. Lichens. Lichens. 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
. Lichens. Lichens. 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 ; 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 als
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