Fig. 105. Multinucleate spore of Lecidea (Mycoblastus) sanguinaria Ach. × 540 (after Zopf).
Fig. 106. Germination of multinucleate spore of Ochrolechia pallescens Koerb. × 390 (after de Bary).
e. Multinucleate Spores. These spores, which are all very large, occur in several genera or subgenera: in Lecidea subg. Mycoblastus ([Fig. 105]), Lecanora subg. Ochrolechia and in Pertusariaceae. Tulasne[677] in his experiments with germinating spores found that in Lecanora parella (Ochrolechia pallescens?) germinating tubes were produced all over the surface of the spore ([Fig. 106]). De Bary[678] verified his observations in that and other species and added considerable detail: about twenty-four hours after sowing spores of Ochrolechia pallescens, numerous little warts arose on the surface of the spore which gradually grew out into delicate hyphae. All these spores contain fat globules and finely granular protoplasm with a very large number of minute nuclei; the presence of the latter has been demonstrated by Haberlandt[679] and later by Zopf[680] who reckoned about 200 to 300 in the spore of Mycoblastus sanguinarius. These nuclei had continued to multiply during the ripening of the spore while it was still contained in the ascus[680]. Owing to the presence of the large fat globules the plasma is confined to an external layer close to the spore wall; the nuclei are embedded in the plasma and are connected by strands of protoplasm. The epispore in some of these large spores is extremely developed: in some Pertusariae it measures 4-5 µ in thickness.
f. Polaribilocular Spores. The most peculiar of all lichen spores are those termed polaribilocular—signifying a two-celled spore of which the median septum has become so thickened that the cell-cavities with their contents are relegated to the two poles of the spore, an open canal frequently connecting the two cell-spaces ([Fig. 107]). Other terms have been suggested and used by various writers to describe this unusual character such as blasteniospore[681], orculiform[682] and placodiomorph[683] or more simply polarilocular.
The polarilocular colourless spore is found in a connected series of lichens—crustaceous, foliose and fruticose (Placodium, Xanthoria, Teloschistes). In another series with a darker thallus (Rinodina and Physcia) the spore is brown-coloured, and the median septum cuts across the plasma-connection. In other respects the brown spore is similar to the colourless one and possesses a thickened wall with reduced cell-cavities.
Fig. 107. Polarilocular spores. a, Xanthoria parietina Th. Fr.; b, Rinodina roboris Th. Fr.; c, Physcia pulverulenta Nyl.; d, Physcia ciliaris DC. × 600.
The method of cell-division in these spores resembles that known as “cleavage by constriction,” in which the cross wall arises by an ingrowth from all sides of the cell; in time the centre is reached and the wall is complete, or an open pore is left between the divided cells. Cell “cleavage” occurs frequently among Thallophytes, though it is unknown among the higher plants. Among Algae it is the normal form of cell-division in Cladophora and also in Spirogyra, though in the latter the wall passes right across and cuts through the connecting plasma threads. Harper[684] found “cleavage by constriction” in two instances among fungi: the conidia of Erysiphe and the gametes of Sporodinia are cut off by a septum which originates as a circular ingrowth of the outer wall, comparable, he considers, with the cell-division of Cladophora.