CHAPTER III

MORPHOLOGY

GENERAL ACCOUNT OF LICHEN STRUCTURE

I. ORIGIN OF LICHEN STRUCTURES

The two organisms, fungus and alga, that enter into the composition

of the lichen plant are each characterized by the simplicity of their

original structure in which there is little or no differentiation into

tissues. The gonidia-forming algae are many of them unicellular, and

increase mainly by division or by sporulation into daughter-cells which

become rounded off and repeat the life of the mother-cell; others,

belonging to different genera, are filaments, mostly of single cell-rows,

with apical growth. The hyphal elements of the lichen are derived from

fungi in which the vegetative body is composed of branching filaments, a

character which persists in the lichen thallus.

The union of the two symbionts has stimulated both, but more especially

the fungus, to new developments of vegetative form, in which the fungus,

as the predominant partner, provides the framework of the lichen

plant-body. Varied structures have been evolved in order to secure life

conditions favourable to both constituents, though more especially to

the alga; and as the close association of the assimilating and growing

tissues is maintained, the thallus thus formed is capable of indefinite

increase.

A. FORMS OF CELL-STRUCTURE

There is no true parenchyma or cellular structure in the lichen thallus

such as forms the ground tissue of the higher plants. The fungal hyphae

are persistently filamentous and either simple or branched. By frequent

and regular cell-division—always at right angles to the long axis—and

by coherent growth, a pseudoparenchyma may however be built up which

functions either as a protective or strengthening tissue (Fig. 36).

[Illustration: Fig. 36. Vertical section of young stage of stratose

thallus (_Xanthoria parietina_ Th. Fr.). _a_, plectenchyma of cortex;

_b_, medullary hyphae; _c_, gonidial zone. × 500 (after Schwendener).]

Lindau[311] proposed the name “plectenchyma” for the tangled weft of

hyphae that is the principal tissue system in fungi as well as lichens.

The more elaborated pseudoparenchyma he designates as “paraplectenchyma,”

while the term “prosoplectenchyma” he reserved for the fibrous or

chondroid strands of compact filaments that occur frequently in

the thallus of the larger fruticose lichens, and are of service in

strengthening the fronds. The term plectenchyma is now generally used for

pseudoparenchyma.

B. TYPES OF THALLUS

Three factors, according to Reinke[312], have been of influence in

determining the thalline development. The first, and most important, is

the necessity to provide for the work of photosynthesis on the part of

the alga. There is also the building up of a tissue that should serve

as a storage of reserve material, essential in a plant the existence

of which is prolonged far beyond the natural duration of either of the

component organisms; and, finally, there is the need of protecting the

long-lived plant as a whole though more particularly the alga.

Wallroth was the first to make a comparative study of the different

lichen thalli. He distinguished those lichens in which the green cells

and the colourless filaments are interspersed equally through the entire

thallus as “homoiomerous” (Fig. 2), and those in which there are distinct

layers of cortex, gonidia, and medulla, as “heteromerous” (Fig. 1),

terms which, though now considered of less importance in classification,

still persist and are of service in describing the position of the alga

with regard to the general structure. A less evident definition of the

different types of thallus has been proposed by Zukal[313] who divides

them into “endogenous” and “exogenous.”

_a._ ENDOGENOUS THALLUS. The term has been applied to a comparatively

small number of homoiomerous lichens in which the alga predominates in

the development, and determines the form of the thallus. These algae,

members of the Myxophyceae, are extremely gelatinous, and the hyphae

grow alongside or within the gelatinous sheath. In the simpler forms the

vegetative structure is of the most primitive type: the alga retains its

original character almost unchanged, and the ascomycetous fungus grows

along with and beside it (Fig. 4). Such are the minutely tufted thalli

of _Thermutis_ and _Spilonema_ and the longer strands of _Ephebe_, in

which the associated _Scytonema_ or _Stigonema_, filamentous blue-green

algae, though excited to excessive growth, scarcely lose their normal

appearance, making it difficult at times to recognize the lichenoid

character unless the fruits also are present.

Equally primitive in most cases is the structure of the thallus

associated with _Gloeocapsa_. The resulting lichens, _Pyrenopsis_,

_Psorotichia_, etc. are simply gelatinous crusts of the alga with a more

or less scanty intermingling of fungal hyphae.

In the Collemaceae, the gonidial cells of which are species of _Nostoc_

(Fig. 2), there appears a more developed thallus; but in general,

symbiosis in _Collema_ has wrought the minimum of change in the habit

of the alga, hence the indecision of the earlier botanists as to the

identification and classification of _Nostoc_ and _Collema_. Though

in many of the species of the genus _Collema_ no definite tissue is

formed, yet, under the influence of symbiosis, the plants become moulded

into variously shaped lobes which are specifically constant. In some

species there is an advance towards more elaboration of form in the

protective tissues of the apothecia, a layer of thin-walled plectenchyma

being occasionally formed beneath or around the fruit as in _Collema

granuliferum_.

In all these lichens, it is only the thallus that can be considered as

primitive: the fruit is a more or less open apothecium—more rarely a

perithecium—with a fully developed hymenium. Frequently it is provided

with a protective thalline margin.

_b._ EXOGENOUS THALLUS. In this group, composed almost exclusively of

heteromerous lichens, Zukal includes all those in which the fungus takes

the lead in thalline development. He counts as such _Leptogium_, a genus

closely allied to _Collema_ but with more membranous lobes, in which

the short terminal cells of the hyphae have united to form a continuous

cortex. A higher development, therefore, becomes at once apparent, though

in some genera, as in _Coenogonium_, the alga still predominates, while

the simplest forms may be merely a scanty weft of filaments associated

with groups of algal cells. Such a thallus is characteristic of the

Ectolechiaceae, and some Gyalectaceae, etc., which have, indeed, been

described by Zahlbruckner[314] as homoiomerous though their gonidia

belong to the non-gelatinous Chlorophyceae.

Heteromerous lichens have been arranged by Hue[315] according to their

general structure in three great series:

1. _Stratosae._ Crustaceous, squamulose and foliose lichens with a

dorsiventral thallus.

2. _Radiatae._ Fruticose, shrubby or filamentous lichens with a

strap-shaped or cylindrical thallus of radiate structure.

3. _Stratosae-Radiatae._ Primary dorsiventral thallus, either crustaceous

or squamulose, with a secondary upright thallus of radiate structure

called the podetium (Cladoniaceae).

II. STRATOSE THALLUS

1. CRUSTACEOUS LICHENS

A. GENERAL STRUCTURE

In the series “Stratosae,” the plant is dorsiventral, the tissues

forming the thallus being arranged more or less regularly in strata one

above the other (Fig. 37). On the upper surface there is a hyphal layer

constituting a cortex, either rudimentary or highly elaborated; beneath

the cortex is situated the gonidial zone composed of algae and hyphae in

close association; and deeper down the medulla, generally a loose tissue

of branching hyphae. The lower cortex which abuts on the medulla may be

as fully developed as the upper or it may be absent.

[Illustration: Fig. 37. Vertical section of crustaceous lichen (_Lecanora

subfusca_ var. _chlarona_ Hue) on bark. _a_, lichen cortex; _b_, gonidia;

_c_, cells of the periderm. × 100.]

The growing tissue is chiefly marginal; the hyphae on the outer edge

remain “meristematic”[316] and provide for horizontal as well as vertical

extension; and there is also continual increase of the algal cells. There

is in addition a certain amount of intercalary growth due to the activity

of the gonidial tissue, both algal and fungal, providing for the renewal

of the cortex, and even interposing new tissue.

B. SAXICOLOUS LICHENS

_a._ EPILITHIC LICHENS. The crustaceous lichens forming this group spread

over the rock surfaces. The support must be stable to allow the necessary

time for the slowly developing organism, and therefore rocks that are

friable or subject to continual weathering are bare of lichens.

_aa._ =Hypothallus or Prothallus.= The first stage of growth in the

lichen thallus can be most easily traced in epilithic crustaceous

species, especially in those that inhabit a smooth rock surface. The

spore, on germination, produces a delicate branching septate mycelium

which radiates on all sides, as was so well observed and recorded by

Tulasne[317] in _Verrucaria muralis_ (Fig. 14). Zukal[318] has called

this first beginning the prothallus. In time the cell-walls of the

filaments become much thicker and though, in some species, they remain

colourless, in others they become dark-coloured, all except the extreme

tips, owing to the presence of lichen pigments—a provision, Zukal[319]

considers, to protect them against the ravages of insects, etc. The

prothallic filaments adhere closely to the substratum and the branching

becomes gradually more dendroid in form, though sometimes hyphae are

united into strands, or even form a kind of plectenchymatous tissue.

This purely hyphal stage may persist for long periods without much

change. In time there may be a fortuitous encounter with the algae (Fig.

38 A) which become the gonidia of the plant. Either these have been

already established on the substratum as free-growing organisms, or, as

accidentally conveyed, they alight on the prothallus. The contact between

alga and hypha excites both to active growth and to cell-division;

and the rapidly multiplying gonidia are as speedily surrounded by the

vigorously growing hyphal filaments.

[Illustration: Fig. 38 A. Hypothallus of _Rhizocarpon confervoides_ DC.,

from the extreme edge, with loose gonidia × 600.]

[Illustration: Fig. 38 B. Young thallus of _Rhizocarpon confervoides_

DC., with various centres of gonidial growth on the hypothallus × 30.]

Schwendener[320] has thus described the origin and further development

of prothallus and gonidia: on the dark-coloured proto- or prothallus,

he noted small nestling groups of green cells which he, at that time,

regarded as direct outgrowths from the lichen hyphae. These gonidial

cells, increasing by division, multiplied gradually and gathered into

a connected zone. He also observed that the hyphae in contact with the

gonidia became more thin-walled and produced many new branches. Some of

these newly formed branches grow upwards and form the cortex, others

grow downwards and build up the medulla or pith; the filaments at the

circumference continue to advance and may start new centres of gonidial

activity (Fig. 38 B). In many species, however, this prothallus or, as

it is usually termed at this stage, the hypothallus, becomes very soon

overgrown and obscured by the vigorous increase of the first formed

symbiotic tissue and can barely be seen as a white or dark line bordering

the thallus (Fig. 39). Schwendener[321] has stated that probably only

lichens that develop from the spore are distinguished by a protothallus,

and that those arising from soredia do not form these first creeping

filaments.

[Illustration: Fig. 39. _Lecanora parella_ Ach. Determinate thallus with

white bordering hypothallus, reduced (M. P., _Photo._).]

_bb._ =Formation of crustaceous tissues.= Some crustaceous lichens have a

persistently scanty furfuraceous crust, the vegetative development never

advancing much beyond the first rather loose association of gonidia and

hyphae; but in those in which a distinct crust or granules are formed,

three different strata of tissue are discernible:

1st. An upper cortical tissue of interlaced hyphae with frequent

septation and with swollen gelatinous walls, closely compacted and with

the lumen of the cells almost obliterated, not unfrequently a layer of

mucilage serving as an outer cuticle. This type of cortex has been called

by Hue[322] “decomposed.” It is subject to constant surface weathering,

thin layers being continually peeled off, but it is as continually being

renewed endogenously by the upward growth of hyphae from the active

gonidial zone. Exceptions to this type of cortex in crustaceous lichens

are found in some _Pertusariae_ where a secondary plectenchymatous cortex

is formed, and in _Dirina_ where it is fastigiate[323] as in _Roccella_.

2nd. The gonidial zone—a somewhat irregular layer of algae and hyphae

below the cortex—which varies in thickness according to the species.

3rd. The medullary tissue of somewhat loosely intermingled branching

hyphae, with generally rather swollen walls and narrow lumen. It rests

directly on the substratum and follows every inequality and crack so

closely, even where it does not penetrate, that the thallus cannot be

detached without breaking it away.

In _Verrucaria mucosa_, a smooth brown maritime lichen found on rocks

between tide-levels, the thallus is composed of tightly packed vertical

rows of hyphae, slender, rather thin-walled, and divided into short

cells. The gonidia are chiefly massed towards the upper surface, but they

also occur in vertical rows in the medulla. One or two of the upper cells

are brown and form an even cortex. The same formation occurs in some

other sea-washed species; the arrangement of the tissue elements recalls

that of crustaceous Florideae such as _Hildenbrandtia_, _Cruoria_, etc.

[Illustration: Fig. 40. Young thallus of _Rhizocarpon geographicum_ DC.,

with primary and subsequent (dotted lines) areolation × 5.]

_cc._ =Formation of areolae.= An “areolate” thallus is seamed and

scored by cracks of varying width and depth which divide it into minute

compartments. These cracks or fissures or chinks originate in two ways

depending on the presence or absence of hypothallic hyphae. Where the

hypothallus is active, new areolae arise when the filaments encounter new

groups of algae. More vigorous growth starts at once and proceeds on all

sides from these algal centres, until similarly formed areolae are met, a

more or less pronounced fissure marking the limits of each. This primary

areolation, termed rimose or rimulose, is well seen in the thin smooth

thallus of _Rhizocarpon geographicum_ (Fig. 40); but the first-formed

areolae are also very frequently slightly marked by subsequent cracks

due to unequal growth. The areolation caused by primary growth conditions

tends to become gradually less obvious or to disappear altogether.

Secondary areolation is due to unequal intercalary growth of the

otherwise continuous thallus[324]. A more active increase of any minute

portions provokes a tension or straining of the cortex between the

swollen areas and the surrounding more sluggish tissues; the surface

layers give way and chinks arise, a condition described by older

lichenologists as “rimose-diffract” or sometimes as “rhagadiose.” The

thallus is generally thicker, more broken and granular in the older

central parts of the lichen. Towards the circumference, where the tissue

is thinner and growth more equal, the chinks are less evident. Sometimes

the more vigorously growing areolae may extend over those immediately

adjoining, in which case the covered portions become brown and their

gonidia gradually disappear.

Strongly marked intersecting lines, similar to those round the margin

of the thallus, are formed when hypothalli that have themselves started

from different centres touch each other. A large continuous patch of

crustaceous thallus may thus be composed of many individuals (Fig. 41).

[Illustration: Fig. 41. _Rhizocarpon geographicum_ DC. on boulder,

reduced (M. P., _Photo._).]

_b._ ENDOLITHIC LICHENS. In many species, only the lower hyphae penetrate

the substratum either of rock or soil. In a few, more especially

those growing on limestone, the greater part or even the whole of the

vegetative thallus and sometimes also the fruits are, to some extent,

immersed in the rock. It has now been demonstrated that a number

of lichens, formerly described as athalline, possess a considerable

vegetative body which cannot be examined until the limestone in which

they are embedded is dissolved by acids. One such species, _Petractis_

(_Gyalecta_) _exanthematica_, studied by Steiner[325] and later by

Fünfstück[326], is associated with the blue-green filamentous alga,

_Scytonema_, and is homoiomerous in structure, the alga growing through

and permeating the whole of the embedded thallus. A partly homoiomerous

thallus, associated with _Trentepohlia_, has been described by

Bachmann[327]. He found the bright-yellow filaments of the alga covering

the surface of a calcareous rock. By reason of their apical growth,

they pierced the rock and dissolved a way for themselves, not only

among the loose particles, but right through a clear calcium crystal

reaching generally to a depth of about 200µ, though isolated threads

had gone 350µ below the surface. Near the outside the tendency was for

the algae to become stouter and to increase by intercalary growth and

by budded yeast-like outgrowths; lower down they were somewhat smaller.

The hyphae that became united with the algae were unusually slender and

were characterized by frequent anastomoses. They closely surrounded the

gonidia and also filled the loose spaces of the limestone with their fine

thread-like strands. Though oil was undoubtedly present in the lower

hyphae there were no swollen nor sphaeroid cells[328]. Some interesting

experiments with moisture proved that the part of the rock permeated with

the lichen absorbed much more water and retained it longer than the part

that was lichen-free.

Generally the embedded tissues follow the same order as in other

crustaceous lichens: an upper layer of cortical hyphae, next a gonidial

zone, and beneath that an interlaced tissue of medullary or rhizoidal

hyphae which often form fat-cells[328]. Friedrich[329] has given

measurements of the immersed thallus of _Lecanora_ (_Biatorella_)

_simplex_: under a cortical layer of hyphae there was a gonidial zone

600-700µ thick, while the lower hyphae reached a depth of 12 mm.; he has

also recorded an instance of a thallus reaching a depth of 30 mm.

On siliceous rocks such as granite, rhizoidal hyphae penetrate the rock

chiefly between the thin separable flakes of mica. Bachmann[330] has

recognized in these conditions three distinct series of cell-formations:

(1) slender long-celled sparsely branched hyphae which form a network

by frequent anastomoses; (2) further down, though only occasionally,

hyphae with short thick-walled bead-like cells; and (3) beneath these,

but only in or near mica crystals, spherical cells containing oil or some

albuminous substance.

_c._ CHEMICAL NATURE OF THE SUBSTRATUM. Lichens growing on calcareous

rocks or soils are more or less endolithic, those on siliceous rocks

are largely epilithic, but Bachmann[331] found that the mica crystals

in granite were penetrated, much in the same way as limestone, by the

lichen hyphae. These travel through the mica in all directions, though

they tend to follow the line of cleavage, thus taking the direction of

least cohesion. He found that oil-hyphae were formed, and also certain

peculiar bristle-like terminal branches; in other cases there were thin

layers of plectenchyma, and gonidia were also present. If however felspar

or quartz crystals, no matter how thin, blocked the way, further growth

was arrested, the hyphae being unable to pierce through or even to leave

any trace on the quartz[332]. On granite containing no mica constituents

the hyphae can only follow the cracks between the different impenetrable

crystals.

Stahlecker[333] has confirmed Bachmann’s observations, but he considers

that the difference in habit and structure between the endolithic and

epilithic series of lichens is due rather to the chemical than to the

physical nature of the substratum. Thus in a rock of mixed composition

such as granite, the more basic constituents are preferred by the hyphae,

and are the first to be surrounded: mica, when present, is at once

penetrated; particles of hornblende, which contain 40 to 50 per cent.

only of silicic acid, are laid hold of by the filaments of the lichen

before the felspar, of which the acid content is about 60 per cent.;

quartz grains which are pure silica are attacked last of all, though in

the course of time they also become corroded.

The character of the substratum also affects to a great extent the

comparative development of the different thalline layers: the hyphal

tissues in silicicolous lichens are much thinner than in lichens on

limestone, and the gonidial zone is correspondingly wider. In a species

of _Staurothele_ on granite, Stahlecker[333] estimated the gonidial

zone to be about 600 µ thick, while the lower medullary hyphae, partly

burrowing into the rock, measured about 6 mm. Other measurements at

different parts of the thallus gave a rhizoidal depth of 3 mm., while on

a more finely granular substratum, with a gonidial zone of 350 µ, the

rhizoidal hyphae measured only 1-1/2 mm. On calcareous rocks, on the

contrary, with a gonidial zone that is certainly no larger, the hyphal

elements penetrate the rock to varying depths down to 15 mm. or even more.

Lang[334] has recorded equally interesting measurements for _Sarcogyne_

(_Biatorella_) _latericola_: on slaty rock which contained no mixture

of lime, the gonidial zone had a thickness of 80 µ, a considerable

proportion of the very thin thallus. Fünfstück[335] has indeed suggested

that this lichen on acid rocks is only a starved condition of

_Sarcogyne_ (_Biatorella_) _simplex_, which on calcareous rocks, though

with a broader gonidial zone, has, as noted above, a correspondingly much

larger hyphal tissue.

Stahlecker’s theory is that the hyphae require more energy to grow in the

acid conditions that prevail in siliceous rocks, and therefore they make

larger demands on the algal symbionts. It follows that the latter must be

stimulated to more abundant growth than in circumstances favourable to

the fungus, such as are found in basic (calcareous) rocks; he concludes

that on the acid (siliceous) rocks, the epilithic or superficial

condition is not only a physical but a biological necessity, to enable

the algae to grow and multiply in a zone well exposed to light with full

opportunity for active photosynthesis and healthy increase.

C. CORTICOLOUS LICHENS

The crustaceous lichens occurring on bark or on dead wood, like those

on rocks, are either partly or wholly immersed in the substratum

(hypophloeodal), or they grow on the surface (epiphloeodal); but even

those with a superficial crust are anchored by the lower hyphae which

enter any crack or crevice of wood or bark and so securely attach the

thallus, that it can only be removed by cutting away the underlying

substance.

_a._ EPIPHLOEODAL LICHENS. These lichens originate in the same way as the

corresponding epilithic series from soredia or from germinating spores,

and follow the same stages of growth; first a hypothallus with subsequent

colonization of gonidia, the formation of granules, areolae, etc. The

small compartments are formed as primary or secondary areolae; the larger

spaces are marked out by the encounter of hypothalli starting from

different centres.

The thickness of the thallus varies considerably according to the

species. In some _Pertusariae_ with a stoutish irregular crust there is

a narrow amorphous cortical layer of almost obliterated cells, a thin

gonidial zone about 35 µ in width and a massive rather dense medulla

of colourless hyphae. Darbishire[336] has described and figured in

_Varicellaria microsticta_, one of the Pertusariaceae, single hyphae

that extend like beams across the wide medulla and connect the two

cortices. In some _Lecanorae_ and _Lecideae_ there is, on the contrary,

an extremely thin thallus consisting of groups of algae and loose fungal

filaments, which grow over and between the dead cork cells of the

outer bark. On palings, there is often a fairly substantial granular

crust present, with a gonidial zone up to about 80 µ thick, while the

underlying or medullary hyphae burrow among the dead wood fibres.

_b._ HYPOPHLOEODAL LICHENS. These immersed lichens are comparable

with the endolithic species of the rock formations, as their thallus

is almost entirely developed under the outer bark of the tree. They

are recognizable, even in the absence of any fructification, by the

somewhat shining brownish, white or olive-green patches that indicate

the underlying lichen. This type of thallus occurs in widely separated

families and genera, _Lecidea_, _Lecanora_, etc., but it is most constant

in Graphideae and in those Pyrenolichens of which the algal symbiont

belongs to the genus _Trentepohlia_. The development of these lichens is

of peculiar interest as it has been proved that though both symbionts

are embedded in the corky tissues, the hyphae arrive there first, and,

at some later stage, are followed by the gonidia. There is therefore no

question of the alga being a “captured slave” or “unwilling mate.”

Frank[337] made a thorough study of several subcortical forms. He found

that in _Arthonia radiata_, the first outwardly visible indication of the

presence of the lichen on ash bark was a greenish spot quite distinct

from the normal dull-grey colour of the periderm. Usually the spots are

round in outline, but they tend to become ellipsoid in a horizontal

direction, being influenced by the growth in thickness of the tree.

At this early stage only hyphae are present; Bornet[338] as well as

Frank described the outer periderm cells as penetrated and crammed with

the colourless slender filaments. Lindau[339], in a more recent work,

disputes that statement: he found that the hyphae invariably grew between

the dead cork cells, splitting them up and disintegrating the bark, but

never piercing the membranes. The purely prothallic condition, as a weft

of closely entangled hyphae, may last, Frank considers, for a long period

in an almost quiescent condition—possibly for several years—before the

gonidia arrive.

It is always difficult to observe the entrance of the gonidia but they

seem to spread first under the second or third layers of the periderm.

With care it is possible to trace a filament of _Trentepohlia_ from

the surface downwards, and to see that the foremost cell is really the

growing and advancing apex of the creeping alga. Both symbionts show

increased vigour when they encounter each other: the thallus at once

develops in extent and in depth, and, ultimately, reproductive bodies are

formed. In some species the apothecia or perithecia alone emerge above

the bark, in others the outer peridermal cells are thrown off, and the

thallus thus becomes superficial to some extent as a white scurfy or

furfuraceous crust.

The change from a hypophloeodal to a partly epiphloeodal condition

depends largely on the nature of the bark. Frank[337] found that

_Lecanora pallida_ remained for a long time immersed when growing on the

thick rugged bark of oak trunks. When well lighted, or on trees with a

thin periderm, such as the ash, the lichen emerges much earlier and

becomes superficial.

Black (or occasionally white) lines intersect the thallus and mark, as

in saxicolous lichens (Fig. 41), the boundary lines between different

individuals or different species. The pioneer hyphae of certain lichens

very frequently become dark-coloured, and Bitter[340] has suggested

as the reason for this that in damp weather the hypothallic growth is

exceptionally vigorous. When dry weather supervenes, with high winds or

strong sunshine, the outlying hyphae, unprotected by the thallus, become

dark-coloured. On the return of more normal conditions the blackened

tips are thrown off. Bitter further states that species of Graphideae do

not form a permanent black limiting line when they grow in an isolated

position: it is only when their advance is checked by some other thallus

that the dark persistent edge appears, a characteristic also to be seen

in the crust of other lichens. The dark boundary is always more marked

in sunny exposed situations: in the shade, the line is reduced to a mere

thread.

Bitter’s restriction of black boundary lines to cases of encountering

thalli only, would exclude the comparison one is tempted to make between

the advancing hyphae of lichens and those of many woody fungi where the

extreme edge of the white invaded woody tissue is marked by a dark line.

In the latter case however it is the cells of the host that are stained

black by the fungus pigment.

2. SQUAMULOSE LICHENS

A. DEVELOPMENT OF THE SQUAMULE

The crustaceous thallus is more or less firmly adherent to, or confused

with, the substratum. Further advance to a new type of thallus is made

when certain hyphal cells of soredium or granule take the lead in

an ascending direction both upwards and outwards. As growth becomes

definitely apical or one-sided, the structure rises free from the

substratum, and small lobules or leaflet-like squamules are formed. Each

squamule in this type of thallus is distinct in origin and not merely the

branch of a larger whole.

In a few lichens the advance from the crustaceous to the squamulose

structure is very slight. The granules seem but to have been flattened

out at one side, and raised into minute rounded projections such as those

that compose the thallus of _Lecanora badia_ generally described as

“subsquamulose.” The squamulose formation is more pronounced in _Lecidea

ostreata_, and in some species of _Pannaria_; and the whole thallus may

finally consist of small separate lobes as in _Lecidea lurida_, _Lecanora

crassa_, _L. saxicola_, species of _Dermatocarpon_ and the primary

thallus of the _Cladoniae_. Most of these squamules are of a firm texture

and more or less round in outline; in some species of _Cladonia_, etc.,

they are variously crenate, or cut into pinnate-like leaflets. Squamulose

lichens grow mostly on rocks or soil, occasionally on dead wood, and are

generally attached by single rhizoidal hyphae, either produced at all

points of the under surface, or from the base only, growth in the latter

case being one-sided. In a few instances, as in _Heppia Guepini_, there

is a central hold-fast.

A frequent type of squamulose thallus is that termed “placodioid,” or

“effigurate,” in which the squamulose character is chiefly apparent at

the circumference. The thallus is more or less orbicular in outline; the

centre may be squamulose or granular and cracked into areolae; the outer

edge is composed of radiating lobules closely appressed to the substratum

(Fig. 42).

[Illustration: Fig. 42. _Placodium murorum_ DC. Part of placodioid

thallus with apothecia × 2.]

All lichens with this type of thallus were at one time included in the

genus _Placodium_, now restricted by some lichenologists to squamulose

or crustaceous species with polarilocular spores. Many of them rival

_Xanthoria parietina_ in their brilliant yellow colouring.

[Illustration: Fig. 43. _Lecania candicans_ A. Zahlbr., with placodioid

thallus, reduced (S. H., _Photo._).]

There are also greyish-white effigurate lichens such as _Lecanora

saxicola_, _Lecania candicans_ (Fig. 43) and _Buellia canescens_,

well-known British species.

B. TISSUES OF SQUAMULOSE THALLUS

The anatomical structure of the squamules is in general somewhat similar

to that of the crustaceous thallus: an upper cortex, a gonidial zone,

and below that a medullary layer of loose hyphae with sometimes a lower

cortex.

1. The upper cortex, as in crustaceous lichens, is generally of

the “decomposed”[341] or amorphous type: interlaced hyphae with

thick gelatinous walls. A more highly developed form is apparent in

_Parmeliella_ and _Pannaria_ where the upper cortex is formed of

plectenchyma, while in the squamules of _Heppia_ the whole structure is

built up of plectenchyma, with the exception of a narrow band of loose

hyphae in the central pith.

2. The gonidia are Myxophyceae or Chlorophyceae; the squamules in some

instances may be homoiomerous as in _Lepidocollema_, but generally they

belong to the heteromerous series, with the gonidia in a circumscribed

zone, and either continuous or in groups. Friedrich[342] held that,

as in crustaceous lichens the development of the gonidial as compared

with the other tissues depended on the substratum. The squamules of

_Pannaria microphylla_ on sandstone were 100 µ thick, and the gonidial

layer occupied 80 or 90 µ of the whole[343]. With that may be compared

_Placodium Garovagli_ on lime-containing rock: the gonidial layer

measured only 50 µ across, the pith hyphae 280 µ and the rhizoidal hyphae

that penetrated the rock 500 µ.

3. The medullary layer, as a rule, is of closely compacted hyphae which

give solidity to the squamules; in those of _Heppia_ it is almost

entirely formed of plectenchyma.

4. The lower cortex is frequently little developed or absent, especially

when the squamules are closely applied to the support as in some

species of _Dermatocarpon_. In some of the squamulose _Lecanorae_ (_L.

crassa_ and _L. saxicola_) the lowest hyphae are somewhat more closely

interwoven; they become brown in colour, and the lichen is attached to

the substratum by rhizoid-like branches. In _Lecanora lentigera_ there is

a layer of parallel hyphae along the under surface. Further development

is reached when a plectenchyma of thick-walled cells is formed both above

and below, as in _Psoroma hypnorum_, though on the under surface the

continuity is often broken. The squamules of _Cladoniae_ are described

under the radiate-stratose series.

3. FOLIOSE LICHENS

A. DEVELOPMENT OF FOLIOSE THALLUS

The larger leafy lichens are occasionally monophyllous and attached at

a central point as in _Umbilicaria_, but mostly they are broken up into

lobes which are either imbricate and crowded, or represent the dividing

and branching of the expanding thallus at the circumference. They are

horizontal spreading structures, with marginal and apical growth. The

several tissues of the squamule are repeated in the foliose thallus,

but further provision is made to meet the requirements of the larger

organism. There is the greater development of cortical tissue, especially

on the lower surface, and the more abundant formation of rhizoidal organs

to attach the large flat fronds to the support. There are also various

adaptations to secure the aeration of the internal tissues[344].

B. CORTICAL TISSUES

Schwendener[345] was the first who, with the improved microscope, made

a systematic study of the minute structure of lichens. He examined

typical species in genera of widely different groups and described their

anatomy in detail. The most variable and perhaps the most important of

the tissues of lichens is the cortex, which is most fully developed in

the larger thalli, and as the same type of cortical structures recurs in

lichens widely different in affinity as well as in form, it seems well to

group together here the ascertained facts about these covering layers.

_a._ TYPES OF CORTICAL STRUCTURE. Zukal[346], and more recently Hue[347],

have made independent studies in the comparative morphology of the

thallus and have given particular attention to the different varieties of

cortex. They each find that the variations come under a definite series

of types. Zukal recognized five of these:

1. =Pseudoparenchymatous= (plectenchyma): by frequent septation of

regularly arranged hyphae and by coalescence a kind of continuous

cell-structure is formed.

2. =Palisade cells=: the outer elongate ends of the hyphae lie close

together in a direction at right angles to the surface of the thallus and

form a coherent row of parallel cells.

3. =Fibrous=: the cortical hyphae lie in strands of fine filaments

parallel with the surface of the thallus.

4. =Intricate=: hyphae confusedly interwoven and becoming dark in colour

form the lower cortex of some foliose lichens.

These four types, Zukal finds, are practically without interstices in the

tissue and form a perfect protection against excessive transpiration. He

adds yet another form:

5. =A cortex formed of hyphae with dark-coloured swollen cells=, which is

not a protection against transpiration. It occurs among lower crustaceous

forms.

Hue has summed up the different varieties under four types, but as he has

omitted the “fibrous” cortex, we arrive again at five different kinds of

cortical formation, though they do not exactly correspond to those of

Zukal. A definite name is given to each type:

1. =Intricate=: an intricate dense layer of gelatinous-walled hyphae,

branching in all directions, but not coalescent (Fig. 44). This rather

unusual type of cortex occurs in _Sphaerophorus_ and _Stereocaulon_, both

of which have an upright rigid thallus (fruticose).

[Illustration: Fig. 44. _Sphaerophorus coralloides_ Pers. Transverse

section of cortex and gonidial layer near the growing point of a frond ×

600.]

[Illustration: Fig. 45. _Roccella fuciformis_ DC. Transverse section of

cortex near the growing point of a frond × 600.]

2. =Fastigiate=: the hyphae bend outwards or upwards to form the cortex.

A primary filament can be distinguished with abundant branches, all

tending in the same direction; anastomosis may take place between the

hyphae. The end branches are densely packed, though there are occasional

interstices (Fig. 45). Such a cortex occurs in _Thamnolia_; in several

genera of Roccellaceae—_Roccellographa_, _Roccellina_, _Reinkella_,

_Pentagenella_, _Combea_, _Schizopelte_ and _Roccella_—and also in the

crustaceous genus _Dirina_. The fastigiate cortex corresponds with

Zukal’s palisade cells.

3. =Decomposed=: in this, the most frequent type of cortex, the hyphae

that travel up from the gonidial layer become irregularly branched and

frequently septate. The cell-walls of the terminal branches become

swollen into a gelatinous mass, the transformation being brought about by

a change in the molecular constituents of the cell-walls which permits

the imbibition and storage of water. The tissue, owing to the enormous

increase of the wall, is so closely pressed together that the individual

hyphae become indistinct; the cell-lumen finally disappears altogether,

or, at most, is only to be detected in section as a narrow disconnected

dark streak. The decomposed cortex is characteristic of many lichens,

crustaceous (Fig. 46) and squamulose, as well as of such highly developed

genera as _Usnea_, _Letharia_, _Ramalina_, _Cetraria_, _Evernia_ and

certain _Parmeliae_.

[Illustration: Fig. 46. _Lecanora glaucoma_ var. _corrugata_ Nyl.

Vertical section of cortex × 500 (after Hue).]

Zukal took no note of the decomposed cortex but the omission is

intentional and is due to his regarding the structure of the youngest

stages of the thallus near the growing point as the most typical and as

giving the best indication as to the true arrangement of hyphae in the

cortex. He thus describes palisade tissue as the characteristic cortex

of _Evernia_, since the formation near the growing point of the fronds

is somewhat palisade-like and he finds fibrous cortex at the tips of

_Usnea_ filaments. In both these instances Hue has described the cortex

as decomposed because he takes account only of the fully formed thallus

in which the tissues have reached a permanent condition.

[Illustration: Fig. 47. _Peltigera canina_ DC. Vertical section of cortex

and gonidial zone × 600.]

4. =Plectenchymatous=: the last of Hue’s types corresponds with the

first described by Zukal. It is the result of the lateral coherence and

frequent septation of the hyphae into short almost square or rounded

cells (Fig. 47). The simplest type of such a cortex can be studied in

_Leptogium_, a genus of gelatinous lichens in which the tips of the

hyphae are cut off at the surface by one or more septa. The resulting

cells are wider than the hyphae and they cohere together to form, in

some species, disconnected patches of cells; in others, a continuous

cortical covering one or more cells thick, while in the margin of the

apothecium they form a deep cellular layer. The cellular type of cortex

is found also, as already stated, in some crustaceous _Pertusariae_, and

in a few squamulose genera or species. It forms the uppermost layer of

the _Peltigera_ thallus and both cortices of many of the larger foliose

lichens such as _Sticta_, _Parmelia_, etc.

5. The “fibrous” cortex must be added to this series, as was pointed

out by Heber Howe[348] who gave the less appropriate designation of

“simple” to the type. It consists of long rather sparingly branched

slender hyphae that grow in a direction parallel with the surface of the

thallus (Fig. 48). It is characteristic of several fruticose and foliose

lichens with more or less upright growth, such as we find in several of

the _Physciae_, and in the allied genus _Teloschistes_, in _Alectoria_,

several genera of Roccellaceae, in _Usnea_ _longissima_ and in _Parmelia

pubescens_, etc. Zukal would have included all the _Usneae_ as the tips

are fibrous.

[Illustration: Fig. 48. _Physcia ciliaris_ DC. Vertical section of

thallus. _a_, cortex; _b_, gonidial zone; _c_, medulla. × 100.]

More than one type of cortex, as already stated, may appear in a genus: a

striking instance of variability occurs in _Solorina_ where, as Hue[349]

has pointed out, the cortex of _S. octospora_ is fastigiate, that of

all the other species being plectenchymatous. Cortical development is a

specific rather than a generic characteristic.

_b._ ORIGIN OF VARIATION IN CORTICAL STRUCTURE. The immediate causes

making for differentiation in cortical development are: the prevailing

direction of growth of the hyphae as they rise from the gonidial zone;

the amount of branching and the crowding of the filaments; the frequency

of septation; and the thickening or degeneration of the cell-walls which

may become almost or entirely mucilaginous. In the plectenchymatous

cortex, the walls may remain quite thin and the cells small as in

_Xanthoria parietina_, or the walls may be much thickened as in both

cortices of _Sticta_. As a result of stretching the cell may increase

enormously in size: in some instances where the internal hyphae are about

3 µ to 4 µ in width, the cortical cells formed from these hyphae may have

a cell cavity 15 µ to 16 µ in diameter.

_c._ LOSS AND RENEWAL OF CORTEX. Very frequently the cortex is covered

over by a layer of homogeneous mucilage which forms an outer cuticle.

It arises from the continual degeneration of the outer cell-walls and

it is liable to friction and removal by atmospheric agency as was

first described by Schwendener[350] in the weather-beaten cortex of

_Umbilicaria pustulata_. He had noted the irregular jagged outline of

the cross section of the thallus, and he then suggested, as the probable

reason, the decay of the outer rind with the constant renewal of it

by the hyphae from the underlying gonidial zone, though he was unable

definitely to prove his theory. The peeling of the dead outer layer (with

its replacement by new tissue) has however been observed many times

since his day. It has been described by Darbishire[351] in _Pertusaria_:

in that genus there is at first a primary cortex formed of hyphae that

grow in a radial direction, parallel to the surface of the thallus. The

walls of these hyphae become gradually more and more mucilaginous till

the cells are obliterated. Meanwhile short-celled filaments grow up in

serried ranks from the gonidial layer and finally push off the dead

“fibrous” cortex. The new tissue takes on a plectenchymatous character,

and the outer cells in time become decomposed and provide a mucilaginous

cuticle which in turn is also subject to wasting.

The same process of peeling was noted by Rosendahl[352] in some species

of brown _Parmeliae_, where the dead tissues were thrown off in shreds,

though only in isolated patches. But whether in patches or as a

continuous sheath, there is constant degeneration, with continual renewal

of the dead material from the internal tissues.

The cortex is the most highly developed of all the lichen structures

and is of immense importance to the plant as may be judged from the

various adaptations to different needs[353]. The cortical cell-walls

are frequently impregnated with some dark-coloured substance which, in

exposed situations, must counteract the influence of too direct sunlight

and be of service in sheltering the gonidia. Lichen acids—sometimes very

brightly coloured—and oxalic acid are deposited in the cortical tissues

in great abundance and aid in retaining moisture; but the two chief

functions to which the cortex is specially adapted are the checking

of transpiration and the strengthening of the thallus against external

strains.

_d._ CORTICAL HAIRS OR TRICHOMES. Though somewhat rare, cortical hairs

are present on the upper surface of several foliose lichens. They take

rise, in all the instances noted, as a prolongation of one of the

cell-rows forming a plectenchymatous cortex.

In _Peltidea_ (_Peltigera_) _aphthosa_ they are especially evident near

the growing edges of the thallus; and they take part in the development

of the superficial cephalodia[354] which are a constant feature of the

lichen. They tend to disappear with age and leave the central older

parts of the thallus smooth and shining. In several other species of

_Peltigera_ (_P. canina_, etc.) they are present and persist during the

life of the cortex. In these lichens the cells of the cortical tissue are

thin-walled, all except the outer layer, the membranes of which are much

thicker. The hairs rising from them are also thick-walled and septate.

Generally they branch in all directions and anastomose with neighbouring

hairs so that a confused felted tangle is formed; they vary in size but

are, as a rule, about double the width of the medullary hyphae as are the

cortical cells from which they rise. They disappear from the thallus,

frequently in patches, probably by weathering, but over large surfaces,

and especially where any inequality affords a shelter, they persist as a

soft down.

Hairs are also present on the upper surface of some _Parmeliae_.

Rosendahl[355] has described and figured them in _P. glabra_ and _P.

verruculifera_—short pointed unbranched hyphae, two or more septate and

with thickened walls. They are most easily seen near the edge of the

thallus, though they persist more or less over the surface; they also

grow on the margins of the apothecia. In _P. verruculifera_ they arise

from the soredia; in _P. glabra_ a few isolated hairs are present on the

under surface.

In _Nephromium tomentosum_ there is a scanty formation of hairs on the

upper surface. They are abundant on the lower surface, and function as

attaching organs. A thick tomentum of hairs is similarly present on the

lower surface of many of the Stictaceae either as an almost unbroken

covering or in scattered patches. In several species of _Leptogium_ they

grow out from the lower cortical cells and attach the thin horizontal

fronds; and very occasionally they are present in _Collema_.

C. GONIDIAL TISSUES

With the exception of some species of _Collema_ and _Leptogium_ lichens

included under the term foliose, are heteromerous in structure, and the

algae that form the gonidial zone are situated below the upper cortex

and, therefore, in the most favourable position for photosynthesis.

Whether belonging to the Myxophyceae or the Chlorophyceae, they form a

green band, straight and continuous in some forms, in others somewhat

broken up into groups. In certain species they push up at intervals among

the cortical cells, as in _Gyrophora_ and in _Parmelia tristis_. In

_Solorina crocea_ a regular series of gonidial pyramids rises towards the

upper surface. The green cells are frequently more dense at some points

than at others, and they may penetrate in groups well into the medulla.

The fungal tissue of the gonidial zone is composed of hyphae which

have thinner walls, and are generally somewhat loosely interlacing.

In _Peltigera_[356] the gonidial hyphae are so connected by frequent

branching and by anastomosis that a net-like structure is formed, in the

meshes of which the algae—a species of _Nostoc_—are massed more or less

in groups. In lichens with a plectenchymatous cortex, the cellular tissue

may extend downwards into the gonidial zone and the gonidia thus become

enmeshed among the cells, a type of formation well seen in the squamulose

species, _Dermatocarpon lachneum_ and _Heppia Guepini_, where the massive

plectenchyma of both the upper and lower cortices encroaches on the pith.

In _Endocarpon_ and in _Psoroma_ the gonidia are also surrounded by short

cells.

A similar type of structure occurs in _Cora Pavonia_, one of the

Hymenolichenes: the gonidial hyphae in that species form a cellular

tissue in which are embedded the blue-green _Chroococcus_ cells[357].

D. MEDULLA AND LOWER CORTEX

_a._ MEDULLA. The hyphal tissue of the dorsiventral thallus that lies

between the gonidial zone and the lower cortex or base of the plant is

always referred to as the medulla or pith. It is, as a rule, by far the

most considerable portion of the thallus. In _Parmelia caperata_ (Fig.

49), for instance, the lobes of which are about 300 µ thick, over 200 µ

of the space is occupied by this layer. It varies however very largely

in extent in different lichens according to species, and also according

to the substratum. In another _Parmelia_ with a very thin thallus, _P.

alpicola_ growing on quartzite, the medulla measures scarcely twice the

width of the gonidial zone. It forms a fairly massive tissue in some of

the crustaceous lichens—in some _Pertusariae_ and _Lecanorae_—attaining a

width of about 600 µ.

Nylander[358] distinguished three types of medullary tissue in lichens:

(1) _felted_, which includes all those of a purely filamentous structure;

(2) _cretaceous_ or _tartareous_, more compact than the felted, and

containing granular or crystalline substances as in some _Pertusariae_;

and lastly

(3) the _cellular_ medulla in which the closely packed hyphae are

divided into short cells and a kind of plectenchyma is formed, as in

_Lecanora_ (_Psoroma_) _hypnorum_, in _Endocarpon_, etc.

[Illustration: Fig. 49. _Parmelia caperata_ Ach. (S. H., _Photo._).]

The felted medulla is characteristic of most lichens and is formed

of loose slender branching septate hyphae with thickish walls. This

interwoven hyphal texture provides abundant air-spaces.

Hue[359] has noted that the walls of the medullary hyphae in _Parmeliae_

are smooth, unless they have been exposed to great extremes of heat or

cold, when they become wrinkled or scaly. They are very thick-walled in

_Peltigera_ (Fig. 50).

[Illustration: Fig. 50. Hyphae from lower medulla of _Peltigera canina_

DC. × 600.]

_b._ LOWER CORTEX. In some foliose lichens such as _Peltigera_ there is

no special tissue developed on the under surface. In _Lobaria pulmonaria_

large patches of the under surface are bare, and the medulla is exposed

to the outer atmosphere, sheltered only by its position. In some other

lichens the lowermost hyphae lie closer together and a kind of felt of

almost parallel filaments is formed, generally darker in colour, as in

_Lecanora lentigera_, and in some species of _Physcia_.

Most frequently however the tissues of the upper cortex are repeated

on the lower surface, though differing somewhat in detail. In all of

the brown _Parmeliae_, according to Rosendahl[360], the structure is

identical for both cortices, though the upper develops now hairs, now

isidia, breathing pores, etc., while the lower produces rhizinae. The

amorphous mucilaginous cuticle so often present on the upper surface

is absent from the lower, the walls of the latter being often charged

instead with dark-brown pigments.

_c._ HYPOTHALLIC STRUCTURES. An unusual development of hyphae from the

lower cortex occurs in the genera _Anzia_ and _Pannoparmelia_—both

closely related to _Parmelia_—whereby a loose sponge-like hypothallus of

anastomosing reticulate strands is formed. In one of the simpler types,

_Anzia colpodes_, a North American species, the hyphae passing out from

the lower medulla become abruptly dark-brown in colour, and are divided

into short thick-walled cells. Frequent branching and anastomosis of

these hyphae result in the formation of a cushion-like structure about

twice the bulk of the thallus. In another species from Australia (_A.

Japonica_) there is a lower cortex, distinct from the medulla, consisting

of septate colourless hyphae with thick walls. From these branch out free

filaments, similar in structure but dark in colour, which branch and

anastomose as in the previous species.

[Illustration: Fig. 51. _Pannoparmelia anzioides_ Darb. Vertical

section of thallus and hypothallus. _a_, cortex; _b_, gonidial zone;

_c_, medulla; _d_, lower cortex; _e_, hypothallus. × ca. 450 (after

Darbishire).]

In _Pannoparmelia_ the lower cortex and the outgrowths from it are

several cells thick; they may be thick-walled as in _Anzia_, or they

may be thin-walled as described and figured by Darbishire[361] in

_Pannoparmelia anzioides_, a species from Tierra del Fuego (Fig. 51). A

somewhat dense interwoven felt of hyphae occurs also in certain parts of

the under surface of _Parmelia physodes_[362].

This peculiar structure, regarded as a hypothallus, is probably of

service in the retention of moisture. The thick cell-walls in most of the

forms suggest some such function.

E. STRUCTURES FOR PROTECTION AND ATTACHMENT

Such structures are almost wholly confined to the larger foliose and

fruticose lichens and are all of the same simple type; they are fungal in

origin and very rarely are gonidia associated with them.

[Illustration: Fig. 52. _Usnea florida_ Web. Ciliate apothecia (S. H.,

_Photo._).]

_a._ CILIA. In a few widely separated lichens stoutish cilia are borne,

mostly on the margins of the thallus lobes, or on the margins of the

apothecia (Fig. 52). They arise from the cortical cells or hyphae,

several of which grow out in a compact strand which tapers gradually

to a point. Cilia vary in length up to about 1 cm. or even longer. In

some lichens they retain the colour of the cortex and are greyish

or whitish-grey, as in _Physcia ciliaris_ or in _Physcia hispida_

(Fig. 110). They provide a yellow fringe to the apothecia of _Physcia

chrysophthalma_ and a green fringe to those of _Usnea florida_. They

are dark-brown or almost black in _Parmelia perlata_ var. _ciliata_ and

in _P. cetrata_, etc. as also in _Gyrophora cylindrica_. The fronds of

_Cetraria islandica_ and other species of the genus are bordered with

short spinulose brown hairs whose main function seems to be the bearing

of “pycnidia” though in many cases they are barren (Fig. 128).

Superficial cilia are more rarely formed than marginal ones, but they

are characteristic of one not uncommon British species, _Parmelia

proboscidea_ (_P. pilosella_ Hue). Scattered over the surface of that

lichen are numerous crowded groups of isidia which, frequently, are

prolonged upwards as dark-brown or blackish cilia. Nearly every isidium

bears a small brown spot on the apex at an early stage of growth. Similar

cilia are sparsely scattered over the thallus, but their base is always

a rather stouter grey structure, which suggests an isidial origin. Cilia

also occur on the margin of the lobes.

As lichens are a favourite food of snails, insects, etc., it is

considered that these structures are protective in function, and that

they impede, if they do not entirely prevent, the larger marauders in

their work of destruction.

[Illustration: Fig. 53. Rhizoid of _Parmelia exasperata_ Carroll (_P.

aspidota_ Rosend.). A, hyphae growing out from lower cortex × 450. B, tip

of rhizoid with gelatinous sheath × 335 (after Rosendahl).]

_b._ RHIZINAE. Lichen rootlets are mainly for the purpose of attachment

and have little significance as organs of absorption. They have been

noted in only one crustaceous lichen, _Varicellaria microsticta_[363],

an alpine species that spreads over bark or soil, and which is further

distinguished by being provided with a lower cortex of plectenchyma.

In foliose lichens they are frequently abundant, though by no means

universal, and attach the spreading fronds to the support. They

originate, as Schwendener[364] pointed out, from the outer cortical

cells, exactly as do the cilia, and are scattered over the under

surface or are confined to special areas. Rosendahl[365] has described

their development in the brown species of _Parmeliae_: the under cortex

in these lichens is formed of a cellular plectenchyma with thickish

walls; the rootlets arise by the outgrowth of several neighbouring cells

from some slight elevation near the edge of the thallus. Branching and

interlacing of these growing rhizinal hyphae follow, the outermost

frequently spreading outwards at right angles to the axis, and forming

a cellular cortex. The apex of the rhizoid is generally an enlarged

tuft of loose hyphae involved in mucilage (Fig. 53), a provision for

securing firmer cohesion to the support; or the tips spread out as a

kind of sucker. Not unfrequently neighbouring “rootlets” are connected

by mucilage at the tips, or by outgrowths of their hyphae, and a rather

large hold-fast sheath is formed.

[Illustration: Fig. 54. _Peltigera canina_ DC. (S. H., _Photo._).]

[Illustration: Fig. 55. _Peltigera canina_ DC. Under surface with veins

and rhizoids (after Reinke).]

In species of _Peltigera_ (Fig. 54) the rhizinae are confined to the

veins or ridges (Fig. 55); they are thickish at the base, and are

generally rather long and straggling. Meyer[366] states that the central

hyphae are stoutish and much entangled owing to the branching and

frequent anastomosis of one hypha with another; the peripheral terminal

branches are thinner-walled and free. These rhizinae vary in colour from

white in _Peltigera canina_ to brown or black in other species. Most

species of _Peltigera_ spread over grass or mosses, to which they cling

by these long loose “rootlets.”

Lichen rhizinae, distinguished by Reinke[367] as “aerial rhizinae,” are

more or less characteristic of all the species of _Parmelia_ with the

exception of those belonging to the subgenus _Hypogymnia_ in which they

are of very rare occurrence, arising, according to Bitter[368], only in

response to some external friction. They are invariably dark-coloured,

rather short, about one to a few millimetres in length, and are simple

or branched. The branches may go off at any angle and are sometimes

curved back at the ends in anchor-like fashion. The _Parmeliae_ grow on

firm substances, trees, rocks, etc., and the irregularities of their

attaching structures are conditioned by the obstacles encountered on the

substratum. Not unfrequently the lobes are attached by the rhizinae to

underlying portions of the thallus.

In the genus _Gyrophora_, the rhizinae are simple strands of hyphae

(_G. polyrhiza_) or they are corticate structures (_G. murina_, _G.

spodochroa_ and _G. vellea_). They are also present in species of

_Solorina_, _Ricasolia_, _Sticta_ and _Physcia_ and very sparingly in

_Cetraria_ (_Platysma_).

_c._ HAPTERA. Sernander[369] has grouped all the more distinctively

aerial organs of attachment, apart from rhizinae, under the term

“hapteron” and he has described a number of instances in which cilia and

even the growing points of the thallus may become transformed to haptera

or sucker-like sheaths.

The long cilia of _Physcia ciliaris_ occasionally form haptera at their

tips where the hyphae are loose and in active growing condition. Contact

with some substance induces branching by which a spreading sheath arises;

a plug-like process may also be developed which pierces the substance

encountered—not unfrequently another lobe of its own thallus. The long

flaccid fronds of _Evernia furfuracea_ are frequently connected together

by bridge-like haptera which rise at any angle of the thallus or from any

part of the surface.

The spinous hairs that border the thalline margins in _Cetraria_ may

also, in contact with some body—often another frond of the lichen—form a

hapteron, either while the spermogonium, which occupies the tip of the

spine, is still in a rudimentary stage, or after it has discharged its

spermatia. The small sucker sheath may in that case arise either from

the apex of the cilium, from the wall of the spermogonium or from its

base. By means of these haptera, not only different individuals become

united together, but instances are given by Sernander in which _Cetraria

islandica_, normally a ground lichen, had become epiphytic by attaching

itself in this way to the trunk of a tree (_Pinus sylvestris_).

In _Alectoria_, haptera are formed at the tip of the thallus filament as

an apical cone-like growth from which hyphae may branch out and penetrate

any convenient object. A species of this genus was thus found clinging

to stems of _Betula nana_. Apical haptera are very frequent in _Cladonia

rangiferina_ and _Cl. sylvatica_, induced here also by contact. These

two plants, as well as several species of _Cetraria_, tend, indeed, to

become entirely epiphytic on the heaths of the _Calluna_ formations.

Haptera similar to those of _Alectoria_ occur in _Usnea_, _Evernia_,

_Ramalina_ and _Cornicularia_ (_Cetraria_). In _Evernia prunastri_ var.

_stictoceros_, a heath form, the fronds become attached to the stems and

branches of _Erica tetralix_ by hapteroid strands of slender glutinous

hyphae which persist on the frond of the lichen after it is detached

as small very dark tubercles surmounted, as Parfitt[370] pointed out,

by a dark-brown grumous mass of cells. Plug-like haptera may be formed

at the base of _Cladoniae_ which attach them to each other and to the

substratum. The brightly coloured fronds of _Letharia vulpina_ are

attached to each other in somewhat tangled fashion by lateral bridges

or by fascicles of hyphae dark-brown at the base but colourless at the

apices, exactly like aerial adventitious rhizinae. They grow out from the

fronds generally at or near the tips and lay hold of a neighbouring frond

by means of mucilage. These haptera are evidently formed in response

to friction. Haptera along with other lichen attachments have received

considerable attention from Galløe[371]. He finds them arising on various

positions of the lichen fronds and has classified them accordingly.

After the haptera have become attached, they increase in size and

strength and supply a strong anchorage for the plant; the point of

contact frequently forms a basis for renewed growth while the part

beneath the hapteron may gradually die off. Haptera are more especially

characteristic of fruticose lichens, but Sernander considers that the

rhizinae of foliose species may function as haptera. They are important

organs of tundra and heath formations as they enable the lichens to get a

foothold in well-lighted positions, and by their aid the fronds are more

able to resist the extreme tearing strains to which they are subjected in

high and unsheltered moorlands.

F. STRENGTHENING TISSUES OF STRATOSE LICHENS

Squamulose and foliose lichens grow mostly in close relation with the

support, and the flat expanding thallus, as in the _Parmeliae_, is

attached at many points to the substance—tree, rock, etc.—over which the

plants spread. Special provision for support is therefore not required,

and the lobes remain thin and flaccid. Yet, in a number of widely

different genera the attachment to the substratum is very slight, and in

these we find an adaptation of existing tissues fitted to resist tearing

strains, resistance being almost invariably secured by the strengthening

of the cortical layers.

_a._ BY DEVELOPMENT OF THE CORTEX. Such a transformation of tissue is

well illustrated in _Heppia Guepini_. The thallus consists of rigid

squamules which are attached at one point only; the cortex of both

surfaces is plectenchymatous and very thick and even the medulla is

largely cellular.

The much larger but equally rigid coriaceous thallus of _Dermatocarpon

miniatum_ (Fig. 56) has also a single central attachment or umbilicus,

and both cortices consist of a compact many-layered plectenchyma. The

same structure occurs in _Umbilicaria pustulata_ and in some species of

_Gyrophora_, which, having only a single central hold-fast, gain the

necessary stiffening through the increase of the cortical layers.

[Illustration: Fig. 56. _Dermatocarpon miniatum_ Th. Fr. (S. H.,

_Photo._).]

In the Stictaceae there are a large number of widely-expanded forms,

and as the attachment depends mostly on a somewhat short tomentum,

strength is obtained here also by the thick plectenchymatous cortex of

both surfaces. When areas denuded of tomentum and cortex occur, as in

_Lobaria pulmonaria_, the under surface is not sensibly weakened, since

the cortical tissue remains connected in a stout and firm reticulation.

_b._ BY DEVELOPMENT OF VEINS OR NERVES. Certain ground lichens belonging

to the Peltigeraceae have a wide spreading thallus often with very large

lobes. The upper cortex is a many-layered plectenchyma, but the under

surface is covered only by a loose felt of hyphae which branch out into

a more or less dense tomentum. As the firm upper cortex continues to

increase by intercalary growth from the branching upwards of hyphae

from the meristematic gonidial zone, there occurs an extension of the

upper thallus with which the lower cannot keep pace[372]. A little

way back from the edge, the result of the stretching is seen in the

splitting asunder of the felted hyphae of the under surface, and in

the consequent formation of a reticulate series of ridges known as the

veins or nerves; they represent the original tomentose covering, and are

white, black or brown, according to the colour of the tomentum itself.

The naked ellipsoid interstices show the white medulla, and, if the veins

are wide, the colourless areas are correspondingly small. Rhizinae are

formed on the nerves in several of the species, and anchor the thallus to

the support. In _Peltigera canina_, the under surface is almost wholly

colourless, the veins are very prominent (Fig. 55), and are further

strengthened by the growth and branching of the parallel hyphae of which

they are composed. They serve to strengthen the large and flabby thallus

and form a rigid base for the long rhizinae by which the lichen clings to

the grass or moss over which it grows.

The most perfect development of strengthening nerves is to be found in

_Hydrothyria venosa_[373], a rather rare water lichen that occurs in

the streams of North America. It consists of fan-like lobes of thin

structure, the cortex being only about one cell thick. The fronds are

about 3 cm. wide and they are contracted below into a stalk which serves

to attach the plant to the substratum. Several fronds may grow together

in a dense tuft, the expanded upper portion floating freely in the water.

Frequently the plants form a dense growth over the rocky beds of the

stream.

At the point where the stalk expands into the free erect frond, there

arise a series of stout veins which spread upwards and outwards. They

are definitely formed structures and not adaptations of pre-existing

tissues: certain hyphae arise from the medulla at the contracted base of

the frond, take a radial direction and, by increase, become developed

into firm strands. The individual hyphae also increase in size, and the

swelling of the nerve gives rise to a ridge prominent on both surfaces.

They seldom anastomose at first but towards the tips they become smaller

and spread out in delicate ramifications which unite at various points.

There is no doubt, as Bitter[372] points out, that the nerves function as

strengthening tissues and preserve the frond from the strain of the water

currents which would, otherwise, tear apart the delicate texture.

III. RADIATE THALLUS

1. CHARACTERS OF RADIATE THALLUS

In the stratose dorsiventral thallus, there is a widely extended growing

area situated round the free margins of the thallus. In the radiate

thallus of the fruticose or filamentous lichens, growth is confined to an

apical region. Attachment to the substratum is at one point only—the base

of the plant—thus securing the exposure of all sides equally to light.

The cortex surrounds the fronds, and the gonidia (mostly Protococcaceae)

lie in a zone or in groups between the cortex and the medulla. It is

the highest type of vegetative development in the lichen kingdom, since

it secures the widest room for the gonidial layer, and the largest

opportunity for photosynthesis.

[Illustration: Fig. 57. _Roccella fuciformis_ DC.]

Shrubby upright lichens consist mostly of strap-shaped fronds, either

simple or branched, which may be broadened to thin bands (Fig. 57) or may

be narrowed and thickened till they are almost cylindrical. The fronds

vary in length according to the species from a few millimetres upwards:

those of _Roccella_ have been found measuring 30 cm. in length; those of

_Ramalina reticulata_, the largest of all the American lichens, extend to

considerably more.

Lichens of filamentous growth are more or less cylindrical (Fig. 58).

They are in some species upright and of moderate length, but in a

few pendulous forms they grow to a great length: specimens of _Usnea

longissima_ have been recorded that measured 6 to 8 metres from base to

tip.

[Illustration: Fig. 58. _Usnea barbata_ Web. (S. H., _Photo._).]

The radiate type of thallus occurs in most of the lichen groups but

most frequently in the Gymnocarpeae. In gelatinous Discolichens it is

represented in the Lichinaceae. It is rare among Pyrenocarpeae: there is

one very minute British lichen in that series, _Pyrenidium actinellum_,

and one from N. America, _Pyrenothamnia_, that are of fruticose habit.

2. INTERMEDIATE TYPES OF THALLUS

Between the foliose and the fruticose types, there are intermediate forms

that might be, and often are, classified now in one group and now in

the other. These are chiefly: _Physcia_ (_Anaptychia_) _ciliaris_, _Ph.

leucomelas_ and the species of _Evernia_.

In the two former the habit is more or less fruticose as the plants are

affixed to the substratum at a basal point, but the fronds are decumbent

and the internal structure is of the dorsiventral type: there is an

upper “fibrous” cortex of closely compacted parallel hyphae, a gonidial

zone—the gonidia lying partly in the cortex and partly among the loose

hyphae of the medulla—and a lower cortex formed of a weft of hyphae which

also run somewhat parallel to the surface. Both species are distinguished

by the numerous marginal cilia, either pale or dark in colour. These two

lichens are greyish-coloured on the upper surface and greyish or whitish

below.

_Evernia furfuracea_ with a basal attachment[374], and with a partly

horizontal and partly upright growth, has a dorsiventral thallus, dark

greyish-green above and black beneath, with occasional rhizinae towards

the base. The cortex of both surfaces belongs to the “decomposed” type;

the gonidial zone lies below the upper surface, and the medullary tissue

is of loose hyphae. In certain forms of the species isidia are abundant

on the upper surface, a character of foliose rather than of fruticose

lichens. _E. furfuracea_ grows on trees and very frequently on palings.

[Illustration: Fig. 59. _Evernia prunastri_ Ach. (M. P., _Photo._).]

_E. prunastri_, the second species of the genus, is more distinctly

upright in habit, with a penetrating basal hold-fast and upright

strap-shaped branching fronds, light-greyish green on the “upper” surface

and white on the other (Fig. 59). The internal structure is sub-radiate;

both cortices are “decomposed”; the gonidial zone consists of somewhat

loose groups of algae, very constant below the “upper” surface, with an

occasional group in the pith near to the lower cortex in positions that

are more exposed to light. There is also a tendency for the gonidial zone

to pass round the margin and spread some way along the under side. The

medulla is of loose arachnoid texture and the whole plant is very limp

when moist. It grows on trees, often in dense clusters.

3. FRUTICOSE AND FILAMENTOUS

A. GENERAL STRUCTURE OF THALLUS

The conditions of strain and tension in the upright plant are entirely

different from those in the decumbent thallus, and to meet the new

requirements, new adaptations of structure are provided either in the

cortex or in the medulla.

CORTICAL STRUCTURES. With the exception of the distinctly

plectenchymatous cortex, all the other types already described recur in

fruticose lichens; in various ways they have been modified to provide not

only covering but support to the fronds.

_a._ =The fastigiate cortex.= This reaches its highest development in

_Roccella_ in which the branched hyphal tips, slightly clavate and

thick-walled, lie closely packed in palisade formation at right angles

to the main axis (Fig. 45). They afford not only bending power, but give

great consistency to the fronds. The cortex is further strengthened

in _R. fuciformis_[375] by the compact arrangement of the medullary

hyphae that run parallel with the surface, and among which occur single

thick-walled filaments. The plant grows on maritime rocks in very exposed

situations; and the narrow strap-shaped fronds, as stated above, may

attain a length of 30 cm., though usually they are from 10 to 18 cm. in

height. The same type of cortex, but less highly differentiated, affords

a certain amount of stiffness to the cylindrical much weaker fronds of

_Thamnolia_.

_b._ =The fibrous cortex.= This type is found in a number of lichens

with long filamentous hanging fronds. It consists of parallel hyphae,

rarely septate and rarely branched, but frequently anastomosing and

with strongly thickened “sclerotic” walls. Such a cortex is the only

strengthening element in _Alectoria_, and it affords great toughness and

flexibility to the thong-like thallus. It is also present in _Ramalina_

(_Alectoria_) _thrausta_, a species with slender fronds (Fig. 60).

[Illustration: Fig. 60. _Alectoria thrausta_ Ach. A, transverse section

of frond; _a_, cortex; _b_, gonidia; _c_, arachnoid medulla × 37. B,

fibrous hyphae from longitudinal section of cortex. × 430 (after Brandt).]

In _Usnea longissima_ the cortex both of the fibrillose branchlets and of

the main axis is fibrous, and is composed of narrow thick-walled hyphae

which grow in a long spiral round the central strand. The hyphae become

more frequently septate further back from the apex (Fig. 61). Such a type

of cortex provides an exceedingly elastic and efficient protection for

the long slender thallus.

[Illustration: Fig. 61. _Usnea longissima_ Ach. Longitudinal sections of

outer cortex. A, near the apex; B, the middle portion of a fibril. × 525

(after Schulte).]

The same type of cortex forms the strengthening element in the fruticose

or partly fruticose members of the family Physciaceae. One of these,

_Teloschistes flavicans_, is a bright yellow filamentous lichen with a

somewhat straggling habit. The fronds are very slender and are either

cylindrical or slightly flattened. The hyphae of the outer cortex are

compactly fibrous; added toughness is given by the presence of some

longitudinal strands of hyphae in the central pith.

Another still more familiar grey lichen, _Physcia ciliaris_, has long

flat branching fronds which, though dorsiventral in structure, are partly

upright in habit. Strength is secured as in _Teloschistes_ by the fibrous

upper cortex. Other species of _Physciae_ are somewhat similar in habit

and in structure.

In _Dendrographa leucophaea_, a slender strap-shaped rock lichen,

Darbishire[376] has described the outer cortex as composed of closely

compacted parallel hyphae resembling the strengthening cortex of

_Alectoria_ and very different from the fastigiate cortex of the

_Roccellae_ with which it is usually classified.

B. SPECIAL STRENGTHENING STRUCTURES

_a._ SCLEROTIC STRANDS. This form of strengthening tissue is

characteristic of _Ramalina_. With the exception of _R. thrausta_ (more

truly an _Alectoria_) all the species have a rather weak cortical layer

of branching intricate thick-walled hyphae, regarded by Brandt[377] as

plectenchymatous, but more correctly by Hue[378] as “decomposed” on

account of the gelatinous walls and diminishing lumen of the irregularly

arranged cells.

[Illustration: Fig. 62. _Ramalina minuscula_ Nyl. A, transverse section

of frond × 37; B, longitudinal strengthening hyphae of inner cortex × 430

(after Brandt).]

In _R. evernioides_, a plant with very wide flat almost decumbent fronds

of soft texture, in _R. ceruchis_ and in _R. homalea_ there is a somewhat

compact medulla which gives a slight stiffness to the thallus. The

other species of the genus are provided with strengthening mechanical

tissue within the cortex formed of closely united sclerotic hyphae that

run parallel to the surface (Fig. 62). In a transverse section of the

thallus, this tissue appears sometimes as a continuous ring which may

project irregularly into the pith (_R. calicaris_); more frequently it

is in the form of strands or bundles which alternate with the groups of

gonidia (_R. siliquosa_, _R. Curnowii_, etc.). In _R. fraxinea_ these

strands may be scarcely discernible in young fronds, though sometimes

already well developed near the tips. Occasionally isolated strands of

fibres appear in the pith (_R. Curnowii_), or the sclerotic projections

may even stretch across the pith to the other side (_R. strepsilis_)

(Fig. 75 B).

In the _Cladoniae_ support along with flexibility is secured to the

upright podetium by the parallel closely packed hyphae that form round

the hollow cylinder a band called the “chondroid” layer from its

cartilage-like consistency.

_b._ CHONDROID AXIS. The central medullary tissue in _Ramalina_ is, with

few exceptions, a loose arachnoid structure; often the fronds are almost

hollow. In one species of _Usnea_, _U. Taylori_, found in polar regions,

there is a similar loose though very circumscribed medullary and gonidial

tissue in the centre of the somewhat cylindrical thallus, and a wide band

of sclerotic fibres towards the cortex.

[Illustration: Fig. 63 A. A, _Usnea barbata_ Web. Longitudinal section

of filament with young adventitious branch. _a_, chondroid axis;

_b_, gonidial tissue; _c_, cortex. × 100 (after Schwendener). B, _U.

longissima_ Ach. Hyphae from central axis × 525 (after Schulte).]

In all other species of _Usnea_ the medulla itself is transformed into

a strong central strand of long-celled thick-walled hyphae closely knit

together by frequent anastomoses (Fig. 63 A). This central strand of

the Usneas is known as the “chondroid axis.” A narrow band of loose

air-containing hyphae and a gonidial zone lie round the central axis

between it and the outer cortex (Fig. 63 A, _b_). At the extreme apex,

the external cortical hyphae grow in a direction parallel with the long

axis of the plant, but further back, they branch out at right angles and

become swollen and mostly “decomposed” as in the cortex of _Ramalina_.

In _Letharia_ (_L. vulpina_, etc.) the structure is midway between

_Ramalina_ and _Usnea_: the central axis is either a solid strand of

chondroid hyphae or several separate strands.

[Illustration: Fig. 63 B. _Usnea longissima_ Ach. A, transverse section

of fibril × 85. B, _a_, chondroid axis; _b_, gonidial tissue; _c_, cortex

× 525 (after Schulte).]

In three other genera with upright fruticose thalli, _Sphaerophorus_,

_Argopsis_ and _Stereocaulon_, rigidity is maintained by a medulla

approaching the chondroid type. In _Sphaerophorus_ the species may have

either flattened or cylindrical branching stalks, but in all of them,

the centre is occupied by longitudinal strands of hyphae. _Argopsis_,

a monotypic genus from Kerguelen, has a cylindrical branching thallus

with a strong solid axis; it is closely allied to _Stereocaulon_, a

genus of familiar moorland lichens. The central tissue of the stalks in

_Stereocaulon_ is also composed of elongate, thick-walled conglutinate

hyphae, formed into a strand which is, however, not entirely solid.

C. SURVEY OF MECHANICAL TISSUES

Mechanical tissues scarcely appear among fungi, except perhaps as

stoutish cartilaginous hyphae in the stalks of some Agarics (_Collybiae_,

etc.), or as a ring of more compact consistency round the central hyphae

of rhizomorphic strands. It is practically a new adaptation of hyphal

structure confined to lichens of the fruticose group, where there is

the same requirement as in the higher plants for rigidity, flexure and

tenacity.

Rigidity is attained as in other plants by groups or strands of

mechanical tissue situated close to the periphery, as they are so

arranged in _Ramalina_ and _Cladonia_; or the same end is achieved by a

strongly developed fastigiate cortex as in _Roccella_. Bending strains

to which the same lichens are subjected, are equally well met by the

peripheral disposition of the mechanical elements.

Tenacity and elasticity are provided for in the pendulous forms either by

a fibrous cortex as in _Alectoria_, or by the chondroid axis in _Usnea_.

Haberlandt[379] has recorded some interesting results of tests made by

him as to the stretching capacity of a freshly gathered pendulous species

in which the central strand was from ·5 to 1 mm. thick. He found he could

draw it out 100 to 110 per cent. of its normal length before it gave way.

In an upright species the frond broke when stretched 60 to 70 per cent.

In both of the plants tested, the central strand retained its elasticity

up to 20 per cent. of stretching. The outer cortical tissue was cracked

and broken in the experiments. Schulte[380] calculated somewhat roughly

the tenacity of _Usnea longissima_ and found that a piece of the main

axis 8 cm. long carried up to 300 grms. without breaking.

D. RETICULATE FRONDS

In the upright radiate thallus, more especially among the _Ramalinae_,

though also among _Cladoniae_[381], there has appeared a reticulate

thallus resulting from the elongate splitting of the tissues, and due

to unequal growth tension and straining of the gelatinous cortex when

swollen with moisture. In several species of _Ramalina_, the strap-shaped

frond is hollow in the centre; and strands of strengthening fibres

give rise to a series of cortical ridges. The thinner tissue between

is frequently torn apart and ellipsoid openings appear which do not

however pierce beyond the central hollow. Such breaks are irregular

and accidental though occurring constantly in _Ramalina fraxinea_, _R.

dilacerata_, etc.

A more complete type of reticulation is always present in a Californian

lichen, _Ramalina reticulata_, in which the large flat frond is a

delicate open network from tip to base (Fig. 64). It grows on the

branches of deciduous trees and hangs in crowded tufts up to 30 cm. or

more in length. Usually it is so torn, that the real size attainable can

only be guessed at. It is attached at the base by a spreading discoid

hold-fast, and, in mature plants, consists of a stoutish main axis from

which side branches are irregularly given off. These latter are firm at

the base like the parent stalk, but soon they broaden out into very wide

fronds. Splitting begins at the tips of the branches while still young;

they are then spathulate in form with a slightly narrower recurved tip,

below which the first perforations are visible, small at first, but

gradually enlarging with the growth of the frond.

[Illustration: Fig. 64. _Ramalina reticulata_ Krempelh. Portion of frond

(after Cramer).]

_Ramalina reticulata_ is an extremely gelatinous lichen and the formation

of the network was supposed by Lutz[382] to be entirely due to the

swelling of the tissues, or the imbibition of water, causing tension

and splitting. A more exact explanation of the phenomenon is given by

Peirce[383]: he found that it was due to the thickened incurved tip,

which, on the addition of moisture, swells in length, breadth and

thickness, causing it to bend slightly upwards and then curve backwards

over the thallus, thus straining the part immediately behind. These

various movements result in the splitting of the frond while it is young

and the cortices are thin and weak.

Peirce made a series of experiments to test the capacity of the tissues

to support tensile strains. In a dry state, a piece of the lichen held a

weight up to 150 grms.; when wet it broke with a weight of 30 grms. It

was also observed that the thickness of the frond doubled on wetting.

E. ROOTING BASE IN FRUTICOSE LICHENS

Fruticose and filamentous lichens are distinguished by their mode of

attachment to the substratum: instead of a system of rhizinae or of hairs

spread over a large area, there is usually one definite rooting base by

which the plant maintains its hold on the support.

Intermediate between the foliose and fruticose types of thallus are

several species which are decumbent in habit, but which are attached at

one (or sometimes more) definite points, with but little penetration of

the underlying substance. One such lichen, _Evernia furfuracea_, has

been classified now as foliose, and again as fruticose. The earliest

stage of the thallus is in the form of a rosette-like sheath which bears

rhizinae on the under surface, very numerous at the centre of the sheath,

but entirely wanting towards the periphery. A secondary thallus of

strap-shaped rather narrow fronds rises from the sheath and increases by

irregular dichotomous branching. These branches, which are considered by

Zopf[384] as adventitious, may also come into contact with the substratum

and produce a few rhizinae at that point; or if the frond is more closely

applied, the irritation thus produced causes a still greater outgrowth

of rhizinae and the formation of a new base from which other fronds

originate. These renewed centres of growth are not of very frequent

occurrence; they were first observed and described by Lindau[385] in

another species, _Evernia prunastri_, and were aptly compared by him to

the creeping stolons of flowering plants.

_Evernia furfuracea_ grows frequently on dead wood, palings, etc., as

well as on trees. _E. prunastri_ grows invariably on trees, and has a

more constantly upright fruticose habit; in this species also, a basal

sheath is present, and the attachment is secured by means of rhizoidal

hyphae which penetrate deeply into the periderm of the tree, taking

advantage of the openings afforded by the lenticels. The sheath hyphae

are continuous with the medullary hyphae of the frond, and gonidia are

frequently enclosed in the tissues; the sheath spreads to some extent

over the surface of the bark, and round the base of the fronds, thus

rendering the attachment of the lichen to the tree doubly secure.

Among _Ramalinae_, the development of the base was followed by

Brandt[386] in one species, _R. Landroensis_, an arboreal lichen from S.

Tyrol. A rosette-like sheath was formed consisting solely of strands of

thick-walled hyphae which spread over the bark. There were no gonidia

included in the tissue.

A different type of attachment was found by Lilian Porter[387] in

corticolous _Ramalinae_—_R. fraxinea_, _R. fastigiata_, and _R.

pollinaria_. The lichens were anchored to the tree by strands of closely

compacted hyphae longitudinally arranged and continuous with the cortical

hyphae. These enter the periderm of the tree by cracks or lenticels,

and by wedge action cause extensive splitting. The strands may also

spread horizontally and give rise to new plants. The living tissues of

the tree were thus penetrated and injured, and there was evidence that

hypertrophied tissue was formed and caused erosion of the wood.

[Illustration: Fig. 65. _Ramalina siliquosa_ A. L. Sm., on rocks, reduced

(M. P., _Photo._).]

Several _Ramalinae_—_R. siliquosa_, _R. Curnowii_, etc.—grow on rocks,

often in extremely exposed situations, in isolated tufts or in crowded

swards (Fig. 65). The separate tufts are not unfrequently connected at

the base by a crustaceous thallus. It is possible also to see on the

rock, here and there, small areas of compact thalline granules that

have scarcely begun to put out the upright fronds. These granules are

corticate on the upper surface and contain gonidia; from the lower

surface, slender branching hyphae in rhizoid-like strands penetrate

down between the inequalities and separable particles of the rock,

if the formation is granitic. They frequently have groups of gonidia

associated with them, and they continue to ramify and spread, the pure

white filaments often enough enclosing morsels of the rock. The upright

fronds are continuous with the base and are thus securely anchored to the

substratum.

On a smooth rock surface such as quartzite a continuous sward of

_Ramalina_ growth is impossible. The basal hyphae being unable to

penetrate the even surface of the rock, the attachment is slight and

the plants are easily dislodged. They do however succeed, sometimes,

in taking hold, and small groups of fronds arise from a crustaceous

base which varies in depth from ·5 to 1 mm. The tissues of this base

are very irregularly arranged: towards the upper surface loose hyphae

with scattered groups of algae are traversed by strands of gelatinized

sclerotic hyphae similar to the strengthening tissues of the upright

fronds, while down below there are to be found not only slender hyphae,

but a layer of gonidia visible as a white and green film on the rock when

the overlying particles are scaled off.

Darbishire[388] found that attachment to the substratum by means of a

basal sheath was characteristic of all the genera of Roccellaceae. He

looks on this sheath, which is the first stage in the development of the

plant, as a primary or proto-thallus, analogous to the primary squamules

of the _Cladoniae_, and he carries the analogy still further by treating

the upright fronds as podetia. The sheath of the Roccellaceae varies

in size but it is always of very limited extent; it is mainly composed

of medullary hyphae, and gonidia may or may not be present. The whole

structure is permanent and important, and is generally protected by a

well-developed upper cortex similar in structure to that of the upright

thallus, _i.e._ of a fastigiate type. There is no lower cortex.

The two British species of _Roccella_—_R. fuciformis_ and _R.

phycopsis_—grow on maritime rocks, the latter also occasionally on trees.

In _R. fuciformis_, the attaching sheath is a flat structure which slopes

up a little round the base of the upright frond. It is about 2 mm. thick,

the cortex occupying about 40 µ of that space; a few scattered gonidia

are present immediately below. The remaining tissue of the sheath is

composed of firmly wefted slender filaments. Towards the lower surface,

there is a more closely compacted dark brown layer from which pass out

the hyphae that penetrate the rock.

The sheath of _R. phycopsis_ is a small structure about 3 to 4 mm. in

width and 1·5 mm. thick. A few gonidia may be found below the dense

cortical layer, but they tend to disappear as the upright fronds become

larger and the shade, in consequence, more dense. Lower down the hyphae

take an intensely yellow hue; mixed with them are also some brown

filaments. A somewhat larger sheath 7 to 8 mm. wide forms the base of

_R. tinctoria_. In structure it corresponds—as do those of the other

species—with the ones already described.

In purely filamentous species such as _Usnea_ there is also primary

sheath formation: the medullary hyphae spread out in radiating strands

which force their way wherever possible into the underlying substance;

on trees they enter into any chink or crevice of the outer bark like

wedges; or they ramify between the cork cells which are split up by the

mere growth pressure. By the vertical increase of the base, the fronds

may be hoisted up and an intercalary basal portion may arise lacking both

gonidia and cortical layer. Very frequently several bases are united and

the lichen appears to be of tufted habit.

A basal sheath provides a similar firm attachment for _Alectoria jubata_

and allied species: these are slender mostly dark brown lichens which

hang in tangled filaments from the branches of trees, rocks, etc.

These attaching sheaths differ in function as well as in structure

from the horizontal thallus of the Cladoniaceae. They may be more

truly compared with the primary thallus of the red algae _Dumontia_

and _Phyllophora_ which are similarly affixed to the substratum, while

upright fronds of subsequent formation bear the fructifications.

IV. STRATOSE-RADIATE THALLUS

1. STRATOSE OR PRIMARY THALLUS

A. GENERAL CHARACTERISTICS

[Illustration: Fig. 66. _Cladonia pyxidata_ Hoffm. Basal squamule and

podetium. _a_, apothecia; _s_, spermogonia (after Krabbe).]

This series includes the lichens of one family only, the Cladoniaceae,

the genera of which are characterized by the twofold thallus, one portion

being primary, horizontal and stratose, the other secondary and radiate,

the latter an upright simple or branching structure termed a “podetium”

which narrows above, or widens to form a trumpet-shaped cup or “scyphus”

(Fig. 66). The apothecia are terminal on the podetium or on the margins

of the scyphi; in a few species they are developed on the primary

thallus. Some degree of primary thallus-formation has been demonstrated

in all the genera, if not in all the species of the family. The genus

_Cladina_ was established to include those species of _Cladonia_ in

which, it was believed, only a secondary podetial thallus was present,

but Wainio[389] found in _Cladonia sylvatica_ a granular basal crust

and, in _Cladonia uncialis_, minute round scales with crenate margins

measuring from ·5 to 1 mm. in width. In some species (subgenus _Cladina_)

the primary thallus is quickly evanescent, in others it is granular or

squamulose and persistent. Where the basal thallus is so much reduced

as to be practically non-existent, apothecia are rarely developed and

soredia are absent. Renewal of growth in these lichens is secured by the

dispersal of fragments of the podetial thallus; they are torn off and

scattered by the wind or by animals, and, if suitable conditions are met,

a new plant arises.