Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/80

68 MUSCINE^E [LIVERWORTS. forms the ramification is of a forked character (dichotomous) (fig. 1). Frequently too, on the older portions, "adventitious" shoots Fio. 4. A. A leafy (foliose) Jungermannia (Chiloscyphus polyanthos corda) with three sporogonia ; on the right above is one with the capsule open, while in the two others it is still closed. B. A "perianthium," with the perichaetial leaves below. are formed ; they may frequently be seen to spring from the margin and the mid-rib of the thallus of Mclzgcria furca ta. The ramifica tion of the foliose Liverworts is a very complex one, but is never of a really forked character. The branchlets commonly stand below the leaves on the sides of the stem. The above-mentioned "adventitious" shoots serve the purpose of vegetative increase. The same end is attained by the gemmae, which are of many different forms. In Aneura individual cells detach themselves from the thallus and grow up into new plants. In the Marchanticx the gemmae are formed in peculiar receptacles which stand on the upper side of the ribbon-shaped thallus. In Marchantia polymorpJia these receptacles have the form of a basket open at the top, while in Lunularia the margin of the same recep tacle is semicircular. The gemmae, which are developed on the base of the receptacle, are flat upright cellular bodies, with two indentations at the sides, out of which at a later period, at the time of germination, proceed two growing points. The gemmae, however, are of similar structure on both sides. Which side is at the time of germination to become dorsal with stomata and air-chambers, and which ventral with root-hairs, is decided by its position with respect to the light. It is always the side turned towards the light which develops itself into that which bears stomata. This occurs even when gemmae that swim on water receive light only from below. In that case the under side becomes that which possesses stomata and the air-chambers described above, while the upper side develops root-hairs (rhizoids). Blasia pusilla has also very pecu liar receptacles for gemmae, provided with a long channel leading outwards, while in many foliose Jungermannicse for example, Jungermannia ventricosa, Scapania ncmorosa, Jungermannia bicus- pidata, and others gemmae consisting of a few cells are formed on the leaves or the stem respectively. (2) The Organs of Sex. The antheridium in Liverworts is a spherical or ellipsoidal cellular body with a shorter or longer stalk (fig. 5, 0), and consists of a wall -layer and an interior tissue, the mother-cells of the spermatozoids. In each of these cells a spermatozoid is formed, and, as it appears, in the following way : the peripheral layer of the cell-nucleus becomes thickened, and splits up into a ribbon, either annular or rolled up spirally. The cilia by means of which the spermatozoid moves in water appear, on the other hand, to be formed from the protoplasm in which the cell - nucleus lies. The process, however, requires still closer investigation. Each antheridium is originally a simple cell, which grows by repeated subdivision and becomes the antheridial body. By the swelling up of the walls of the spermatozoid mother-cells, the cells of the antheridium-wall are forced asunder at the apex, and the spermatozoids, each imbedded in mucilage, are emitted in jets either free from one another or united into a pulp. This pulp is often ejected by an explosion of the antheridinm, as may be easily observed in Frullania dilatata. In water the spermatozoids isolate themselves, and in the most favourable cases are carried over to the archegonia. Since very many spermatozoids are formed in one antheridium, and since one or very few of these suffice for the fertilization of an archegonium, it is evident that an enormous number of them must perish. The archegonia also proceed from one cell, through the division of which their construction is accomplished. They always take their origin from the cells of the upper surface, and the same is the case with those of Anthoceros, which, however, are sunk into the tissue of the thallus, so that their neck and ventral portions remain united with the tissue of the thallus and do not project freely over it. The distribution of the organs of sex is either monoecious or Fio. 5. Marchantia polymorpha. A. Anterior portion of a thallus, t, with two erect antheridial inflorescences hu. B. Longitudinal vertical section through a young antheridial inflorescence. The antheridia are situated in depressions of the upper surface. 6, scales ; h, roots (rhizoids) ; o, o, the openings of the depressions in which the antheridia lie. C. A nearly mature antheri dium ; st, its stalk ; w, the wall. D. Two spermatozoids, magnified 800 dia meters. Eacli possesses two fine cilia. (After Sachs.) dioecious. In the latter case the antheridia and archegonia are found on different individual plants, as in Sphserocarpits terrestris, where the male plant is smaller than the female. In the former case the antheridia and archegonia are found on the same plant. In the thallose forms they are always inserted on the dorsal side of the thallus, and commonly sunk into a cavity in it (fig. 5, B}. In Anthoceros the antheridia even stand in closed cavities, which do not open till the antheridium is mature. The organs of sex stand either on ordinary branches, which at a later period vegetate and grow, or on special sexual shoots. These are of especially remarkable construction in the Marchantiese, of which Marchantia polymorpha may serve as an example. In this species the antheridici are found on the upper side of a peltate radially -lobed disk, supported on a stalk (fig. 5, A). The archegonia stand on the under side of a similar receptacle. These structures may be termed "inflorescences." They are not simple branches, but systems of branches which have arisen from the repeated bifurcation of a thallns-shoot. Each indentation in the disk of the " inflorescence " corresponds to the vegetative point of a branch, and this explains the fact that the oldest antheridia are found in the centre of the disk ; and from this point to the periphery, where the vegetative points are found, are successive groups of antheridia of younger growth, for the youngest organs of sex always stand next to the vegetative point of the shoot. The stalk of the inflorescence is nothing else than the inferior portion of the branch-system greatly prolonged. In the leafy Jungcrmannicie the distribution of the organs of sex is also either monoecious or dioecious. The arche gonia in this case stand either singly (Lejeunia, &c. ) or in larger groups. The first archegonium always arises from the apical cell of the shoot connected with it, so that with it the latter closes its longitudinal growth. This circumstance has been employed in the classification of the foliose Jungermannicas (with the exception of Haplomitrium) as acrogynous in contradistinction to the anacrogy- nous, in which latter the archegonia never proceed from the apex of the shoot itself. Whilst the archegonia of the thallose forms are protected by being lodged in an excavation of the thallns, or surrounded by a luxuriant growth of its tissue, the archegonia of the foliose Junger- mannicie are envelopeol by the leaves of the stem-bud, which on that account are termed perichaetial leaves (folia pericksetialia), or, collectively, perichsetium. Between these pericha-tial leaves and the archegonia there grows up in most forms a second involucre, of a goblet or pitcher shape, the "perianthium" (fig. 4, B}. It springs up in the form of a circular fence, gradually growing up wards. In many Jungcrmannicse (Calypogeia] the archegonia are enclosed in a kind of sack-shaped structure, which forces its way into the ground, in which, in fact, it takes root. This "fruit- sac " in the Jungermannicie gcocalyccse, is the hollowed-out shoot from which the archegonia arise. Its extremity becomes thickened and forms a circular fence about the archegonia ; thus there arises a structure about two twenty-fifths of an inch long, which is con stantly becoming deeper, and at the bottom of which the arche gonia are placed. There is always an endeavour to protect and envelop the archegonia. This is notably conspicuous in the Marchanticaz, where round every archegonium there grows after