196 



REPRODUCTION 



In size they vary from such minute bodies as those in Parmelia exasperata 

 which measure 25-35 /* m diam., up to nearly i mm. in Lobaria laetevirens. 



As a rule, they range from about 150 p 

 to 400 /j. across the widest part, and are 

 generally rather longer than broad. They 

 open above by a small slit or pore called 

 the ostiole about 20 /* to I oo /JL wide which 

 is frequently dark in colour. In one in- 

 stance, in Icmadophila aeruginosa, Nien- 

 burg 1 has described a spermogonium with 

 a wide opening, the spermatiophores 

 being massed in palisade formation along 

 the bottom of a cup-like structure. 



C. COLOUROFSPERMPGONIA. Though 

 usually the ostiole is visible as a darker 

 point than the surrounding tissue, sper- 

 mogonia are often difficult to locate un- 

 less the thallus is first wetted, when they become visible to slight magnification. 

 Theyappear asblack points mmanyParmeliae.Physciae^occellae^c., though 

 even in these cases they are often brown when moistened. They are dis- 

 tinctly brown in some Cladoniae, in Nephromium, and in some Physciae; 

 orange-red or yellow in Placodium and concolorous with the thallus in 

 Usnea, Ramalina, Stereocaulon, etc. 



Fig. 112. Free spermogonia in spinous 

 cilia of Cetraria islandica Ach. A, part 

 of frond; B, cilia, x 10. 



D. STRUCTURE 



a, ORIGIN AND GROWTH. The spermogonia (or pycnidia) of lichens 

 when mature are more or less hollow structures provided with a distinct 

 wall or " perithecium," sometimes only one cell thick and then not easily de- 

 monstrable, as in Physcia speciosa, Opegrapka vulgata, Pyrenula nitida, etc. 

 More generally the " perithecium " is composed of a layer of several cells 

 with stoutish walls which are sometimes colourless, but usually some shade 

 of yellow to dark-brown, with a darker ostiole. The latter, a small slit or 

 pore, arises by the breaking down of some of the cells at the apex. After 

 the expulsion of the spermatia, a new tissue is formed which completely 

 blocks up the empty spermogonium. In filamentous lichens such as Usnea 

 a dangerous local weakening of the thallus is thus avoided. 



Spermogonia originate from hyphae in or near the gonidial zone. The 

 earliest stages have not been seen, but Moller 2 noted as the first recogniz- 

 able appearance or primordium of the "pycnidia" in cultures of Calicium 

 trachelinum a ball or coil of delicate yellowish-coloured hyphae. At a more 



1 Nienburg [908. 2 Moller 1887. 



