become changed beyond recognition both in size and form.” In support of

his theory of the composite constitution of the thallus, Schwendener

pointed out the wide distribution and frequent occurrence in nature

of the algae that become transformed to lichen gonidia. He claimed as

further proof of the presence of two distinct organisms that, while the

colourless filaments react in the same way as fungi on the application of

iodine, the gonidia take the stain of algal membranes.

G. SYNTHETIC CULTURES

Schwendener’s “dual hypothesis,” as it was termed, excited great interest

and no little controversy, the reasons for and against being debated

with considerable heat. Rees[189] was the first who attempted to put the

matter to the proof by making synthetic cultures. For this purpose he

took spores from the apothecium of a _Collema_ and sowed them on pure

cultures of _Nostoc_, and as a result obtained the formation of a lichen

thallus, though he did not succeed in producing any fructification. He

observed further that the hyphal filaments from the germinating spore

died off when no _Nostoc_ was forthcoming.

Bornet[190] followed with his record of successful cultures. He selected

for experiment the spores of _Physcia_ (_Xanthoria_) _parietina_ and was

able to show that hyphae produced from the germinating spore adhered

to the free-growing cells of _Protococcus[191] viridis_ and formed

the early stages of a lichen thallus. Woronin[192] contributed his

observations on the gonidia of _Parmelia_ (_Physcia_) _pulverulenta_

which he isolated from the thallus and cultivated in pure water. He

confirmed the occurrence of cell division in the gonidia and also the

formation of zoospores, these again forming new colonies of algae

identical in all respects with the thalline gonidia. He was able to see

the germinating tube from a lichen spore attach itself to a gonidium,

though he failed in his attempts to induce further growth. In our own

country Archer[193] welcomed the new views on lichens, and attempted

cultures but with very little success. Further synthetic cultures were

made by Bornet[194], Treub[195] and Borzi[196] with a series of lichen

spores. They also were able to observe the first stages of the thallus.

Borzi observed spores of _Physcia_ (_Xanthoria_) _parietina_ scattered

among _Protococcus_ cells on the branch of a tree. The spores had

germinated and the first branching hyphae had already begun to encircle

the algae.

[Illustration: Fig. 5. _Endocarpon pusillum_ Hedw. Asci and spores, with

hymenial gonidia × 320 (after Stahl).]

[Illustration: Fig. 6. _Endocarpon pusillum_ Hedw. Spore germinating in

contact with hymenial gonidia × 320 (after Stahl).]

Additional evidence in favour of the theory of the independent origin

of the colourless filaments and the green cells was furnished by

Stahl’s[197] research on hymenial gonidia in _Endocarpon_ (Fig. 5). By

making synthetic cultures of the mature spores with these bodies, he was

able to observe not only the germination of the spores and the attachment

of the filaments to the gonidia (Fig. 6), but also the gradual building

up of a complete lichen thallus to the formation of perithecia and spores.

[Illustration: Fig. 7. Germination of spore of _Physcia parietina_ De

Not. in contact with _Protococcus viridis_ Ag. × 950 (after Bornet).]

[Illustration: Fig. 8. _Physcia parietina_ De Not. Vertical section of

thallus obtained by synthetic culture × 130 (after Bonnier).]

Some years later Bonnier[198] made an interesting series of synthetic

cultures between the spores of lichens germinated in carefully sterilized

conditions, and algae taken from the open (Figs. 7 and 8). Separate

control cultures of spores and algae were carried on at the same time,

with the result that in one case lichen hyphae alone, in the other algae

were produced. The various lichen spores with which he experimented were

sown in association with the following algae:

(1) PROTOCOCCUS.

Pure synthetic cultures of _Physcia_ (_Xanthoria_) _parietina_ were begun

in August 1884 on fragments of bark. In October 1886 the thallus was

several centimetres in diameter, and some of the lobes were fruited.

_Physcia stellaris_ was also grown on bark; in one case both thallus and

apothecia were developed.

_Parmelia acetabulum_, another corticolous species, formed only a minute

thallus about 5 mm. in diameter, but entirely identical with normally

growing specimens.

(2) PLEUROCOCCUS.

_Lecanora_ (_Rinodina_) _sophodes_, sown on rock in 1883, reached in 1886

a diameter of 13 mm. with fully developed apothecia.

_Lecanora ferruginea_ and _L. subfusca_ after three years’ culture formed

sterile thalli only.

_Lecanora coilocarpa_ in four years, and _L. caesio-rufa_ in three years

formed very small thalli without fructification.

(3) TRENTEPOHLIA (Chroolepus).

_Opegrapha vulgata_ in two years had developed thallus and apothecia. The

control culture of the spores formed, as in nature, a considerable felt

of mycelium in the interstices of the bark, but no pycnidia or apothecia.

_Graphis elegans._ Only the beginning of a differentiated thallus was

obtained with this species.

_Verrucaria muralis_ (?)[199] gave in less than a year a completely

developed thallus.

Bonnier also attempted cultures with species of _Collema_ and _Ephebe_,

but was unsuccessful in inducing the formation of a lichen plant.

H. HYMENIAL GONIDIA

Reference has already been made to the minute green cells which were

originally described by Nylander[200] as occurring in the perithecia

of a few Pyrenolichens as free gonidia, _i.e._ unentangled with lichen

hyphae. Fuisting[201] found them in the perithecium of _Polyblastia_

(_Staurothele_) _catalepta_ at a very early stage of its development

when the perithecial tissues were newly differentiated from those of the

surrounding thallus. The gonidia enclosed in the perithecium differed in

no wise from those of the thallus: they had become mechanically enclosed

in the new tissue; and while those in the outer compact layers died off,

those in the centre of the structure, where a hollow space arises, were

subject to very active division, becoming smaller in the process and

finally filling the cavity. Winter’s[202] researches on similar lichens

confirmed Fuisting’s conclusions: he described them as similar to the

thalline gonidia but lighter in colour and of smaller size, measuring

frequently only 2·3 µ in diameter, though this size increased to about 7

µ when cultivated outside the perithecium.

Stahl[203] sufficiently demonstrated the importance of these gonidia in

supplying the germinating spores with the necessary algae. They come to

lie in vertical rows between the asci and, owing to pressure, assume

an elongate form[204] (Figs. 5 and 6). They have been seen in very few

lichens, in _Endocarpon_ and _Staurothele_, both rather small genera of

Pyrenolichens, and, so far as is known, in two Discolichens, _Lecidea

phylliscocarpa_ and _L. phyllocaris_, the latter recorded from Brazil by

Wainio[205], and, on account of the inclusion of gonidia in the hymenium,

placed by him in a section, _Gonothecium_.

I. NATURE OF ASSOCIATION BETWEEN ALGA AND FUNGUS

_a._ CONSORTIUM AND SYMBIOSIS. These cultures had established

convincingly the composite nature of the lichen thallus, and

Schwendener’s opinion, that the relationship between the two organisms

was some varying degree of parasitism, was at first unhesitatingly

accepted by most botanists. Reinke[206] was the first to point out

the insufficiency of this view to explain the long continued healthy

life of both constituents, a condition so different from all known

instances of the disturbing or fatal parasitism of one individual on

another. He recognized in the association a state of mutual growth and

interdependence, that had resulted in the production of an entirely new

type of plant, and he suggested _Consortium_ as a truer description of

the connection between the fungus and the alga. This term had originally

been coined by his friend Grisebach in a paper[206] describing the

presence of actively growing _Nostoc_ algae in healthy _Gunnera_ stems;

and Reinke compared that apparently harmless association with the similar

phenomenon in the lichen thallus. The comparison was emphasized by him

in a later paper[207] on the same subject, in which he ascribes to

each “consort” its function in the composite plant, and declares that

if such a mutual life of Alga and Ascomycete is to be regarded as one

of parasitism, it must be considered as reciprocal parasitism; and he

insists that “much more appropriate for this form of organic life is the

conception and title of _Consortium_.” In a special work on lichens,

Reinke[208] further elaborated his theory of the physiological activity

and mutual service of the two organisms forming the consortium.

Frank[209] suggested the term _Homobium_ as appropriate, but it is faulty

inasmuch as it expresses a relationship of complete interdependence, and

it has been proved that the fungus partly, and the alga entirely, have

the power of free growth.

A wider currency was given to this view of a mutually advantageous growth

by de Bary[210]. He followed Reinke in refusing to accept as satisfactory

the theory of simple parasitism, and adduced the evident healthy life of

the algal cells—the alleged victims of the fungus—as incompatible with

the parasitic condition. He proposed the happily descriptive designation

of a _Symbiosis_ or conjoint life which was mostly though not always, nor

in equal degree, beneficial to each of the partners or symbionts.

_b._ DIFFERENT FORMS OF ASSOCIATION. The type of association between the

two symbionts varies in different lichens. Bornet[211], in describing the

development of the thallus in certain members of the Collemaceae, found

that though as a rule the two elements of the thallus, as in some species

of _Collema_ itself, persisted intact side by side, there was in other

members of the genus an occasional parasitism: short branches from the

main hyphae applied themselves by their tips to some cell of the _Nostoc_

chain (Fig. 9). The cell thus seized upon began to increase in size, and

the plasma became granular and gathered at the side furthest away from

the point of attachment. Finally the contents were used up, and nothing

was left but an empty membrane adhering to the fungus hypha. In another

species the hypha penetrated the cell. These instances of parasitism are

most readily seen towards the edge of the thallus where growth is more

active; towards the centre the attached cells have become absorbed, and

only the shortened broken chains attest their disappearance. The other

cells of the chains remain uninjured.

[Illustration: Fig. 9. _Physma chalazanum_ Arn. Cells of _Nostoc_ chains

penetrated and enlarged by hyphae × 950 (after Bornet).]

In _Synalissa_, a small shrubby gelatinous genus, the hypha, as described

by Bornet and by Hedlund[212], pierces the outer wall of the gelatinous

alga (_Gloeocapsa_) and swells inside to a somewhat globose haustorium

which rests in a depression of the plasma (Fig. 10). The alga, though

evidently undamaged, is excited to a division which takes place on a

plane that passes through the haustorium; the two daughter-cells then

separate, and in so doing free themselves from the hypha.

[Illustration: Fig. 10. _Synalissa symphorea_ Nyl. Algae (_Gloeocapsa_)

with hyphae from the internal thallus × 480 (after Bornet).]

Hedlund followed the process of association between the two organisms

in the lichens _Micarea_ (_Biatorina_) _prasina_ and _M. denigrata_

(_Biatorina synothea_), crustaceous species which inhabit trunks of trees

or palings. In these the alga, one of the Chlorophyceae, has assumed the

character of a _Gloeocapsa_ but on cultivation it was found to belong

to the genus _Gloeocystis_. The cells are globose and rather small;

they increase by the division of the contents into two or at most four

portions which become rounded off and covered with a membrane before

they become free from the mother-cell. The lichen hypha, on contact with

any one of the green cells, bores through the outer membrane and swells

within to a haustorium, as in the gonidia of _Synalissa_.

[Illustration: Fig. 11. Gonidia from _Ramalina reticulata_ Nyl. A,

gonidium pierced and cell contents shrinking × 560; B, older stage, the

contents of gonidium exhausted × 900 (after Peirce).]

[Illustration: Fig. 12. _Pertusaria globulifera_ Nyl. Fungus and gonidia

from gonidial zone × 500 (after Darbishire).]

Penetrating haustoria were demonstrated by Peirce[213] in his study of

the gonidia of _Ramalina reticulata_. In the first stage the tip of a

hypha had pierced the outer wall of the alga, causing the protoplasm to

contract away from the point of contact (Fig. 11). More advanced stages

showed the extension of the haustorium into the centre of the cell, and,

finally, the complete disappearance of the contents. In many cases it was

found that penetration equally with clasping of the alga by the filament

sets up an irritation which induces cell-division, and the alga, as in

_Synalissa_, thus becomes free from the fungus. Hue[214] has recorded

instances of penetration in an Antarctic species, _Physcia puncticulata_.

It is easy, he says, to see the tips of the hyphae pierce the sheath of

the gonidium and penetrate to the nucleus.

Lindau[215] has described the association between fungus and alga in

_Pertusaria_ and other crustaceous forms as one of contact only (Fig.

12). He found that the cell-membrane of the two adhering organisms was

unbroken. Occasionally the algal cell showed a slight indentation, but

was otherwise unchanged. The hyphal branch was somewhat swollen at the

tip where it touched the alga, and the wall was slightly thinner. The

attachment between the two cells was so close, however, that pressure on

the cover-glass failed to separate them.

Generally the hypha simply surrounds the gonidium with clasping branches.

Many algae also lie free in the gonidial zone, and Peirce[216] claims

that these are larger, more deeply coloured and in every way healthier

looking than those in the grasp of the fungus. He ignores, however, the

case of the soredial algae which though very closely invested by the

fungus are yet entirely healthy, since on their future increase depends

in many cases the reproduction of new individual lichens.

In a recent study of a crustaceous sandstone lichen, “_Caloplaca

pyracea_,” Claassen[217] has sought to prove a case of pure parasitism.

The rock was at first covered with the green cells of _Cystococcus_ sp.

Later there appeared greyish-white patches on the green, representing the

invasion of the lichen fungus. These patches increased centrifugally,

leaving in time a bare patch in the centre of growth which was again

colonized by the green alga. The lichen fruited abundantly, but wherever

it encroached the green cells were more or less destroyed. The true

explanation seems to be that the green cells were absorbed into the

lichen thallus, though enough of them persisted to start new colonies

on any bare piece of the stone. In the same way large patches of

_Trentepohlia aurea_ have been observed to be gradually invaded by the

dark coloured hyphae of _Coenogonium ebeneum_. In time the whole of the

alga is absorbed and nothing is to be seen but the dark felted lichen.

The free alga as such disappears, but it is hardly correct to describe

the process as one of destruction.

This algal genus _Trentepohlia_ (_Chroolepus_) forms the gonidia of

the Graphideae, Roccelleae, etc. It is a filamentous aerial alga which

increases by apical growth. In the Graphideae, many of which grow on

trees beneath the outer bark (hypophloeodal), the association between the

two symbionts may be of the simplest character, but was considered by

Frank[218] to be of an advanced type. According to his observations and

to those of Lindau[219], the fungal hyphae penetrate first between the

cells of the periderm. The alga, frequently _Trentepohlia umbrina_, tends

to grow down into any cracks of the surface. It goes more deeply in when

preceded by the hyphae. In some species both organisms maintain their

independent growth, though each shows increased vigour when it comes into

contact with the other. In some instances the cells of the alga are

clasped by the fungus which causes the disintegration of the filament.

The cells lose their bright yellow or reddish colour and are changed in

appearance to greenish lichen gonidia; but no penetration by haustoria

has ever been observed in _Trentepohlia_.

Bachmann’s[220] study of a similar gonidium in a calcicolous species of

_Opegrapha_ confirms Frank’s results. The algae had pierced not only

between the looser lime granules but also through a crystal of calcium

carbonate, and occupied nests scooped out in the rock by means of acid

formed and excreted by their filaments. When association took place with

the fungus, the algal cells were more restricted to a gonidial zone; but

some of the cells, having been pushed aside by the hyphae, had started

new centres of gonidia. On contact with the hyphae there was a tendency

to bud out in a yeast-like growth.

In the thallus of the Roccelleae, the algal filament, also a

_Trentepohlia_, is broken up into separate cells, but in the

Coenogoniaceae, whether the gonidium be a _Cladophora_ as in _Racodium_,

or a _Trentepohlia_ as in _Coenogonium_, the filaments remain intact and

are invested more or less closely by the hyphae.

[Illustration: Fig. 13. Outer edge of _Phycopeltis expansa_ Jenn., the

alga attacked by hyphae and passing into separate gonidia × 500 (after

Vaughan Jennings).]

A somewhat different type of association takes place between alga and

fungus in _Strigula complanata_, an epiphyllous lichen more or less

common in tropical regions. Cunningham[221], who found it near Calcutta,

described the algal constituent and placed it in a new genus, _Mycoidea_

(_Cephaleuros_). It forms small plate-like expansions on the surface

of the leaf, and also penetrates below the cuticle, burrowing between

that and the epidermal cells; occasionally, as observed by Cunningham,

rhizoid-like growths pierce deeper into the tissue—into and below the

epidermal layer. Very frequently, in the wet season, a fungus takes

possession of the alga and slender colourless hyphae creep along its

surface by the side of the cell rows, sending out branches which grow

downwards. Marshall Ward[222] described the same lichen from Ceylon. He

states that the alga may be attacked at any stage, and if it is in a very

young condition it is killed by the fungus; at a more advanced period of

growth it continues to develop as an integral part of the lichen thallus,

but with more frequently divided and smaller cells. Vaughan Jennings[223]

observed _Strigula complanata_ in New Zealand associated with a closely

allied chroolepoid alga _Phycopeltis expansa_. He also noted the growth

of the fungus over the alga breaking up the plates of tissue and

separating the cells which, from yellow, change to a green colour and

become rounded off (Fig. 13). The mature lichen, a white thallus dotted

with black fruits, contrasts strikingly with the yellow membranous alga.

Lichen formation usually begins near the edge of the leaf and the margin

of the thallus itself is marked by a green zone showing where the fungus

has recently come into contact with the alga.

More recently Hans Fitting[224] has described “_Mycoidea parasitica_”

as it occurs on evergreen leaves in Java. The alga, a species of

_Cephaleuros_, though at first an epiphyte, becomes partially parasitic

at maturity. It penetrates below the cuticle to the outer epidermal cells

and may even reach the tissue below. When it is joined by the lichen

fungus, both constituents grow together to form the lichen. Fitting adds

that the leaf is evidently but little injured. In this lichen the alga in

the grip of the fungus loses its independence and may be killed off: it

is an instance of something like intermittent parasitism.

J. RECENT VIEWS ON SYMBIOSIS AND PARASITISM

No hyphal penetration of the bright-green algal cell by means of

haustoria had been observed by the earlier workers, Bornet[225],

Bonnier[226] and others, though they followed Schwendener[227] in

regarding the relationship as one of host and parasite. Lindau, also,

after long study accepted parasitism as the only adequate explanation of

the associated growth, though he never found the fungus actually preying

on the alga.

In recent years interest in the subject has been revived by the

researches of Elenkin[228], a Russian botanist who claims to have

established a case for parasitism or rather “endosaprophytism.” He has

demonstrated by means of staining reagents the presence in the thallus

of large numbers of dead algal cells. A few empty membranes are to be

found in the cortex and in the gonidial zone, but the larger proportion

occur below the gonidial zone and partly in the medulla. He describes

the lower layer as a “necral” or “hyponecral” zone, and he considers

that the hyphae draw their nourishment chiefly from dead algal material.

The fungus must therefore be regarded in this case as a saprophyte

rather than a parasite. The algae, he considers, may have perished

from want of sufficient light and air or they may have been destroyed

by an enzyme produced by the fungus. The latter he thinks is the more

probable, as dead cells are frequently present among the living algae

of the gonidial zone. To the action of the enzyme he also attributes

the angular deformed appearance of many gonidia and the paler colour

and gradual disintegration of their contents which are finally used up

as endosaprophytic nourishment by the fungus. Dead algal cells were

more easily seen, he tells us, in crustaceous lichens associated with

“_Pleurococcus_” or “_Cystococcus_”; they were much less frequent in the

larger foliose or fruticose lichens. Dead cells of _Trentepohlia_ were

also difficult to find.

In a second paper Elenkin records one clear instance of a haustorium

entering an algal cell, and says he found some evidence of hyphal

branches penetrating otherwise uninjured gonidia, round holes being

visible in their outer wall, but he holds that it is the cell-wall of the

alga that is mainly dissolved by the ferment and then used as food by the

hyphae.

No allowance has been made by Elenkin for the normal wasting common to

all organic beings: the lichen fungus is continually being renewed,

especially in the cortical structures, and the alga must also be subject

to change. He[229] claims, nevertheless, that his observations have

proved that the one symbiont is always preying on the other, either as a

parasite or as a saprophyte. He has likened the conception of symbiosis

to that of a balance between two organisms, “a moveable equilibrium

of the symbionts.” If, he says, we could conceive a state where the

conditions of life would be equally favourable for both partners there

would be true mutualism, but in practice one only is favoured and gains

the upper hand, using its advantage to prey on the other. Unless the

balance is redressed, the complete destruction of the weaker is certain,

and is followed in time by the death of the stronger. The fungus being

the dominant partner, the balance, he considers, is tipped in its favour.

Elenkin’s conclusions are not borne out by the long continued and

healthy life of the lichen. There is no record of either symbiont having

succumbed to the other, and the alga, when set free, is unchanged and

able to resume its normal development. Without the alga the fungus cannot

form the ascigerous fruit. Is that because as a parasite within the

lichen it has degenerated past recovery, or has it become so adapted to

symbiosis that in saprophytic conditions it fails to develop?

Another Russian lichenologist, U. N. Danilov[230], records results which

would seem to support the theory of parasitism. He found that from the

clasping hyphae minute haustoria were produced, which penetrate the algal

cell-wall, and branch when within the outer membrane, thus forming a

delicate network over the plasma; secondary haustoria arising from this

network protrude into the interior and rob the cell-contents. He observed

gonidia filled with well-developed hyphae and these, after having

exhausted one cell, travel onwards to others. Some gonidia under the

influence of the fungus had become deformed and were finally killed. As

a proof of this latter statement he adduces the presence in the thallus

of some gonidia containing shrivelled protoplasm, of others entirely

empty. He considers, as further evidence in favour of parasitism, the

finding of empty membranes as well as of colourless gonidia filled

with the hyphal network. This description hardly tallies with the usual

healthy appearance of the gonidial zone in the normal thallus, and it has

been suggested that where the fungus filled the algal cell, it was as a

saprophyte preying on dead material.

The gradual perishing of algal cells in time by natural decay and their

subsequent absorption by the fungus is undisputed. It is open to question

whether the varying results recorded by these workers have any further

significance.

These observations of Elenkin and Danilov have been proved to be

erroneous by Paulson and Somerville Hastings[231]. They examined the

thalli of several lichens (_Xanthoria parietina_, _Cladonia_ sp., etc.)

collected in early spring when vegetative growth in these plants was

found to be at its highest activity. They found an abundant increase of

gonidia within the thallus, which they regarded as sporulation of the

algae, and the most careful methods of staining failed to reveal any case

of penetration of the gonidia by the hyphae.

Nienburg[232] has published some recent observations on the association

of the symbionts. In the wide cortex of a _Pertusaria_ he found not only

the densely compact hyphae, but also isolated gonidia. In front of these

latter there was a small hollow cavity and, behind, parallel hyphae

rich in contents. These gonidia had originated from the normal gonidial

zone. They were moved upward by special hyphae called by Nienburg

“push-hyphae.” After their transportation, the algae at once divide and

the products of division pass to a resting stage and become the centre

of a new thalline growth. A somewhat similar process was noted towards

the apex of _Evernia furfuracea_. Radial hyphae pushed up the cortex,

leaving a hollow space over the gonidial zone. Into the space isolated

algae were thrust by “push-hyphae.” In this lichen he also observed the

penetration of the algal cell by haustoria of the fungus. He considers

that the alga reaps advantage but also suffers harm, and he proposes the

term _helotism_ to express the relationship.

An instructive case of the true parasitism of a fungus on an alga has

been described by Zukal[233] in the case of _Endomyces scytonemata_ which

he calls a “half-lichen.” The mature fungus formed small swellings on the

filaments of the _Scytonema_ and, when examined, the hyphae were seen to

have attacked the alga, penetrating the outer gelatinous sheath and then

using up the contents of the green cells. It is only after the alga has

been destroyed and absorbed, that asci are formed by the fungus. Zukal

contrasts the development of this fungus with the symbiotic growth of the

two constituents in _Ephebe_ where both grow together for an indefinite

time.

Mere associated growth however even between a fungus and an alga does

not constitute a lichen. An instance of such growth is described by

Sutherland[234] in an account of marine microfungi. One of these, a

species of _Mycosphaerella_, was found on _Pelvetia canaliculata_, and

Sutherland claims that as no apparent injury was done to the alga, it

was a case of symbiosis and that there was formed a new type of lichen.

The mycelium, always intercellular, pervaded the whole host-plant, and

the fungal fruits were invariably formed on the algal receptacles close

to the oogonia. Their position there is, of course, due to the greater

food supply at that region. Both fungus and alga fruited freely. A closer

analogy could have been found by the writer in the smut fungus which

grows with the host-cereal until fruiting time; or with the mycorrhiza

of _Calluna_ which also pervades every part of the host-plant without

causing any injury. In the true lichen, the alga, though constituting an

important part of the vegetative body, takes no part in reproduction,

except by division and increase of the vegetative cells within the

thallus. The fruiting bodies are always of a modified fungal nature.

2. PHYSIOLOGY OF THE SYMBIONTS

The occurrence of isolated cases of parasitism—the fungus preying on the

alga—in any case leaves the general problem unsolved. The whole question

turns on the physiological activity and requirements of the two component

elements of the thallus. From what sources do they each procure the

materials essential to them as living organisms? It is chiefly a question

of nutrition.

A. NUTRITION OF ALGAE

_a._ CHARACTER OF ALGAL CELLS. Gonidia are chlorophyll-containing bodies

and assimilate carbon-dioxide from the atmosphere by photosynthesis

as do the chlorophyll cells of other plants. They also require water

and mineral salts which, in a free condition, they absorb from their

immediate surroundings, but which, in the lichen thallus, they must

obtain from the fungal hyphae. If the nutriment supplied to them in

their inclosed position be greater or even equal to what the cells could

procure as free-living algae, then they undoubtedly gain rather than lose

by their association with the fungus, and are not to be considered merely

as victims of parasitism.

_b._ SUPPLY OF NITROGEN. Important contributions on the subject of algal

nutrition have been made by Beyerinck[235] and Artari[236]. The former

conducted a series of culture experiments with green algae, including the

gonidia of _Physcia_ (_Xanthoria_) _parietina_. He successfully isolated

the lichen gonidia and, at first, attempted to grow them on gelatine with

an infusion of the Elm bark from which he had taken the lichen. Growth

was very slow and very feeble until he added to the culture-medium a

solution of malt-extract which contains peptones and sugar. Very soon

he obtained an active development of the gonidia, and they multiplied

rapidly by division[237] as in the lichen thallus. This proved to him

conclusively the great advantage to the algae of an abundant supply of

nitrogen.

Artari in his work has demonstrated that there are two different

physiological races of green algae: (1) those that absorb peptones—which

he designates peptone-algae—and (2) those that do not so absorb peptones.

He tested the cells of _Cystococcus humicola_ taken from the thallus

of _Physcia parietina_, and found that they belonged to the peptone

group and were therefore dependent on a sufficiency of nitrogenous

material to attain their normal vigorous growth. It was also discovered

by Artari that the one race can be made by cultivation to pass over to

the other: that ordinary algae can be educated to live on peptones, and

peptone-algae to do without.

We learn further from Beyerinck’s researches that Ascomycetes, the group

of fungi from which the hyphae of most lichens are derived, are what

he terms ammonia-sugar fungi; that is to say, the hyphae can abstract

nitrogen from ammonia salts and, with the addition of sugar, can form

peptones. The lichen peptone-algae are thus evidently, by their contact

with such fungi, in a favourable position for securing the nitrogenous

food supply most suited to their requirements. In their deep-seated

layers, they are to a large extent deprived of light, but it has been

proved by Artari[238] in a series of culture experiments extending over

a long period, that the gonidia of _Xanthoria parietina_ remain green in

the dark under very varied conditions of nutriment, though the colour is

distinctly fainter.

Recently Treboux[239] has revised the work done by Artari and

Beyerinck in reference to _Cystococcus humicola_. He denies that two

physiological races are represented in this alga, the lichen gonidia,

in regard to the nitrogen that they absorb, behaving exactly as do the

free-living forms of the species. He finds that the gonidium is not a

peptone-carbohydrate organism in the sense that it requires nitrogen in

the form of peptones, inorganic ammonia salts being a more acceptable

food supply. Treboux concludes that his results favour the view that

the gonidia are in an unfavourable situation for receiving the kind of

nitrogenous compound most advantageous to them, that they are therefore

in a sense “victims” of parasitism, though he qualifies the condition as

being a lichen-parasitism or helotism. This view does not accord with

Chodat’s[240] results: in his cultures of gonidia he observed that with

glycocoll or peptone, which are nearly equivalent, they developed four

times better than with potassium nitrate as their nitrogenous food, and

he concluded that they assimilated nitrogen better from bodies allied to

peptides.

_c._ EFFECT OF SYMBIOSIS ON THE ALGA. Treboux’s observations however

convinced him that the alga leads but a meagre existence within the

thallus. Cell-division—the expression of active vitality—was, he held,

of rare occurrence in the slowly growing lichen-plant, and zoospore

formation in entire abeyance. He contrasts this sluggish increase[241]

with the rapid multiplication of the free-living algal cells which cover

whole tree-trunks with their descendants in a comparatively short time.

These latter cells, he finds, are indeed rather smaller, being generally

the products of recent division, but mixed with them are numbers of

larger resting cells, comparable in size with the lichen gonidia. He

states further, that the gonidia are less brightly green and, as he

judges, less healthy, though in soredial formation or in the open they

at once regain both colour and power of division. Treboux had entirely

failed to observe the sporulation which is so abundant at certain seasons.

Their quick recovery seems also a strong argument in favour of the

absolutely normal condition of metabolism within the gonidial cell;

and the paler appearance of the chlorophyll is doubtless associated

with the acquisition of carbohydrates from other sources than by

photosynthesis. There is a wide difference between any degree of

unfavourable life-conditions and parasitism however slight, even

though the balance of gain is on the side of the fungus. It is not too

fanciful to conclude that the demand for nitrogen on the part of the

alga has influenced its peculiar association with the fungus. In the

thallus of hypophloeodal lichens it has been proved indeed that the alga

_Trentepohlia_ with apical growth is an active agent in the symbiotic

union. _Cystococcus_ and other green algal cells are stationary, but they

are doubtless equally ready for—as many of them are equally benefited

by—the association. Keeble[242] has pointed out in the case of _Convoluta

roscoffensis_ that nitrogen-hunger induces the green algae to combine

forces with an animal organism, though the benefit to them is only

temporary and though they are finally sacrificed. The lichen gonidia, on

the contrary, persist for a long time, probably far beyond their normal

period of existence as free algae.

Examples of algal association with other plants might be cited here: of

_Nostoc_ in the roots of _Cycas_ and in the cells of _Anthoceros_, and

of _Anabæna_ in the leaf-cells of _Azolla_, but in these instances it is

generally held that the alga secures only shelter. It was by comparing

the lichen-association with the harmless invasion of _Gunnera_ cells by

_Nostoc_ that Reinke[243] arrived at his conception of “consortism.”

_d._ SUPPLY OF CARBON. Carbon, the essential constituent of all organic

life, is partly drawn from the carbon-dioxide of the air, and assimilated

by the green cells; it is also partly contributed by the fungus as a

product of its metabolism. A proof of this is afforded by Dufrenoy[244]:

he found a _Parmelia_ growing closely round pine needles and even sending

suckers into the stomata. He covered the lichen with a black cloth and

after seven weeks found that the gonidia had remained very green. That

growth had not been checked was evidenced by an unusual development

of soredia and of spermogonia. Dufrenoy describes the condition as

a parasitism of the algae on the fungus which in turn was drawing

nourishment from the pine needles.

Artari[245] has proved that lichen gonidia can obtain carbohydrates

from the substratum as well as by photosynthesis. He cultivated the

gonidia of _Xanthoria parietina_ and _Placodium murorum_ on media which

contained organic substances as well as mineral salts, while depriving

them of atmospheric carbon-dioxide and in some cases of light also. The

gonidia not only grew well but, even in the dark, they remained normally

green, a phenomenon coinciding with Etard and Bouilhac’s[246] experience

in growing _Nostoc_ in the dark: with suitable culture media the alga

retained its colour. _Nostoc_ also grows in the dark in the rhizome of

_Gunnera_. Radais’[247] experiments with _Chlorella vulgaris_ confirmed

these results. On certain organic media growth and cell-division were as

rapid in the dark as in the light, and chlorophyll was formed. The colour

was at first yellowish and the full green arrived slowly, especially on

sugar media, but in ten days it was uniform and normal.

When making further experiments with the alga, _Stichococcus bacillaris_,

Artari[248] found that it also grew well on an organic medium and that

grape sugar was the most valuable carbonaceous food supply. Chodat[249]

also found that sugar or glucose was a desirable ingredient of culture

media.

Treboux[250], in his work on organic acids, has also proved by

experimental cultures with a large series of algae, including the gonidia

of _Peltigera_, that these green plants in the absence of light and in

pure cultures would grow and form carbohydrates if the culture medium

contained a small percentage of organic acids. The acids he employed

were combined with potassium and were thus rendered neutral or slightly

alkaline; acetate of potash proved to be the most advantageous compound

of any that was tested. Amino-acids and ammonia salts were added to

provide the necessary nitrogen. Oxalic acid and other organic acids of

varying composition are peculiarly abundant in lichen tissues and may be

a source of carbon supply. Marshall Ward[251] has found calcium carbonate

crystals in the lower air-containing tissues of _Strigula complanata_.

Treboux finally concluded from his researches that just as fungi can

extract carbohydrates from many sources, so algae can secure their carbon

supply in a variety of ways. He affirms that the metabolic activity of

the alga in these cultural conditions is entirely normal, and the various

cell-contents are formed as in the light. Whether, in this case, starch

is formed directly from the acids or through a series of combinations

has not been determined. Uhlir[252], with electric lighting, made

successful cultures of _Nostoc_ isolated from Collemaceae on silicic

acid, proving thereby that these gonidia do not require a rich nutriment.

A certain definite humidity was however essential, and bacteria were

never eliminated as they are associated with the gelatinous membranes of

Nostocaceae.

_e._ NUTRITION WITHIN THE SYMBIOTIC PLANT. Culture experiments bearing

more directly on the nutrition of lichens as a whole were carried out

by F. Tobler[253]. He proved that the gonidia had undoubtedly drawn on

the calcium oxalate secreted by the hyphae for their supply of carbon.

In a culture medium of poplar-bark gelatine he grew hyphae of _Xanthoria

parietina_, and noted an abundant deposit of oxalate crystals on their

cell-walls. A piece of the lichen thallus including both symbionts and

grown on a similar medium formed no crystals, and microscopic examination

showed that crystals were likewise absent from the hyphae of the thallus

that had grown normally on the tree, the inference being that the gonidia

used them up as quickly as they were deposited. It must be remembered in

this connection, however, that Zopf[254] has stated that where lichen

acids are freely formed as, for instance, in _Xanthoria parietina_, there

is always less formation and deposit of calcium oxalate crystals, which

may partly account for their absence in the normal thallus so rich in

parietin.

Tobler next introduced lichen gonidia into a culture medium in which the

isolated hyphal constituent of a thallus had been previously cultivated,

and placed the culture in the dark. In these circumstances he found that

the gonidia were able to thrive but formed no colour: they were obtaining

their carbohydrates, he decided, not from photosynthesis, but from the

excretory products such as calcium oxalate that had been deposited in the

culture medium by the lichen hyphae. We may conclude with more or less

certainty that the loss of carbohydrates, due to the partial deprivation

of light and air suffered by the alga owing to its position in the

lichen thallus, is more than compensated by a physiological symbiosis

with the fungus[255]. It has indeed been proved that in the absence of

free carbon-dioxide, algae may utilize the half-bound CO₂ of carbonates,

chiefly those of calcium and magnesium, dissolved in water.

_f._ AFFINITIES OF LICHEN GONIDIA. Chodat[256] has, in recent years, made

cultures of lichen gonidia with a view to discovering their relation to

free-living algae and to testing at the same time their source of carbon

supply. He has come to the conclusion that lichen gonidia are probably in

no instance the normal _Protococcus viridis_: they differ from that alga

in the possession of a pyrenoid and in their reproduction by zoospores

when free.

Careful cultures were made of different _Cladonia_ gonidia which were

morphologically indistinguishable, and which varied in size from 10 to

16µ in diameter, though smaller ones were always present. He recognized

them to be species of _Cystococcus_: they have a pyrenoid[257] in the

centre and a disc-like chromatophore more or less starred at the edge.

These gonidia grew well on agar, still better on agar-glucose, but best

of all with an addition of peptone to the culture. There was invariably

at first a slight difference in form and colour in the mass between the

gonidia of one species and those of another, but as growth continued they

became alike.

In testing for carbon supply, he found that gonidia grew slowly without

sugar (glucose), and that, as sources of carbon, organic acids could not

entirely replace glucose though, in the dark, the gonidia used them to

some extent; the colony supplied with potassium nitrate, and grown in the

dark, had reached a diameter of only 2 mm. in three months. With glucose,

it measured 5 mm. in three weeks, while in three months it formed large

culture patches.

A further experiment was made to test their absorption of peptones by

artificial cultures carried out both in the light and the dark. The

gonidia grew poorly in all combinations of organic nitrogen compounds.

When combined with glucose, growth was at once more vigorous though only

half as much in the dark as in the light, the difference in this respect

being especially noticeable in the gonidia from _Cladonia pyxidata_. He

concludes that as gonidia in these cultures are saprophytic, so in the

lichen thallus also they are probably more or less saprophytic, obtaining

not only their nitrogen in organic form but also, when possible, their

carbon material as glucose or galactose from the hyphal symbiont which in

turn is saprophytic on humus, etc.

B. NUTRITION OF FUNGI

Fungi being without chlorophyll are always indebted to other organisms

for their supply of carbohydrates. There has never therefore been any

question as to the advantage accruing to the hyphal constituent in the

composite thallus. The gonidia, as various workers have proved, have also

a marked preference for organized nourishment, and, in addition, they

obtain carbon by photosynthesis. Chodat[258] considers that probably

they are thus able to assimilate carbon-dioxide in excess, a distinct

advantage to the hyphae. In some instances the living gonidium is invaded

and the contents used up by the fungus and any dead gonidia are likewise

utilized for food supply. It is also taken for granted that the fungus

takes advantage of the presence of humus whether in the substratum or

in aerial dust. In such slow growing organisms, there is not any large

demand for nourishment on the part of the hyphae: for many lichens it

seems to be mere subsistence with a minimum of growth from year to year.

C. SYMBIOSIS OF OTHER PLANTS

The conception of an advantageous symbiosis of fungi with other

plants has become familiar to us in Orchids and in the mycorhizal

formation on the roots of trees, shrubs, etc. Fungal hyphae are also

frequent inhabitants of the rhizoids of hepatics though, according to

Gargeaune[259], the benefit to the hepatic host-plant is doubtful.

An association of fungus and green plant of great interest and bearing

directly on the question of mutual advantage has been described

by Servettaz[260]. In his study of mosses, he was able to confirm

Bonnier’s[261] account of lichen hyphae growing over such plants as

_Vaucheria_ and the protonema of mosses, which is undoubtedly hurtful;

but he also found an association of a moss with one of the lower fungi,

_Streptothrix_ or _Oospora_, which was distinctly advantageous. In

separate cultivation the fungus developed compact masses and grew well in

peptone agar broth.

Cultures of the moss, _Phascum cuspidatum_, were also made from the

spores on a glucose medium. The specimens in association with the fungus

were fully grown in two months, while the control cultures, without any

admixture of the fungus, had not developed beyond the protonema stage.

Servettaz draws attention to the proved fact that, in certain instances,

plants benefit when provided with substances similar to their own decay

products, and he considers that the fungus, in addition to its normal

gaseous products, has elaborated such substances, as acid products, from

the glucose medium to the great advantage of the moss plant.

A symbiotic association of _Nostoc_ with another alga, described by

Wettstein[262], is also of interest. The blue-green cells were lodged in

the pyriform outgrowths of the siphoneous alga, _Botrydium pyriforme_

Kütz., which the author of the paper places in a new genus, _Geosiphon_.

The sheltering _Nostoc symbioticum_ fills all of the host left vacant

by the plasma, and when the season of decay sets in, it forms resting

spores which migrate into the rhizoids of the host, so that both plants

regenerate together.

Wettstein has compared this symbiotic association with that of lichens,

and finds the analogy all the more striking in that the membrane of his

new alga had become chitinous, which he thinks may be due to organic

nutrition.

II. LICHEN HYPHAE

A. ORIGIN OF HYPHAE

Lichen hyphae form the ground tissue of the thallus apart from the

gonidia or algal cells. They are septate branched filaments of single

cell rows and are colourless or may be tinged by pigments or lichen acids

to some shade of yellow, brown or black. They are of fungal nature, and

are produced by the mature lichen spore.

The germination of the spore was probably first observed by Meyer[263].

His account of the actual process is somewhat vague, and he

misinterpreted the subsequent development into thallus and fruit entirely

for want of the necessary magnification; but that he did succeed in

germinating the spores is unquestionable. He cultivated them on a smooth

surface and they grew into a “dendritic formation”—a true hypothallus.

Many years later the development of hyphae from lichen spores was

observed by Holle[264] who saw and figured the process unmistakably in

_Borrera_ (_Physcia_) _ciliaris_.

A series of spore cultures was undertaken by Tulasne[265] with the

twofold object of discovering the exact origin of hyphae and gonidia

and of their relationship to each other. The results of his classical

experiment with the spores of _Verrucaria muralis_—as interpreted by

him—were accepted by the lichenologists of that time as conclusive

evidence of the genetic origin of the gonidia within the thallus.

[Illustration: Fig. 14. Germinating spores of _Verrucaria muralis_ Ach.

after two months’ culture × ca. 500 (after Tulasne).]

The spores of the lichen in large numbers had been sown by Tulasne in

early spring on the smooth polished surface of a piece of limestone,

and were covered with a watch-glass to protect them from dust, etc.

At irregular intervals they were moistened with water, and from time

to time a few spores were abstracted from the culture and examined

microscopically. Tulasne observed that the spore did not increase or

change in volume in the process of germination, but that gradually the

contents passed out into the growing hyphae, till finally a thin membrane

only was left and still persisted after two months (Fig. 14). For a

considerable time there was no septation; at length cross-divisions were

formed, at first close to the spore, and then later in the branches. The

hyphae meanwhile increased in dimension, the cells becoming rounder and

somewhat wider, though always more slender than the spore which had given

rise to them. In time a felted tissue was formed with here and there

certain cells, filled with green colouring matter, similar to the gonidia

of the lichen and thus the early stages at least of a new thallus were

observed. The green cells, we now know, must have gained entrance to the

culture from the air, or they may have been introduced with the water.

B. DEVELOPMENT OF LICHENOID HYPHAE

Lichen hyphae are usually thick-walled, thus differing from those of

fungi generally, in which the membranes, as a rule, remain comparatively

thin. This character was adduced by the so-called “autonomous” school as

a proof of the fundamental distinction between the hyphal elements of

the two groups of plants. It can, however, easily be observed that, in

the early stages of germination, the lichen hyphae, as they issue from

the spore, are thin-walled and exactly comparable with those of fungi.

Growth is apical, and septation and branching arise exactly as in fungi,

and, in certain circumstances, anastomosis takes place between converging

filaments. But if algae are present in the culture the peculiar lichen

characteristics very soon appear.

Bonnier[266], who made a large series of synthetic cultures,

distinguishes three types of growth in lichenoid hyphae (Fig. 15):

1. Clasping filaments, repeatedly branched, which attach and surround the

algae.

2. Filaments with rather short swollen cells which ultimately form the

hyphal tissues of cortex and medulla.

3. Searching filaments which elongate towards the periphery and go to the

encounter of new algae.

In five days after germination of the spores, the clasping hyphae had

laid hold of the algae which meanwhile had increased by division;

the swollen cells had begun to branch out and ten days later a

differentiation of tissue was already apparent. The searching filaments

had increased in number and length, and anastomosis between them had

taken place when no further algae were encountered. The cell-walls of

the swollen hyphae and their branches had begun to thicken and to become

united to form a kind of cellular tissue or “paraplectenchyma[267].” At

a later date, about a month after the sowing of the spores, there was a

definite cellular cortex formed over the thallus. The hyphal cells are

uninucleate, though in the medulla they may be 1-2-nucleate.

[Illustration: Fig. 15. Synthetic culture of _Physcia parietina_ spores

and _Protococcus viridis_ five days after germination. _s_, lichen-spore;

_a_, septate filaments; _b_, clasping filaments; _c_, searching

filaments. × 500 (after Bonnier).]

The hyphae in close contact with the gonidia remain thin-walled, and

have been termed by Wainio[268] “meristematic.” They furnish the growing

elements of the lichen either apical or intercalary. In most genera the

organs of fructification take rise from them, or in their immediate

neighbourhood, and isidia and soredia also originate from these gonidial

hyphae.

As the filaments pass from the gonidial zone to other layers, the

cell-walls become thicker with a consequent reduction of the cell-lumen,

very noticeable in the pith, but carried to its furthest extent in the

“decomposed” cortex where the cells in the degenerate tissue often become

reduced to disconnected streaks indicating the cell-lumen, and the outer

cortical layer is merely a continuous mass of mucilage.

All lichen tissues arise from the branching and septation of the hyphae,

the septa always forming at right angles to the long axis of the

filaments. There is no instance of longitudinal cell-division except in

the spores of certain genera (_Collema_, _Urceolaria_, _Polyblastia_,

etc.). The branching of the hypha is dichotomous or lateral, and very

irregular. Frequent septation and coherent growth result in the formation

of plectenchyma.

C. CULTURE OF HYPHAE WITHOUT GONIDIA

Artificial cultures had demonstrated the germination of lichen spores,

with the formation of hyphae, and from synthetic cultures of fungus and

alga complete lichen plants had been produced. To Möller[269] we owe

the first cultures of a thalline body from the fungus alone, both from

spermatia and from ascospores. The germination of the spermatia has a

direct bearing on their function as spores or as sexual organs and is

described in a later chapter.

The ascospores of _Lecanora subfusca_ were caught in a drop of water on

a slide as they were ejaculated from the ascus, and, on the following

day, a very fine germinating tube was seen to have pierced the exospore.

The hypha became slightly thicker, and branching began on the third day.

If in water alone the culture soon died off, but in a nutrient solution

growth slowly continued. The hyphae branched out in all directions from

the spore as a centre and formed an orbicular expansion which in fourteen

days had reached a size of ·1 mm. in diameter. After three weeks’ growth

it was large enough to be visible without a lens; the mycelial threads

were more crowded, and certain terminal hyphae had branched upwards in

an aerial tuft, this development taking place from the centre outwards.

Möller marked this stage as the transition from a mere protothallus to a

thallus formation. In three months a diameter of 1·5-2 mm. was reached;

a transverse section gave a thickness of ·86 mm. and from the under

side loose hyphae branched downwards and attached the thallus, when it

had been transferred to a solid substratum such as cork. Above these

rhizoidal hyphae, a stratum of rather loose mycelium represented the

medulla, and, surmounting that, a cortical layer in which the hyphae were

very closely compacted. Delicate terminal branches rose into the air over

the whole surface, very similar in character to hypothallic hyphae at the

margin of the thallus.

_Lecanora subfusca_ has a rather small simple spore; it emitted

germinating tubes from each end, and a septum across the middle of the

spore appeared after germination had taken place. Another experiment

was with a much larger muriform spore measuring 80 µ in length and 20 µ

in thickness. On germination about 20 tubes were formed, some of them

rising into the air at once, the others encircling the spore, so that the

thallus took form immediately; growth in this case also was centrifugal.

In three months a diameter of 6 mm. was reached with a thickness of 1 to

2 mm. and showing a differentiation into medulla and cortex. The hyphae

did not increase in width, but frequently globose or ovate swellings

arose in or at the ends, a character which recurs in the natural growth

of hyphae both of lichens and of Ascomycetes. These swellings depend on

the nutrition.

_Pertusaria communis_ possesses a very large simple spore, but it is

multinucleate and germinates with about 100 tubes which reach their

ultimate width of 3 to 4 µ before they emerge from the exospore. The

hyphae encircle the spore, and an opaque thalline growth is quickly

formed from which rise terminal hyphal branches. In ten weeks the

differentiation into medulla and cortex was reached, and in five months

the hyphal thallus measured 4 mm. in diameter and 1 to 2 mm. in thickness.

Möller instituted a comparison between the thalli he obtained from the

spores and those from the spermatia of another crustaceous lichen,

_Buellia punctiformis_ (_B. myriocarpa_). After germination had taken

place the hyphae from the spermatia grew at first more quickly than those

from the ascospores, but as soon as thallus formation began the latter

caught up and, in eight weeks, both thalli were of equal size.

Another comparative culture with the spermatia and ascospores of

_Opegrapha subsiderella_ gave similar results: the spores of that species

are elongate-fusiform and 6-to 8-septate; germination took place from

the end cells in two to three days after sowing. The germinating hyphae

corresponded exactly with those from the spermatia and growth was equally

slow in both. The middle cells of the spores may also produce germinating

tubes, but never more than about five were observed from any one spore.

A browning of the cortical layer was especially apparent in the hyphal

culture from another lichen, _Graphis scripta_: a clear brown colour

gradually changing to black appeared about the same period in all the

cultures.

The hyphae from the spores of _Arthonia_ developed quickest of all:

the hyphae were very slender, but in three to four months the growth

had reached a diameter of 8 mm. In this plant there was the usual

outgrowth of delicate hyphae from the surface; no definite cortical layer

appeared, but only a very narrow line of more closely interwoven somewhat

darker hyphae. Frequently, from the surface of the original thallus,

excrescences arose which were the beginnings of further thalli.

Tobler[270] experimenting with _Xanthoria parietina_ gained very similar

results. The spores were grown in malt extract for ten days, then

transferred to gelatine. In three to five weeks there was formed an

orbicular mycelial felt about 3 mm. in diameter and 2 mm. thick. The

mycelium was frequently brownish even in healthy cultures, but the aerial

hyphae which, at first, rose above the surface were always colourless.

After these latter disappeared a distinct brownish tinge of the thallus

was visible. In seven months it had increased in size to 15 mm. in

length, 7 mm. in width and 3 mm. thick with a differentiation into three

layers: a lower rather dense tissue representing the pith, above that a

layer of loose hyphae where the gonidial zone would normally find place,

and above that a second compact tissue, or outer cortex, from which arose

the aerial hyphae. The culture could not be prolonged more than eight

months.

D. CONTINUITY OF PROTOPLASM IN HYPHAL CELLS

Wahrlich[271] demonstrated that continuity of protoplasm was as constant

between the cells of fungi as it has been proved to be between the cells

of the higher plants. His researches included the hyphae of the lichens,

_Cladonia fimbriata_ and _Physcia_ (_Xanthoria_) _parietina_.

Baur[272] and Darbishire[273] found independently that an open connection

existed between the cells of the carpogonial structures in the lichens

they examined. The subject as regards the thalline hyphae was again

taken up by Kienitz-Gerloff[274] who obtained his best results in the

hypothecial tissue of _Peltigera canina_ and _P. polydactyla_. Most of

the cross septa showed one central protoplasmic strand traversing the

wall from cell to cell, but in some instances there were as many as four

to six pits in the walls. The thickening of the cell-walls is uneven

and projects variously into the cavity of the cell. Meyer’s[275] work

was equally conclusive: all the cells of an individual hypha, he found,

are in protoplasmic connection; and in plectenchymatous tissue the side

walls are frequently perforated. Cell-fusions due to anastomosis are

frequent in lichen hyphae, and the wall at or near the point of fusion

is also traversed by a thread of protoplasm, though such connections are

regarded as adventitious. Fusions with plasma connections are numerous

in the matted hairs on the upper surface of _Peltigera canina_ and they

also occur between the hyphae forming the rhizoids of that lichen. The

work of Salter[276] may also be noted. He claimed that his researches

tended to show complete anatomical union between all the tissues of the

lichen plant, not only between the hyphae of the various tissues but also

between hyphae and gonidia.

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_).

[Illustration: Fig. 16. Examples of _Chroococcus_. A, _Ch. giganteus_

West; B, _Ch. turgidus_ Naeg.; C and D, _Ch. schizodermaticus_ West × 450

(after West).]

[Illustration: 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.

[Illustration: Fig. 18. Examples of _Nostoc_. _N. Linckia_ Born. A, nat.

size; B, small portion × 340; C, _N. coerulescens_ Lyngbye, nat. size

(after West).]

[Illustration: 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).

[Illustration: Fig. 20. _Stigonema_ sp. × 200 (after Comère).]

[Illustration: 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.

[Illustration: 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.

[Illustration: Fig. 23. _Cystococcus Cladoniae pyxidatae_ Chod. from

culture × 800 (after Chodat).]

[Illustration: 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.

[Illustration: Fig. 24. _Coccobotrys Verrucariae_ Chod. from culture ×

800 (after Chodat).]

[Illustration: 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.

[Illustration: Fig. 26. _Urococcus_ sp. Group of cells much magnified

(after Hassall).]

[Illustration: Fig. 27. _Palmella_ sp. × 400 (after Comère).]

[Illustration: 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.

[Illustration: Fig. 29. A, _Trentepohlia umbrina_ Born.; B, _T. aurea_

Mart. × 300 (after Kütz.).]

[Illustration: 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.

[Illustration: 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).

[Illustration: Fig. 32. _Phycopeltis expansa_ Jenn. much magnified (after

Vaughan Jennings).]

[Illustration: 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 botryoide_s 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.

[Illustration: 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.

[Illustration: 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.