LICHENS 555
a homogeneous protoplasm, molecular granulations (often abundant), and a pale-yellow oily substance, which in the fully developed spore often becomes condensated into one or more globules, visible generally near its ends or in the middle. The number of the spores in each theca varies very much in different species, being in most lichens 8, occasionally 6, 4 or 2, or rarely only 1. In some species, however, they are 20-100, when the thecæ are said to be poly spared. In form they are also very variable, the principal shapes assumed by them in order of frequency being ellipsoid, fusiform, oblongo-cylindrical, sphæroid, and acicular, with several intermediate forms. They are still more variable as to size, being smallest in polyspored (e.g. , Lecanora fuscata, &c. ), and largest in monospored species (e.g., Pertusaria communis, &c.). The largest spores of all are found in Varicellaria amicrosticta, where they are 225-0 030 mil limetre long, and 095-0 115 millim. thick. As to colour, they are either colourless (white, nearly hyaline or pale yellow under the microscope), or coloured (brownish, or brown, or blackish), while in Sphærophoron there is also a bluish-black superficial pigment. The spore-wall varies in thickness, and is typically composed of two strata, the outer of which is termed the epispore, and is coloured, while the inner is termed the endospore and is colourless, gelatinous, and generally less distinct. Notwithstanding that in these respects the spores are so variable in different species, they are when mature remarkably constant in the same species, so that their characters afford valuable specific diagnoses. Apparently it is only those lichens which germinate from spores that occur in a fertile condition.
2. The spermogones, which are the presumed male
organs of reproduction, at once differ in appearance from
the apothecia in being very minute corpuscles. In many
cases their outline is invisible to the naked eye, unless the
thallus has been previously moistened, when they appear
as minute points or papillte. When magnified they ex
ternally bear a resemblance to the apothecia of the Pyreno-
carpei, but internally, on microscopical examination, they
are seen to differ essentially from these. In form they are
nucleiform, round, or oblong, and are either sessile on the
surface of the thallus, or more or less immersed in its sub
stance, or sometimes enclosed in prominent thalline verruca?.
Usually they are simple, though occasionally two or several
become continent or aggregated into little groups. They
almost always occur on the same thallus as the apothecia,
or rarely on different thalli (e.y., Eplitbe jmbescens), so that
lichens are consequently monoecious and dioecious. In
colour the spermogones are black or brown, or concolorous
with the thallus itself. They are composed of two parts,
viz., a shell or conceptacle and a nucleus.
(1) The conceptacle, which is analogous to the hypothecium of
the apothecium, is composed of a tissue formed of very small
cellules, which are cemented together and have thick walls. The
ostiolc at its summit is generally similar to that of nucleiform
apothecia, and in the case of entirely immersed spermogones is the
only portion visible.
(2) The nucleus consists of the sterigmata anil spermatia, and of a mucilaginous substance (the spermatic gelatin) in the cavity between them, which very greedily imbibes water and aids in the expulsion of the spermatia. Sometimes also there are present in the nucleus a few elongated, articulated, and occasionally branching filaments intermixed with the sterigmata which are comparable with the paraphyses of the apothecia (e.g., Ilamalina}. (a) The sterig mata are elongated cellules filled with a colourless fluid, and are attached to the interior face of the conceptacle growing from it con- vergently towards the centre, and often nearly filling its entire cavity. They present two important modifications, viz., simple sterigmata and articulated sterigmata or artkrosterigmatn, both of which may become somewhat branched. When arrived at maturity they possess the faculty of producing from their apices or also from thearticuli the spermatia, one at a time, though many in succession, as may be inferred from the immense number of them lying in the cavity of the nucleus, (b) The spermatia are very small, slender, colourless corpuscles, which at first appear as minute protrusions on the apices of the sterigmatic cells. They subsequently become gradually elongated, and, on reaching maturity, detach themselves from the sterigmata and lie free in the cavity of the nucleus, till in wet weather they are expelled through the ostiole of the spermo gones. In form they present two principal modifications, viz., adcular and cllipsni/l, of which the former, the more frequent, is rather variable. They may be slightly fusiformi-incrassate at one apex (e.g. , Usnca}, or at both apices (e.g., Parmelia), or they may be cylindrical and straight, the most common shape, or cylindrical and arcuate (e.g. , Roccclla}. The spermatia of artlirosterigmata are cylindrical and straight, but slightly thickened and obtuse at the
apices. In size they also vary, though more in length than in thick ness, the arcuate spermatia being sometimes very long (0 040 milli metre). These differences in form and size are often very useful in the discrimination of species, just as the two types of the sterigmata are sometimes of great service in the distinction of genera. The sper-
Fig. 4. – Vertical Section of a Spermogone of Parmelia physodes, showing the Sterigmata and Spermatia.
matia frequently exhibit a Brownian movement, but they possess no faculty of germination, their functions, as generally acknowledged, being the fertilization or fecundation of the spores. That the spermogones are rightly presumed to be the male organs of reproduction may legitimately be inferred alike from their relation to and their antagonism with the apothecia. In the former respect their position on the thallus relatively to the apothecia is similar to that of the sexual organs in other classes of plants; while in the latter respect spermogones are plentiful on thalli destitute of apothecia, and on those with apothecia are much more sparingly present. The manner, however, in which the spermatia impregnate the spores has not yet been definitely ascertained. Most probably, as suggested by Nylander (in Syn., p. 40, note 1), the spermatia do not exercise any direct influence on the female organ in the hymenium, but their fecundating influence is rather exercised on the prothalline elements of the growing thallus. M. Stahl indeed has recorded and illustrated (Beitr. zur Entwick. der Flechten, 1877) the result of certain researches on Collema microphyllum, in which he supposed that he had detected sexual organs in the shape of an ascogonium and a trichogyne, regarded by him as a kind of carpogonium. These observations, however, have not been confirmed by subsequent researches on the same or any other species of Collema, while his attributing to the hyphæ a faculty of "contortion" or spirally coiling themselves, which from their nature they do not and cannot possess, is calculated to invalidate all that he otherwise observed and depicted.
3. The pycnides are minute, dark-coloured pyrenodean conceptacles which occur on the thalli of various lichens, especially such as are crustaceous. In external appearance they resemble to some extent the spermogones, from which, however, they at once differ in their internal organization. They consist of simple filaments composed of narrow (often short) cellules, termed basidia, bearing on their apices bodies called stylospores, which are colourless, usually oblong, but variable in form and size, and filled with organic matter (in part at least oily) similar to the spores. Each basidium produces only a single stylospore, which, unlike the spermatia, has a germinative property. Their occurrence in lichens was first pointed out by Tulasne, who showed their affinity to certain analogous fruits (Diplodia, Phoma, Septoria, &c.) in various thecaspored fungi, and regarded them as supplementary or secondary sporiferous reproductive organs. Considering the number of parasitic fungilli
