ORGANS OF EEPEODUCTION. 



375 



and 3. Pycnidia, containing stylospores. Although it is gene- 

 rally believed that in Lichens we have a true sexual reproduc- 

 tion taking place, at present we are ignorant of the mode in 

 which fecundation is effected. 



The ajpothecia are of various forms, and have received different 

 names accordingly. The more usual forms are the round {fig. 

 822, cp) and linear ; in the latter case they are commonly 

 termed Urella {Jig. 821). The apothecia maybe either sessile or 

 stalked ; in the latter case the stalk has received the name of 

 jpodetium. The apothecium is either composed of two parts, 

 called the thalamium and excipulinn, or, of the former only. The 

 latter, when present, forms a partial or entire covering to the 

 thalamium. The body of the apothecium constitutes the thala- 

 miunt, and the layer of cells at the bottom of this, upon which 

 the thecse and paraphyses are placed, is termed the hypothecium. 

 When the apothecium is divided by a vertical section, it is seen 



Fig. 821. 



Figs. 823, 824. 

 t 



Fig.%ri. Thallus of Opegrapha afra showing lirell^. Fig.S^. Portion of 



the th^Wus ot Parmeiia parietina, with young apothecia, ap,RnAsper- 

 mugcmia, sp. After Henfrey. Pigr. 823. A theca,t, of a Lichen, sur- 

 rounded by paraphyses. The theca contains four spores or sportdia, p. 

 Fig. 824. One of the spores of the above divided into two cells, 



to contain a number of spore-cases called asci or thec<s {fig. 823, 

 t), surrounded by thread-like or somewhat club-shaped filaments, 

 called jparaphyses, p, which are usually regarded as abortive 

 asci ; the asci and the paraphyses are placed perpendicularly 

 upon the hypothecium. The apothecia are frequently of a 

 different colour from the surrounding thallus ; this is due either 

 to the paraphyses or the excipulum. Each of the asci generally 

 contains eight spores, but in some cases only four {fig. 823), 

 and in others sixteen ; thus the spores are generally a multiple 

 of two, and the number is always constant for each species. In 

 rare cases the asci have a large number of spores, and are hence 

 said to be polysporous. The spores are sometimes termed 

 sporidia or sporides. Some of these spores are of a very complex 

 structure, being divided into two {fig. 824), four, or many cells. 



