REPRODUCTION IN DISCOLICHENS 



169 



rufescens^ and P. malacea} the two systems — paraphyses and ascogonium — 

 grow simultaneously, though in P. horizontalis the ascogonium has dis- 

 appeared by the time the paraphyses are formed. In the genus Nephroma, 

 in Physcia stellaris and in Xanthorina parietina the paraphyses are also late 

 in making their appearance. 



In most instances, the paraphyses push their way up between the cortical 

 cells which gradually become absorbed, or they may stop short of the sur- 

 face as in Nephromium tomentosum'^. The overlying layer of cortical cells in 

 that case dies off gradually and in time disappears. Such an apothecium is 

 said to be '' at first veiled." Later formed paraphyses at the circumference 

 of the apothecium form the parathecium, which is thus continuous with the 

 hypothecium. 



e. Variations in apothecial Development. Lichens are among 

 the least stereotyped of plants : instances of variation have been noted in 

 several genera. 



aa. Parmeltae. A somewhat complicated course of development has 

 been traced by Baur'^ in Parmelia acetabulum. In that lichen the group of 

 three to six carpogonia do not lie free in 

 the gonidial tissue, but originate nearer 

 the surface (Fig. i-)6) and are surrounded 

 from the first by a tissue connected with, 

 and resembling the tissue of the cortex. 

 In the several ascogonia, there are more 

 cells and more spirals than in Collema 

 or in Physcia, and all of them are some- 

 what confusedly intertwined. The tri- 

 chogynes are composed of three to five 

 cells and project 10 to 15^ above the 

 surface. When further development be- 

 gins, the ascogonial cells branch out and 

 form a primary darker layer or hypo- 

 thecium above which extends the subhymenium, a 

 loosely woven hyphae. Branches from the ascogonial hyphae at a later stage 

 push their way up through this tissue and form above it a second plexus of 

 hyphae — the base of the hymenium. Baur considers this a very advanced 

 type of apothecium; he found it also present in Parmelia saxatilis, though, 

 in that species, the further growth of the first ascogonial layer was more 

 rapid and the secondary plexus and hymenium were formed earlier in the 

 life of the apothecium. He has also stated that a similar development occurs 

 in other genera such as Usnea, though Nienburg's^ work scarcely confirms 

 that view. 



Fig. 96. Parmelia acetabulum Dut). Vertical 

 section of thallus and carpogonial group 

 X 550 (after Baur). 



ight-coloured band of 



1 Funfstiick 1884. 



Baur 1904. 



'Nienburg 1908. 



