Lichens

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.

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.

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.

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).

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).

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.

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).

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.

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.

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.

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.