Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/171

Rh IN CRYPTOGAMS.] termed elatcrs (fig. 3), which are very hygroscopic. By germination of these spores a sexual generation or pro- thallus is formed. In Lycopodiacese a sexual process has not been observed in all genera ; but in Selaginella and Isoetes, where it has been observed, it is exceedingly instructive, as it forms a connecting link between the process as seen in Cryptogams and that observed in Angiosperms. The asexual generation produces the fructification which consists in the formation of a single sporangium in the axil of each leaf of a certain part of the axis. In those genera in which sexual organs have been discovered these sporangia are of two sizes, termed macrosporangict (fig. 347) and microsporauyia respectively. The whole fructification frequently forms a spike. In the macrosporangium or oophoridiwm few, usually four, largo spores (macrosporcs) are formed, having each an exospore and an endosporc. In the microsporan- gium (fig. 248) numerous small spores or microspores are formed of a similar structure. When the sporangia are shed the capsule decays, and the spores escape. Within the endospore of the macrospore, a development of cellular tissue takes place which eventually completely fills it and constitutes tliejwotkallus, termed sometimes on this account cndotkallus (fig. 348). As it increases in size the exospore is ruptured triradiately, and the endospore projecting is also ruptured, and thus the prothallus is protruded, in the sub stance of which archegonia are formed. The microspores, on the other hand, produce within their endospore, some times after the formation of a rudimentary prothallus, antherozoid mother-cells, which are discharged by the rupture of the exospore and endospore (fig. 249). By fertilization of the oosphere of the archegonium an oospore is formed, from which the sexual generation arises. The embryo is formed from the lower part of the oospore, the upper half elongates and forms a sns2)cnsor (fig. 349) of several cells, by which the embryo is pushed downwards into the substance of the prothallium, and there develops into a young plant, rudimentary cotyledons being formed upon the young axis. It will be observed that this process resembles very much the mode of embryogeny in Phanerogams, and, indeed, from this it is but a step to the form found in Gymnospertns and thence to Phanerogams. In Ilhizocarps the sporangia are formed in the interior of ovoid sacs, termed sporocarps (fig. 350), which are usually supported on stalks, and rise from the leaves near their base, one or many on the same stalk. In each sporo- carp, microsporangia and macrosporangia are formed in various positions ; and a prothallium (fig. 351) is formed i;i the macrospore, as in Lycopodiaceso. From the oospores formed in it the embryos a^e produced, but without suspensors. In Characere there is an alternation of a sexual with an asexual condition. From the oospore a small proembryo is first formed, from a cell of which the sexual plant is developed. In Mosses the sexual generations are produced as lateral shoots from a filamentous proembryo or protonema (fig. 352). Upon this leaf-bearing axis either at the apex (acrocarpous) or laterally (pleurocarpous) the sexual organs arise. The asexual generation rises from the oospore and constitutes what is usually termed the fruit (fig. 353) of the moss. It has been designated the sporogonium. The sporogonium is formed by ths enlargement of the oospore into an ovoid embryo which presses against the epigone, finally rupturing it, and carries the upper portion upwards with it as the calyptra or cap, the lower portion forming a sheath or vaginula at the base. This calyptra may be cither split on ono side, dimidiate, or entire, then termed mitriform. The apper part of the sporogonium forms a capsule, urn, or thtca ; the lower portion acts as a 161 supporting stalk or seta, which is often swollen, just beneath the capsule forming the apophysis (fig. 354, a). In Sphagnum the sporogonium is fully developed within the epigonal leaves, and when complete the axis beneath it elongates, forming the pseudopodmm, and projects con siderably beyond the pcricluetium. The walls of the capsule of the sporogonium consist of several layers of cells. It has a central axis or columella, surrounding which aro the spores. When mature the capsule usually dehisces, Fig. 352. Fig. 353. FIG. a r &amp;gt;2. Moss (Funatia hyyronifttica), showing the cellular protonema/), or proembryo, with its roots r, and buds, b, whence arise leafy stems bearing the organs of reproduction. FIG. 358. Hygrometrlc Cord-Moss (Funaria hygrometricd), with its urn-like theca u supported on stalks /&amp;gt;, which arise from perich^tial leaves /. Tho thecse are covered by a calyptra c, which splits on one side. The opcrculum or lid o is seen in one of the thecaj from which the calyptra has fallen. Tho seta is twisted, and displays hygrometric properties. either by four longitudinal valves, as in Andraea, or most commonly by a lid or operculum (fig. 355), between which and the capsule an annulus or ring of cells is separated. On removal of the operculum the stoma or opening of the capsule is seen. The margin of this is sometimes entire (gymnostomi), more usually fringed with teeth or cellular prolongations, constituting a peristome (fig. 355, p), either in one row (aplopcristomi) or in two rows (diploperiatomi), Stretching across the mouth of the capsule is frequently seen a thin cellular membrane, the epiphragm or Fig. 354. Fig. 355. Fig. 356. FIG. 354. Fructification of the Hair-Moss (Polytrictium), consisting of an urn- like (urceolatc) case or theca o, supported on a stalk (seta) s, and covered by a calyptra c, which splits at the base. Fio. M5. Theca u of the Extinguisher-Moss (Encdtiiptci), showing the teeth of the pciistomcp, which fold inwards when moisture is applied. FiG.. Uifi. Vertical section of a Mushroom (Agaricus canipestn s). my, mycelium or spawn ; to?, remains of volva or wrapper; st, stipe or stalk: an, annulus or ring, being the remains of the velum, veil, orcortina; /a, lamella) or gills of tho hynienium h ; p, the pilcus. tympanum, as in Polytrichum. The spores shed from the capsule germinate and give rise to the proembryo which by budding gives off the sexual plants. j n jj e a . In Hepaticrc the sexual generation arises either directly tic. IV. 21