CARPOSPORE^. 
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, Rbizocarpon 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 hypotballus^. 
Ihe 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, 
Fig. 218.— Vertical section of the apothecium of Aitaptychia cihai-is (\ about 50); h the liymenium, ji/ sub-hyiiienial 
layer and excipulum ; all the rest belongs to the thallus ; 711 its medullary layer, r its cortex, g its gonidia ; zX 1 1 the thallus 
forms a cup-shaped rim round the apothecium. 
and only appear above its surface at a later period, and then, in the one case they expand 
their hymenial layer to the air (Gymnocarpous Lichens), and in the other, they allow the 
spores to escape through an orifice (Angiocarpous Lichens). In all Lichens without 
exception the receptacle and all its essential parts take origin exclusively from the hyphal 
tissue ; it is the Fungus alone that produces the receptacles ; the nourishing Algae, 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. 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 the 
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 obscure^. It originates, in heteromerous 
^ See Schwendener, Flora, 1865, no. 26. 
^ [See Archer, On Apothecia in some Algre, Quart. Journ. Micr. Sei. 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.] 
Y 2 
