Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/576

556 LICHENS

which frequently occur on lichens, it might be supposed that the pycnides in reality belonged to the same category. From their constant occurrence, however, on the same species, and the evident correlation between them and the accompanying fructifications, as also from the resemblance of their stylospores to the spores of the apothecia, there are good grounds for adopting the conclusion come to by Tulasne. They are very common on the margin of the thallus of isidiiferous states of Peltigera canina and P. rufescens, where they have often been mistaken for spermogones, which in this genus have not yet been detected. Pycnides occur also in Lecidea vermifera, and abundantly in L. tantilla, in Habrothallus, in several species of Strigula, in Spilonema revertens, and will probably be yet observed in other lichens.

Fig. 5 – Pycnides of Peltigera rufescens. a, basidia; b, stylospores

The Gonidia of Lichens.

In view of the important place occupied by the gonidia in the structure of lichens, and of the discussions that have recently taken place concerning them, they require to be considered somewhat in detail in order that their real nature and relation to the hyphaj, or the thalline filaments, may become apparent. The gonidia are spherical, ellipsoid, or variously rounded cellules, with thin, colourless walls composed of cellulose, containing chlorophyll (or a subsimilar colouring matter), homogeneous or granulose, with generally a solid nucleus in the centre. As to the origin of the chlorophyll, it may be observed in passing that this is the same in lichens as in other cryptogamic plants, e.g., mosses and Hepaticæ, in which it occurs, the only visible difference being that gonidia often occur as discrete cellules. The gonidia increase by binary (very rarely by ternary or quarternary) division, the nucleus also dividing into two portions, each of which forms the centre of a secondary gonidial cellule. In the gonidial stratum, where they are arranged between the radicles of the hyphæ, their division necessarily proceeds only slowly, but in ecorticated thalli, leprose and others, in which they are free, they are readily multiplied by repeated division. In gonidia isolated from the thallus of some species belonging to Cladonia, Evernia, and Physcia, zoospores have been detected by M. Famintzin and M, Boranetzky (see Ann. Sc. Nat., 1863, p. 137), and, although Nylander failed to perceive such in subsequent experiments (Flora, 1877, No. 23), he adds that it is possible they may be generated in free gonidia (i.e., in unstratified thalli), which could not be the case in gonidia closely surrounded by thalline filaments. The subject will well repay further investigation. Other matters relating to the character and relations of the gonidia will be best elucidated by considering the forms which they present, and their origin in the thallus.

I. The Forms of the Gonidia. – These have been fully treated by Nylander in the Flora, loc. cit., where also the first scientific exposition and classification of them have been given. According to the views there propounded, gonidia in their wider acceptation include three very distinct types: – (1) Eugonidia (or gonidia proper), which are involved in a distinct cellular membrane, and are usually bright green; (2) Gonimia (or the gonidial granules already mentioned), which are naked, pale greenish, glaucous greenish or bluish; and (3) Gonidimia (or Leptogonidia), which are intermediate between the two preceding, smaller, and of an oblong form. Of these the distinction between eugonidia and gonimia is fundamental, and of "so great weight that lichens seem to present a twofold parallel series" according to the presence of the one or the other in their texture. These two different anatomical elements, as observed by Nylander, have a certain biological analogy with the blood globules in animals, and similarly afford absolute characters. The principal forms presented by these three kinds of gonidia are the following. 1. Eugonidia consist of – (a) Haplogonidia, the most frequent, simple, of a protococcoid form, or sometimes glomerulose (as in granulosoleprose thalli); (b) Platygonidia, being depressed and variously membranosely connated gonidia (Syngonidia), as in some foliicolous species (e.g., Platygramma phyllosema); (c) Chroolepogonidia (or Chrysogonidia), containing chlorophyll and orange grains (endochrome) in the same cellule, more or less similar to Chroolepa (as in Gyalecta chlorobæa, Arthonia pruiosa, Platygrapha periclea, Verrucaria insiliens, and the genus Thelopsis); (d) Confervogonidia, somewhat resembling Confervæ, and forming the chief element of the thallus of Cœnogonium. 2. Gonidimia are smaller than gonidia proper, with the wall of the cellules less distinct. They occur in Peltidea, Solorina, and Nephroma expallidum. To these belong also hymenial gonidia, which are often very minute, and are present in the thalamium (destitute of paraphyses) of various Pyrcnocarpei (e.g., Verrucaria pallida, V. umbrina, and V. hymenogonia), rarely of Arthoniæ as in A. chroolepida. 3. Gonimia (including the gonimia of Cephalodia) consist of (a) Haplogonimia, which are somewhat large (very large in Phylliscum), and either simple, or two or several aggregated; (b) Sirogonimia, which are scytonemoid or sirosiphoid gonimia, distinguished by the gonimia being tunicated, and are characteristic of the family Ephebacei; (c) Hormogonimia, the most common form, smaller, moniliformly arranged, and contained in syngonimia, especially characteristic of the family Collemacci, whence Collema (or Nostoc) itself, according to Nylander, is to be considered but as a single syngonimium; (d) Speirogonimia, which are similar to the preceding, but are not moniliform, with the syngonimia subglobose, smaller and more scattered, as in Omphalaria and Synalissa. It will be perceived from the above that many of these forms are more or less similar to "gonidioid" algæ, though, as we shall presently see, they are not identical with these.

II. The Origin of the Gonidia. – By pre-microscopic authors this was a subject necessarily ignored, and indeed it is only within the last thirty years that it has been investigated by Hellenists. The earliest theory as to their origin was that propounded by Bayrhoffer (Einiges üb. d. Lichenen und deren Befruchtung, 1851), confirmed by observations of Speerschneider (Bot. Zeit., 1853, &c. ), and supported by Schwendener (Untersuch. üb. d. Flechtenthallus, 1868). This was to the effect that the gonidia derived their origin from the hyphæ (i.e., the thalline filaments), in the way succinctly detailed by M. Fries (in Scand., 1871, p. 7, where it is fully endorsed). "The hyphæ," he says, "are not only elongated into filaments, but also put forth short branchlets, the terminal cell of which is gradually dilated, becomes subglobose, and is at length filled with chlorophyll (or a subsimilar matter); in a few that (terminal cell) is changed into a gonidium, and then by varied division germinates other gonidia." For several years this theory was accepted at second-hand by most authors who referred to the subject, though a different origin of the gonidia, presently to be noticed, was indicated by the celebrated Tulasne so early as 1852, in his "Mémoire sur les lichens" (Ann. Sc. Nat.). The erroneous nature of this theory was well pointed out by Schwendener, who (Die Algentypen d. Flechtengonidien, 1869) very correctly affirmed that the actual development of a gonidium from the terminal cell of a hypha had not been observed, though, strange to say, he had previously him self observed this phenomenon. Not being able otherwise to account for the origin of the gonidia, and following up one of two alternatives put forward by De Bary (Morpholog. und Physiolog. der Pilze, Flechten, &c., 1865, p. 291), he promulgated the hypothesis now familiarly known as Schwendenerism. The conclusion to which De Bary came on noticing the resemblance between the gonidia of Collemaceæ and certain algæ was as follows: – "Either the lichens in question," he says, "are the perfectly developed states of plants, whose imperfect forms have hitherto stood amongst the algæ, as Nostocaceæ, and Chroococcaceæ, or these latter are typical algæ which assume the forms of Collema, Ephebe, &c., through certain parasitic Ascomycetes penetrating into them, spreading their mycelium into the continuously growing thallus, and frequently attached to their phycochrome-bearing cells." Taking this latter suggestion as his starting point, and assuming the identity of certain algal types with the gonidia of lichens, and the identity of the mycelium of fungi with their hyphæ, Schwendener extends the said alternative to various other groups of lichens than the Collemaceæ, and comes to the conclusion that a lichen is composed of a parasitic fungus (the hyphæ) and a number of low algæ (Chlorophyllaceæ and Phycochromaceæ), the former of which produces the reproductive bodies and is nourished by the latter. This theory was subsequently expanded and illustrated at length by Bornet (Recherches sur les Gonidies des Lichens, 1873), who affirms, as the result of numerous investigations, "that the connexion of the hyphæ with the gonidia is of such a nature as to exclude all possibility of the one organ being produced by the other," and that the theory of parasitism can alone explain it satisfactorily. To give any detailed