I 



LIGHENES. 301 



rate from one another. Whilst these changes have been 

 taking place in the ascogonium, it has become invested by a 

 dense felt-work of hyphae, formed by the active growth of 

 the hyphae of the thallus. From this investing layer hyphae 

 grow inward between the separating coils of the ascogo- 

 nium, and bear paraphyses, which form the rudimentary 

 hymenium. At the same time outgrowths have been 

 formed from the cells of the ascogonium, which either are 

 asci, or grow into hyphal filaments, which bear asci as 

 lateral branches. The asci, whether derived directly or in- 

 directly from the cells of the ascogonium, come to lie in the 

 hymenium among the paraphyses." Thus the apothecium 

 is partly developed from the carpogonium, and partly from 

 the hyphse of the thallus, agreeing in this with what is now 

 known to be the mode of formation of the corresponding 

 parts of some, at least, of the Helvellacece. 



Whether there are similar sexual organs in other lichens, 

 is at present unknown ; probably, when discovered, they will 

 be found to bear some resemblance to those of Collema, just 

 described ; but it is altogether likely that, instead of fertili- 

 zation taking place by means of free male elements (sper- 

 matia), it will be shown to be more nearly like that now 

 known in Peziza or Ascobolus. 



399. The Gonidia. The gonidia of lichens are of so 

 much importance that they demand a somewhat extended 

 notice. As above stated (paragraph 392), they are green or 

 greenish cells, or rows of cells, which occur either distributed 

 irregularly through the tissue of the lichen-thallus (the ho- 

 moomerous lichens), or in different layers or regions (the 

 heteromerous lichens). These green bodies are of different 

 forms in different groups of lichens, while in nearly related 

 species they are often exactly alike. They may consist of 

 isolated cells, or groups of cells, as in most fruticose or folia- 

 ceous lichens (e.g., Parmelia, Fig. 202, Sticta, Fig. 201, 

 SphcBropliorus and Usnea, Fig. 208), while, on the other 

 hand, they may be made up of rows or chains of cells 

 (e.g., Lecanactis and Graphis, Fig. 209, Mallotium, Fig. 

 210, and Collema, Figs. 206 and 207). They are known to 

 reproduce by the division (fission) of their cells, and, in 



