CHAPTER VII. PHENOMENA OF VEGETATION. LICHENS. 401 



If the Alga is released from its union with the Fungus by natural or artificial 

 means, it may, as we learn from the researches of Famintzin and Baranetzki, 

 Schwendener, Bornet, Woronin, and Stahl, continue to vegetate, recovering its typical 

 morphological characters as an Alga, if the necessary conditions are present. See for 

 example Fig. 170. 



Where the Alga is much altered in the Lichen, careful examination of liberated 

 cells is often necessary for the certain determination of the species. Such an examina- 

 tion has yet to be made in the case of many of the Lichen-algae, especially the Palmel- 

 laceae, and consequently these forms cannot at present be named with certainty and 

 exactness. 



SECTION CXVI. Form and structure of the Lichen-thallus. The seizure 

 of the Alga by the Fungus is followed by the development of the combined body into 

 its ultimate form, its growth into the perfect Lichen-thallus (the blastema of Wallroth), 

 on which organs of reproduction of the Fungus make their appearance when the 

 highest development has been reached. These organs in the majority of Lichens arc 

 sporocarps (apothecia a&Aperiffieaa) together with spermogonia (see page 211); in a few, 

 which will be briefly noticed again at the close of this account, they are basidia- 

 bearing hymem'a ; in many they are small brood-buds which separate spontaneously 

 from the thallus and are known as soredia. As a general rule the Alga takes part 

 only in the construction of the thallus and soredia and not in that of the sporocarps 

 and spermogonia or the basidia-bearing hymenium. The 'thallodic 'margin of the apo- 

 thecia which is characteristic of many genera and contains Algae (see Figs. 86, 87, 89) 

 belongs, as the name rightly expresses, to the thallus and not to the apothecium. It 

 has often been observed indeed at the first beginning of the formation of the apothecia 

 (see page 213), that algal cells are included in the primordial hyphal weft; but they 

 die away in most species without increasing in size or number in the sporocarp itself. 



Nylander 1 was the first who called attention to a remarkable exception to this 

 rule in the case of the pyrenocarpous species just mentioned, in which the Algae, 

 Pleurococcus and Stichococcus in the best ascertained cases, which are taken into 

 the primordial sporocarp, grow with it and multiply their cells to such an extent 

 that they gather thickly round the asci. This is the origin of the hymenia containing 

 Algae and the hymenial algal cells of Stigmatomma cataleptum, Sphaeromphale, 

 and especially Endocarpon pusillum and Polyblastia rugulosa which have been 

 especially investigated by Stahl, as was mentioned on page 400 2 . The algal cells are 

 distinguished by their considerably smaller size from their sister-cells in the thallus to 

 which they belong. 



The fully developed thallus as regards its outward configuration appears in three 

 chief forms, which are not however sharply distinguished from one another : the 

 fruticose (thallus fruticulosus, filamentosus, thamnodes, Fig. 86 A), which rises with 

 a narrow base from its substratum, and is either simple or more usually branched like a 

 shrub; tiizfoliaceous (thallus foliaceus, frondosus, placodes, Fig 86 /), in form a flat, 

 leaf-like, usually lobed and crisped body which spreads over the surface of the sub- 

 stratum, but is only attached to it at one or at several scattered points and can 



1 Synopsis meth. Lichenum, p. 47. 



" See Beitr. z. Entwicklungsg. d. Flechten, where the literature is cited. 



[4] Dd 



