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 accom panying fructifications, as also from the resem blance of their stylo- spores 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 IIG- thallus of isidiiferous states of Pdtigera 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. 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 sphe rical, ellipsoid, or variously rounded cellules, with thin, colourless walls composed of cellulose, containing chloro phyll (or a subsiinilar 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 crypto- gamic plants, e.g., mosses and Ilepaticx, 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 hyphte, 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, Eoernia, and P/tyseia, 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 goni dia 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. cif&amp;gt;., where also the first scientific expo sition 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 Leptoyonidia), which are intermediate between the two preceding, smaller, and of an oblong form. Of these the dis tinction between eugonidia and gonimia is fundamental, and of &quot; so great weight that lichens seem to present a twofold parallel scries&quot; according to the presence of the one or the other in their texture. These two different anatomical elements, as observed by Nylander, liave a certain biological analogy with the blood globules in animals, uid similarly afford absolute characters. The principal forms pre sented by these three kinds of gonidia are the following. 1. Eu&quot;-omdia consist of (a) Haployonidia, the most frequent, simple, of a protococcoid form, or sometimes glomerulose (as in granuloso- leprose 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 (endo- chrome) in the same cellule, more or less similar to Chroolcpa (as in Gyalccta chlorobasa, Arthonia pruiosa, Platygrapha per idea, Ver- rucaria insiliens, and the genus Thclopsis) ; (d) Confcn-ogonidia, somewhat resembling Confervse, and forming the chief element of the thallus of Ccenogonium. 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 liy menial gonidia, which are often very minute, and are present in the thalamium (destitute of paraphyses) of various Pyrcnocarpc.i (e.g., Vcrrucaria pallida, V. umbrina, and V. hymcnogonia], rarely of Arthonise as in A. chroolcpida. 3. Gonimia (including the gonimia of Ceplicdodia) consist of (a) Hcqrtogonimia, which ave somewhat large (very large in Phylliscum}, and either simple, or two or several aggregated ; (b) Sirogoniinia, which are scytonemoid or sirosiphoid gonimia, distinguished by the gonimia being tunicated, and are characteristic of the family Ephebacei ; (c) Homnogonimia , the most common form, smaller, moniliformly arranged, and con tained in syngonimia, especially characteristic of the family Collcm- acci, whence Collcma (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 Ompha- laria and Synalissa. It will be perceived from the above that many of these forms are more or less similar to &quot;gonidioid &quot; alg&amp;lt;e, 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 ub. d. Lichencn und dcrcn Ucfruchtuiig, 1851), confirmed by observations of Speersehneider (Bot. Zcit., 1853, &c. ), and sup ported by Schwendener (Untersuch. iib. d. Flechtenthallw, 1868). This was to the effect that the gonidia derived their origin from the hypliHJ (i.e., the thalline filaments), in the way succinctly detailed by M. Fries (in Scand., 1871, p. 7, where it is fully endorsed). &quot; The hyphre,&quot; he says, &quot; are not only elongated into filaments, but also put forth short brauchlets, the terminal cell of which is gradu ally 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.&quot; For sereral 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 indi cated by the celebrated Tulasne so early as 1852, in his &quot; Memoire sur les lichens&quot; (Ann. Sc. Nat.). The erroneous nature of this theory was well pointed out by Schwendener, who (T&amp;gt;ie Algcntypcn d. Fleclitengonidicn, 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 (Jforpholog. und PJnjsiolog. dcr Pilze, Flcchten, &c., 1865, p. 291), he promulgated the hypo thesis now familiarly known as Schwendenerism. The conclusion to which De Bary camo on noticing the resemblance between the gonidia of Collcmaccfe and certain algre was as follows : &quot; Either the lichens in question,&quot; he says, &quot;are the perfectly developed states of plants, whose imperfect forms have hitherto stood amongst the algre, as Nostocaccae, and Chroococcaccx, or these latter are typical algas which assume the forms of CoUcnia, Ephcbc, &c., through cer tain parasitic Ascomycetcs penetrating into them, spreading their mycelium into the continuously growing thallus, and frequently attached to their phycochrome-bearing cells.&quot; Taking this latter suggestion as his starting point, and assuming the identity of cer tain algal types with the gonidia of lichens, and the identity of the mycelium of fungi with their hypha), Schwendener extends the said alternative to various other groups of lichens than the Collcmacese, and comes to the conclusion that a lichen is composed of a parasitic fungus (the hyplue) and a number of low alga? (CJiloroplitjUKcrfK and Phycochromaccie), the former of which produces the reproductive bodies and is nourished by the latter. This theory was sub sequently expanded and illustrated at length by Bornet (Eccherches sur les Gonidies dcs Lichens, 1873), who affirms, as the result of numerous investigations, &quot;that the connexion of the hyplne with the gonidia is of such a nature as to exclude all possibility of the one organ being produced by the other,&quot; and that the theory of parasitism can alone explain it satisfactorily. To give any detailed