550 



LICHENE9. 



of the thallus, and of a different colour (fig. 782 c c), and of an internal 

 medullary layer of paraphyses and sporangia, lying perpendicularly to 

 the outer layer (figs. 782 c m, 781 t p). The fructification gradually 

 projects more from the surface, and either remains covered with the 

 outer layer, or bursts through it. When it remains closed, there is a 

 nucleus in the centre. When the fructification bursts through the 

 cortical or outer layer, it expands in the form of shield-like discs, called 

 apothecia (a-o0jj*j, a repository), or patellae (figs. 780 s, 781 a) (patella. 



a 



a hollow disk), or linear expansions called lirellse (lira, a furrow). 

 Sometimes the cortical matter forms a border round the fructification, 

 at other times it grows up in the form of a stalk, so as to give rise to a 

 podetium (TO?, a foot). The young thecae (asci) contain spores, vary- 

 ing from 4 to 8 (fig. 413), or from 12 to 16. Occasionally, the spores 

 are in sets of two (fig. 413, 2). Separated cells of the medullary 

 layer, of a green colour, called gonidia (yo'vj, generation, and el^oc, 

 resemblance), or gongyli, are considered as another kind of reproduc- 

 tive organ. There is much uncertainty as to the real character of the 

 spherical or subspherical green bodies called gonidia, which are char- 

 acteristic of true lichens. When separated from the parent structure, 

 they are capable of forming new plants. Thwaites says that they are 

 not gemmae, but that they are analogous to the vesicles of Nostoc, 

 one of the Algas. Lichens are found in all quarters of the globe, 



Figs. 780-782. Organs of fructification of Parmelia Acetabulum, to illustrate the natural order 

 Lichenes, section Hymenothalameae. 



Fig. 780. t, Thallus of the Lichen, s, Apothecia in the form of shields in different degrees 

 of development. 



Fig. 781. Apothecium, a, cut vertically and magnified in order to show the layer, t p, formed 

 by the union of thecaa and paraphyses. 



Fig. 782. A small portion of the apothecium much more magnified, showing, c m, the central 

 medullary layer, c c, The cortical layer. 1 1, Thecse in different degrees of development, p, 

 Paraphyses. 



