CHAPTER VII. — PHENOMENA OF VEGETATION. — LICHENS. 



415 



conical bodies like small flat apothecia of Peziza with the apex of the cone towards 

 the thallus and produced by suitable branching of a tuft of hyphae ; these bodies 

 when they are present in large numbers unite together at the margins and form 

 patches of some size ; in Dictyonema and Laudatea they are smooth expanded 

 layers usually resembling those of the Thelephoreae. The sporiferous structure of 

 Rhipidonema is not clearly understood. Further details will be found in Mattirolo 

 and Johow. 



Section CXVIT. The thallus of very many Lichens forms small brood- 

 buds which separate spontaneously from the thallus and under favourable conditions 

 may develope into a Lichen-thallus similar to the parent structure. These have been 

 known since Acharius' time by the name of soredia. They appear to be wanting in 

 some species, as in Lecidea geographica and Endocarpon pusillum ; in others they 

 are very abundant, as in many species of Usnea, Bryopogon, Ramalina, Evernia, 

 Imbricaria, Parmelia, Pertusaria and others. 



In their most general characters the soredia are small portions of the thallus 

 which have a distinct form in each case and consist of one or more algal cells and the 

 hyphae which have grown round them. As the soredium developes into a new 

 thallus the two constituents behave 

 in the manner which has been 

 described above in the special 

 types of thallus. Their structure 

 at the moment of separation is 

 in many cases that of the mother- 

 thallus in a rudimentary form, 

 as in Collema where they are 

 roundish branchlets, prolifications 

 of the upper surface or margin of 



the thallus appearing like granules to the naked eye, in the Graphideae also which 

 contain Chroolepus, in Roccella and others. Detailed investigations in many 

 species have not yet been made. 



The soredia have been carefully studied by Tulasne and especially by 

 Schwendener in the typical heteromerous forms which contain Palmellaceae, especially 

 Cystococcus, and there their development and structure usually exhibit the following 

 peculiar features (Fig. 181). 



They are formed at spots in the algal zone beneath the rind which are distributed 

 over the surface or margin of the thallus and vary according to the species. Branches 

 of the hyphae twine round an adjacent algal cell or group of cells which have arisen by 

 division and form a soredium ; if a group of cells is thus attacked the hyphae also 

 insinuate themselves between the cells and twine round each of them. The roundish 

 body thus formed consists entirely of one or more algal cells and an envelope of 

 hyphae which differs in compactness and colour in different species, and in some, as 

 in Bryopogon, is not perfectly closed ; how it is separated from the thallus has not 

 been clearly ascertained. 



By unlimited repetition of this process at one spot the soredia accumulate there 

 beneath the rind, which swells up and at length bursts, the soredia emerging from 



Fig. 181 



1B1. Usnea barbata. c. an isolated mature soredium, with an algal cell 

 [Cystococcus) in the envelope of hyphae. d another with several algal cells 

 in optical longitudinal section, f.ytwo soredia in the act of germinating; the 

 hyphal envelope has grown out below into rhizoid-branches, and above shows 

 already the structure of the apex of the thallus (see Fig. 171). After 

 Schwendener. _ Magn. more than 500 times. 



