Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/574

554 LICHENS

in which latter case they are called pseudo-cyphellæ. Their physiological function is not definitely known, but they are most likely connected with the nutrition of the plants. (3) Isidia constitute an exuberant condition of some foliaceous and crustaceous thalli. They consist of elevated, coralloid, stipitate excrescences, which are sometimes branched, and which are always of the same colour and texture as the thallus itself. This isidioid condition in crustaceous thalli is the basis of the old pseudo-genus Isidium, which is merely an anomalous state of some species of Pertusaria. Isidia have the same functions as the soredia, and serve as propagula. (4) Cephalodia are orbicular and flattened or globular tubercles of a peculiar texture which occur on the thallus of many species belonging to different genera. They are usually epigenous, as in Stereocaulon, Usnea, Lecanora gelida, Lecidea panæola, &c. In a few species, however, they are hypogenous, as in Peltidea venosa and Psoroma euphyllum, while in various Stictei, Nephroma expallidum, &c., they are endogenous, forming pyrenoid protuberances on the lower surface. Recently Nylander has detected both epigenous and hypogenous cephalodia on Psoroma arenarium and Lecanora allorhiza. They are of a paler colour than the rest of the thallus, from which they differ also in structure, being confusedly cellulose, and containing gonidial granules. According to Th. M. Fries (in Flora, 1866, p. 19) they are only morbid excrescences caused by algals intruding themselves under the cortex; but this is at once refuted by the fact of their forming constant characters of so many different species occurring in various situations. In these they evidently constitute normal organs, the use of which, however, is unknown.

II. Reproductive System. – This consists of apothecia or the female organs, of spermogones or the presumed male organs, and probably also of pycnides or a secondary kind of fructification.

1. The apothecia, like the thallus, are very variable in external form and colour, as also in their internal structure. In external form they present three principal modifications, viz., (1) disciform (or gymnocarpous), in which the shape is that of a disc (as in all the higher genera); (2) nucleiform (or angiocarpous), in which the shape is that of a rounded tubercle with an apical ostiole (as in Endocarpon, Verrucarici); and (3) peridiiform, similar in shape to the preceding, but closed, with no ostiole (as in Thelocarpon, Endococcus). The last two are but little variable in figure, and consequently do not in this respect admit of different designations. The disciform apothecia, however, present various shapes, of which the following are the principal: – (a) peltate, which are large, rounded, without any distinct thalline margin (e.g., Usnea, Peltigera); (b) lecanorine, or scutelliform, which are orbicular and surrounded by a distinct, more or less prominent thalline margin (e.g., Parmelia, Lecanora), having sometimes also in addition a proper one (e.g., Thelotrema, Urceolaria); (c) lecideine, or patelliform, which are typically orbicular, with only a proper margin (e.g., Lecidea), sometimes obsolete, and which are occasionally irregular in shape, angular or flexuose (e.g., Lecidea jurana, L. myrmecina), or complicated and gyrose (e.g., Gyrophora), and even stipitate (e.g., Bæomyces); (d) lirelliform, which are of very irregular figure, elongated, branched or flexuose, with only a proper margin (e.g., Xylographa, Graphis, &c.) or none (e.g., some Arthoniæ), and are often very variable even in the same species. It may be here observed that young disciform apothecia are more or less nucleiform. In colour the apothecia are extremely variable, and it is but rarely that they are concolorous or subconcolorous with the thallus (e.g., Usnea, Ramalina). Usually they are discolorous, and may be black, brown, yellowish, or also less frequently rose-coloured, rusty-red, orange-reddish, saffron, or of various intermediate shades. Occasionally in the same species their colour is very variable (e.g., Lecanora metaboloides, Lecidea decolorans), while sometimes they are white or glaucous, rarely greenish, pruinose. Lecideine apothecia, which are not black, but otherwise variously coloured, are termed biatorine.

The two principal parts of which an apothecium consists are the hypotherium and the thecium.

(1) The hypothecium, which corresponds to the hypothallus, is the conceptacle of the apothecia. It is composed of cellular tissue, generally very dense, and often presenting an indistinct stratification. This tissue may in general be distinguished from that of the neighbouring parts of the thallus by its cellules being smaller, more compact, and differently coloured, though in some instances (as in certain Pannariæ) the limits are not determinable. In the apothecia of such genera as Calidum, Bæomyces, &c., the hypothecium is composed of hollow cemented filaments arranged longitudinally and constricted into a stipe for the support of the fruit. The hypothecium in disciform apothecia is usually termed the excipulum proper, while in nucleiform apothecia it is termed the pyrenium, and in peridiiform apothecia the peridium. When the pyrenium quite covers the nucleus it is said to be entire, dimidiate when it covers only the upper portion. The hypothecium is either colourless or dark, or reddish or yellowish, according as its cellules are tinged.

Fig. – 3. Vertical Section of Apothecium of Physcia parictina. a, paraphyses; b, thecæ with bilocular spores; c, hypothecium.

(2) The thecium, or as it is more frequently termed the hymenium, is that part of the apothecium which contains the organs of the fruit, viz., the thalamium and the thecæ, which are placed perpendicularly to the hypothecium. It is penetrated by an amyloid substance, colourless and very greedy of water, termed the hymenial gelatine, formed of the lichenine, which becomes bluish or wine-reddish when tinged with iodine. The thecium itself corresponds to the gonidial-medullary stratum, while its superficial portion, termed the epithecium, corresponds to the epithallus. (a) The thalamium generally consists of paraphyses which are erect colourless filaments arising from the hypothecium, and whose function is to aid in the expulsion of the spores by the pressure which they exercise upon the thecæ. They are of nearly equal height, closely placed together, usually very slender, though slightly variable in thickness, frequently articulated, and rarely branched or anastomosing. Internally they are hollow and filled with protoplasm, which sometimes is separated into little globules. Their apices are generally coloured, in most instances dilated, sometimes clavate, and are cemented together by gelatin. They are frequently confused together; occasionally they are but little evolute; while in many of the Pyrenocarpei they are entirely wanting, though in these the ostiolar filaments of the hypothecium have sometimes been mistaken for them, (b) The thecæ are large, oblong, cylindrical or ovoid cellules or vesicles containing the spores, and are usually more or less attenuated towards the base. In size and shape they vary considerably in the different genera and species according to the size, number, form, and arrangement of their spores. They differ also in the same species, within certain limits, according to age, the young theca being more slender than those which are older. In some genera which have very large spores (e.g., Varicellaria, Pertusaria) the thecæ are distended in proportion, and generally present a saccate or oblongo-ventricose form. The theca itself is a thin membranous cellule, the walls of which are at first of an equal thickness throughout, but in process of development they become gradually thinner, except at the summit, where they retain their original thickness. In some species the wall is remarkably thick at the apex (e.g., Arthonia), and in others it is invested throughout with a kind of external cuticle (e.g., Pertusaria). The thecæ are resorbed after the expulsion of the spores; though where their walls are extremely thin (as in Calicium) they are ruptured and disappear at a very early stage. The spores are the special reproductive organs of lichens, and are produced in the thecæ by free cell-formation, i.e., by the separation and subsequent condensation of the protoplasm of the fully developed theca around certain points in its interior, corresponding in number to that of the spores to be formed. After the formation of the external spore-wall has taken place, the spores have a definite outline, and may consist of only one cellule or loculus (simple spores), or may be divided by one or more transverse partitional membranes (septate spores). Sometimes the loculi are restricted to the two ends or poles of the spore (one in each) and are said to be polari-bilocular, the two loculi being occasionally united by a longitudinal tube. At other times the transverse partitions are further divided by several longitudinal partitions, in which case the spore is said to be murali-divided, from the resemblance it then bears to the stones in a wall. The contents of the spores are