3. Acicular, cylindrical and straight, the most common form; these occur in most of the Lecanorae, Cladoniae, Lecideae, Graphideae, Pyrenocarpeae and occasionally they are budded off from arthrosterigmata.
4. Acicular, cylindrical, bent; sometimes these are very long, measuring up to 40 µ; they are found in various Lecideae, Lecanorae, Graphideae, Pyrenocarpeae, and also in Roccella, Pilophorus and species of Stereocaulon.
5. Ellipsoid or oblong and generally very minute; they are borne on simple sterigmata and are characteristic of the genera Calicium, Chaenotheca, Lichina, Ephebe, of the small genus Glypholecia and of a few species of Lecanora and Lecidea.
In many instances there is more or less variation of form and of size in the species or even in the individual. There are no spherical spermatia.
b. Size and Structure. The shortest spermatia in any of our British lichens are those of Lichina pygmaea which are about 1·4 µ in length and the longest are those of Lecanora crassa which measure up to 39 µ. In width they vary from about 0·5 µ to 2 µ. The mature spermogonium is filled with spermatia and, generally, with a mass of mucilage that swells with moisture and secures their expulsion.
The spermatia of lichens are colourless and are provided with a cell-wall and a nucleus. The presence of a nucleus was demonstrated by Möller[727] in the spermatia of Calicium parietinum, Opegrapha atra, Collema microphyllum, C. pulposum and C. Hildenbrandii, and by Istvanffi[728] in those of Buellia punctiformis (B. myriocarpa), Opegrapha subsiderella, Collema Hildenbrandii, Calicium trachelinum, Pertusaria communis and Arthonia communis (A. astroidea). Istvanffi made use of fresh material, fixing the spermatia with osmic acid, and in all of these very minute bodies he demonstrated the presence of a nucleus which stained readily with haematoxylin and which he has figured in the spermatia of Buellia punctiformis as an extremely small dot-like structure in the centre of the cell. On germination, as in the cell-multiplication of other plants, the nucleus leads the way. Germination is preceded by nuclear division, and each new hyphal cell of the growing mycelium receives a nucleus.
c. Germination of Spermatia (pycnidiospores). The strongest argument in favour of regarding the spermatia of lichens as male cells had always been the impossibility of inducing their germination. That difficulty had at length been overcome by Möller[727] who cultivated them in artificial solutions, and by that means obtained germination in nine different lichen species. He therefore rejected the commonly employed terms spermatia and spermogonia and substituted pycnoconidium and pycnidia. Pycnidiospore has been however preferred as more in accordance with modern fungal terminology. His first experiment was with the “spermatia” of Buellia punctiformis (B. myriocarpa) which measure about 8-10 µ in length and about 3 µ in width, and are borne directly on the septate spermatiophores (arthrosterigmata). In a culture drop, the spore had swelled to about double its size by the second or third day, and germination had taken place at both ends, the membrane of the spore being continuous with that of the germinating tube. In a short time cross septa were formed in the hyphae which at first were very close to each other. While apical growth advanced these first formed cells increased in width to twice the original size and, in consequence, became slightly constricted at the septa. In fourteen days a circular patch of mycelium had been formed about 280 µ in diameter. The development exactly resembled that obtained from the ascospores of the same species grown in the absence of gonidia. The largest thallus obtained in either case was about 2 mm. in diameter after three months’ growth. The older hyphae had a tendency to become brownish in colour; those at the periphery remained colourless. In Opegrapha subsiderella the development, though equally successful, was very much slower. The pycnidiospores (or spermatia) have the form of minute bent rods measuring 5·7 µ × 1·5 µ. Each end of the spore produced slender hyphae about the fifth or sixth day after sowing. In four weeks, the whole length of the filament with the spore in the middle was 300 µ. In four months a patch of mycelium was formed 2 mm. in diameter. Growth was even more sluggish with the pycnidiospores of Opegrapha atra. In that species they are rod-shaped and 5-6 µ long. Germination took place on the fifth or sixth day and in fourteen days a germination tube was produced about five times the length of the spore. In four weeks the first branching was noticed and was followed by a second branching in the seventh week. In three months the mycelial growth measured 200-300 µ across.
Germination was also observed in a species of Arthonia, the spores of which had begun to grow while still in the pycnidium. The most complete results were obtained in species of Calicium: in C. parietinum the spores, which are ovoid, slightly bent, and brownish in colour, swelled to an almost globose shape and then germinated by a minute point at the junction of spore and sterigma, and also at the opposite end; very rarely a third germinating tube was formed. Growth was fairly rapid, so that in four weeks there was a loose felt of mycelium measuring about 2 cm. × 1 cm. and 1 mm. in depth. Parallel cultures were carried out with the ascospores and the results in both cases were the same; in five or six weeks small black points appeared, which gradually developed to pycnidia with mature pycnidiospores from which further cultures were obtained.
On C. trachelinum, which has a thin greyish-white thallus spreading over old trunks of trees, the pycnidia are usually abundant. Lindsay had noted two different kinds and his observation was confirmed by Möller. The spores in one pycnidium are ovoid, measuring 2·5-3 µ × 1·5-2 µ; in the other rarer form, they are rod-shaped and 5-7 µ long. In the artificial cultures they both swelled, the rod-like spores to double their width before germination, and sometimes several tubes were put forth. Growth was slow, but of exactly the same kind from these two types of spores as from the ascospores. At the end of the second month pycnidia appeared on all the cultures, in each case producing the ovoid type of spore.
In a second paper Möller[729] recorded the partially successful germination of the “spermatia” of Collema (Leptogium) microphyllum, the species in which Stahl had demonstrated sexual reproduction. Growth was extraordinarily slow: after a month in the culture solution the first swelling of the spermatium prior to germination took place, and some time later small processes were formed in two or three directions. In the fourth month a branched filament was formed.