CARPOSPOREJE. 



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outwards. Gonidia are also to be found behind 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 hyphae. In other crustaceous Lichens (as 

 Sporastatia Morio, Rhizocarpon subconcentricum, Aspicilia 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 thus arise on a fibrous substratum called the hypothallm ^ 



'^he Formation of the Spores of Lichens takes place in receptacles termed Apothecia, 

 when they are similar to those of the Discomycetes, or Perithecia^ when they are similar to 

 those of some Pyrenomycetes. They are formed in the interior of the tissue of the thallus, 



I Fig. 218.— Vertical section of the apotheciuin o'i Anaptychia ciliaris (X about 50); h the liymenium. jv sub-hynienial 



layer and excipuluin ; all the rest belongs to the thallus ; m its medullary layer, r its cortex, g its gonidia ; at ^ ^ the thallus 

 forms a cup-shaped rim round the apothecium. 

 md only appear above its surface at a later period, and then, in the one case they expand 

 heir hymenial layer to the air (Gymnocarpous Lichens), and in the other, they allow the 

 pores to escape through an orifice (Angiocarpous Lichens). In all Lichens without 

 !xception the receptacle and all its essential parts take origin exclusively from the hyphal 

 issue ; it is the Fungus alone that produces the receptacles ; the nourishing Algae, /. e. the 

 ;onidia, take no part whatever in it ; or only in a secondary manner in so far as the 

 hallus-tissue together with its gonidia grows like a wall round the apothecium and to a 

 certain extent envelopes it (as shown in Fig. 218), or grows luxuriantly beneath the 

 receptacle and raises it upon a kind of stalk above the surrounding thallus. The only 

 exception to this endogenous origin of the receptacle occurs in Ccenogonium and similar 

 forms, where it is impossible, because the hyphae form only a very thin layer round tke 

 filamentous Alga which performs the part of gonidia'^. These forms serve to show with 

 especial clearness, as we know from Schwendener's researches, that the receptacle of 

 Lichens belongs exclusively to the hyphal tissue. 



The investigation of the development of the apothecium is attended with great 

 difficulty, and more than one point is still obscured It originates, in heteromerous 



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



^ [See Archer, On Apothecia in some Algae, Quart. Journ, Micr. Sci. 1875.] 

 ^ What follows is taken from De Bary's account of his own researches, and from those of 

 Schwendener and Fuisting. [See Stahl, loc. czV.] 



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