At the top of the common haircap mossrsquo;s (Polytrichum) stalk in the spring form apical genital organs, hidden among the leaves: on some specimens

Life cycle of moss

At the top of the common haircap moss’s (Polytrichum) stalk in the spring form apical genital organs, hidden among the leaves: on some specimens — male (antheridia), on others — female (archegonium) (diagram 139).

In antheridium, having a shape of bag-elongated bodies, covered with multicellular wall, forms the biflagellate sperm in a large amount. In archegonium at the bottom of their extended part — abdomen — forms one egg. In wet weather, during rain, dew, antheridium opens at the top and the sperm swim out. By the time of oocyte maturation archegonium also opens at the top (neck) with duct, formed by spreading inland, the so-called tubular neck cells. Located above the ovum abdominal tubular cell also spreads. Sperm, swimming to archegonium, penetrate to the ovum due to duct mucus, and one of them merges with it, making the fertilization. The fertilized ovum (zygote), without leaving the mother plant, begins to divide, and further, inside the stretched abdomen, archegonium form new seedlings. The upper, narrower part of archegonium — the neck — after fertilization withers and falls off. The newly formed seedling presented by capsule sitting on the leg (called seta). Seta is embedded in the tip of the stem by its base, sucking nutrients that are necessary for the capsule development. In this state, the plant goes under the snow. Next spring the leg becomes longer and brings a capsule high above the cover sheet. Capsule tears off the top of the archegonium abdomen, which remains on it in the form of fibrous formation — a cap (Calyptra), which protects young capsule from drying, lowering the temperature, etc. The mature capsule has the following structure: on top it is covered with a lid, the middle part is called urn, the lower, slightly extended after the constriction, called apophysis (the neck of the capsule). Inside the capsule, in the center of it, passes cord of fruitless cells — the columnella at the top expanding in epiphragm. From all sides around the columnella the sporangium is hanging on filaments, in which spores are formed (Diag. 139).

On maturation of spores (beginning — middle of summer) columnella and wall of the sporangium destroys, and spores lay in the urn cavity. Then a cap falls off (dropping is due to the development of a special apparatus), and on the upper edge of the urn it may be seen so-called peristome, consisting of teeth arranged in a single row. Enter in urn is closed with a film (epiphragm). Peristome teeth are very hygroscopic and change their position depending on the humidity. In wet weather they are bent inward, pressing the film to the edges of the urn, which prevents the penetration of water into the box. In dry weather peristome teeth are folded out, the film shrinks and spores sow outside through the holes between the teeth.

Diagram 139. Common haircap (Polytrichum commune): 1 — stem with antheridium rosettes; 2 — stems with sporogonia; 3 — a capsule covered with a cap (Calyptra); 4 — a capsule with a lid; 5 — assembly of antheridium; 6 — assembly of archegonia; 7 — a longitudinal cut through the capsule: a — urn, b — neck (apophysis), c — cap, d — columnella, e — sporangium with spores, f — epiphragm, g — rudimentary peristome; 8 — a cross cut of the box
Diagram 140. The diagram of the life cycle of green moss Funaria: A — sperm; B — archegonium; C — fertilized archegonium; D — E — sporogony development in the archegonium abdomen; F — sporogonia; G — a box with sporangium; H — I — formation of spores; J — spore germination; K — protonema; L — young moss stalk; M — antheridium

Spore, hitting the damp earth, germinates and forms a richly branched thread — protonema, on which arise buds, developing in some time leafy moss seedlings (diagram 140).

Thus, the moss has an “alteration of generations”. Leafy plant (moss by itself), which forms the genitals (gametophyte), is replaced by a capsule on the seta, inside of which forms the sporangium with spores (sporophyte), and this new generation stays in touch with the gametophyte. Meiosis takes place during the formation of spores in sporangia capsule.

Gametophyte (moss by itself) prevails in the life cycle. Sporophyte developed poorly and is small in size and has a short existence. In some mosses on the capsule grows an assimilation tissue with stomata, but the main nutrition sporophyte receives from the gametophyte. Taken from http://worldofschool.org

Asexual generation in mosses (capsule with the foot) has a special name — sporogonia. Cap belongs to gametophyte (the rest of the archegonium abdomen) (Diag. 140). The sporophyte of mosses is not capable of self-reproduction, except of spore formation. But if you cut any its part (for example, sporogony leg), cut into small pieces and place them in a bowl with the ground, covering it with glass to create a humid atmosphere, then from each piece will develop protonema, on which buds will arise, and then the latter will develop into leafy shoots. This phenomenon — the formation of the gametophyte from sporophyte cells except spores — was named apospory.

Cells of plants that arose this way will contain, unlike conventional gametophyte, double (2X) set of chromosomes. In one such experiment on “diploid gametophyte” it was able to observe the sex organs and after fertilization — the sporogony development containing quadruple set of chromosomes. The sporogony seta, chopped into pieces, when cultivated in the bowl, gave again a protonema and leafy plants with a quadruple set of chromosomes.

These experiments greatly illustrate the failure of characterization the generations only by a set of chromosomes, as diploid and tetraploid gametophytes retain the structure of a conventional gametophyte.

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