268 THALLOPHYTES. 



form an acute angle. Adventitious branches arise laterally below the apex of the thallus, 

 the cortical fibres forming at a particuFar point a new apex and subsequently growing 

 outwards. Gonidia are also to be found below the new apex, while the base of the 

 branch sends out medullary fibres and an axial bundle into the primary branch, so 

 that the homologous forms of tissue of the two are continuous. The growth of Usnea 

 may be compared, irrespectively of subordinate points, to that of the so-called stroma 

 of the Xylariae ; the formation of the gonidia is a subordinate element in the structure 

 of the whole. 



In some crustaceous Lichens the thallus possesses in general no defined contour, and 

 no external differentiation takes place ; the thallus appears as a somewhat irregular 

 aggregation of masses of gonidia traversed by hyphse. In other crustaceous Lichens (as 

 Sporastatia Morio, Rhizocarpon suhconcentricum^ Aspic'ilia calcarea, &c.), the thallus forms 

 lobed discs which increase by centrifugal growth at the margin ; the growing margin 

 consists altogether of hyphal tissue, in which, further inwards masses of gonidia appear 

 at a few isolated spots and gradually spread ; the cortical tissue is indented at the circum- 

 ference of the spots where the gonidia are formed. Isolated scaly pieces of a true 

 Lichen-thallus then arise on a fibrous substratum called the Hypothallus^. 



7he Formation of the Spores of Lichens takes place in receptacles termed Jpothecia, 

 similar to those of the Discomycetes, or in other cases to those of some Pyrenomycetes. 

 They are formed in the interior of the tissue of the thallus, and only appear above its 

 surface at a later period, when they either expand their hymenial layer to the air (Gymno- 

 carpous Lichens), or allow the spores to escape outwards through an orifice (Angio- 

 carpous Lichens). In all Lichens without exception the apothecium and all its essential 

 parts derive their origin exclusively from the hyphal tissue ; it is the Fungus alone that 

 produces the receptacles ; the nourishing Algee, /. e. the gonidia, take no part whatever in 

 it, or only in a secondary manner in so far as the thallus-tissue together with its gonidia 

 grows like a wall round the apothecium and to a certain extent envelopes it (as shown in 

 Fig. 192), or grows luxuriantly beneath the apothecium and raises it upon a kind of 

 stalk above the surrounding thallus. The only exception to this endogenous origin 

 of the apothecium occurs in Coenogonium and similar forms, where it is impossible, 

 because the hyphae form only a very thin layer round the filamentous Alga which per- 

 forms the part of gonidia. These forms serve to show with especial clearness, as w^e 

 know from Schwendener's researches, that the receptacle of Lichens belongs exclusively 

 to the hyphal tissue. 



The history of the development of the apothecium is a branch of the inquiry 

 attended with great ditTiculty, and in more than one point is still obscure^. It originates, 

 in heteromerous Lichens, beneath the cortical layer, in the lower part of the gonidial 

 zone, or, in some crustaceous Lichens, in the deepest part of the thallus in immediate 

 contact with the substratum; in homoomerous gelatinous Lichens and in Ephebe it 

 arises beneath the surface of the thallus. The commencement of the gymnocarpous 

 apothecium is, in heteromerous Lichens, a very small roundish ball of confused interwoven 

 hyphae, on the outer side of which a tuft of very delicate hyphse — the first paraphyses — 

 rises at a very early period. The most external hyphal investment of this ball, and 

 therefore surrounding the tuft of paraphyses and opening above (outvv-ards), is termed 

 by lichenologists the Excipulum. The further growth of the rudiment of the apothecium 

 is now occasioned by the increase in size of the excipulum by the formation of new 

 fibres, w^hile new paraphyses are intercalated among those already formed and outside 

 the tuft, the extension of the apothecium being the immediate result of the fresh forma- 

 tion of these bodies. The growth is first completed in the centre of the apothecium ; 

 at the outside it continues longer, often even after the appearance of the apothecium 



^ See Schwendener, Flora, 1865, no. 26, 



2 What follows is taken from De Bary's account of his oynH researches, and fiom those of 

 Schwendener and Fuisting. 



