Reproduction in Pogonatum

In this article we will discuss about the vegetative and sexual modes of reproduction in Pogonatum.

Vegetative Reproduction in Pogonatum:

Vegetative propagation of Pogonatum takes place by the following methods:

Gemmae:

Bud-like gemmae develop on the rope-like rhizoids and each gemma forms a gametophyte after germination.

Secondary Protonema:

The secondary protonema develops from any part of the plant other than spores. Many buds are produced on the protonema which are capable of growing into new plants.

Sometimes, a diploid protonema is pro­duced aposporously from the tissue of sporo­phyte without the formation of spores, thus giving rise to a diploid gametophyte.

Sexual Reproduction in Pogonatum:

Pogonatum is usually dioecious i.e., hetero- thallic, although a few monoecious species (P. microstromium) have been reported. The development of sex organs in Pogonatum is similar to that of Funaria.

Antheridial Head:

Antheridia are borne at the apex of the male shoot (Fig. 6.54B). They are surrounded by the perichaetial leaves which are red or orange in colour (Fig. 6.57B). The antheridial head forms a compound cluster cup structure or inflorescence (Fig. 6.57A). At the base of each perichaetial leaf, clusters of antheridia and hairs like multicellular paraphyses are present (Fig. 6.57B).

Antheridium:

The development of antheridium in Pogonatum is similar to that of Funaria. The mature antheridium is made up of a club-shaped body and a short multicellular stalk (Fig. 6.57B). The body has a single-layered jacket surrounding a central mass of androcyte mother cells.

Each androcyte mother cell divides mitotically to form two androcytes and each androcyte is metamor­phosed into a curved biflagellate antherozoid or sperm (Fig. 6.57C).

Archegonial Head:

Archegonia also develop in clusters of 3-6 at the top of the female shoot (Fig. 6.57D). They are also surrounded by paraphyses and perichaetial leaves.

Archegonium:

The development of archegonia in Pogo­natum is similar to that of Funaria.

The mature archegonium is a shortly stalked flask-shaped body, differentiates into a massive venter and a long neck (Fig. 6.57E). The archego­nial jacket is single-layered in neck region, while it is multilayered in venter region. The neck con­sists of six vertical rows of neck cells and 6-9 neck canal cells. The venter contains a ventral canal cell and a large egg or oosphere.

Fertilisation of Archegonium:

The fertilisation process in Pogonatum is like that of other mosses. At the top of the archegonia a clear passage is formed due to the dissolution of neck canal cells and ventral canal cell. Like other bryophytes, water is essential for free swimming of sperms towards the neck of the archegonium.

Finally, one antherozoid fuses with the egg to form a diploid zygote or oospore, thus the sporophytic generation begins. More than one archegonia may be fertilised, but usual­ly one sporophyte is matured in the archegonial branch.

Sporophyte:

Development of the Sporophyte:

The development of sporophyte begins with the enlargement of the zygote which secretes a wall around itself. The zygote divides transverse­ly to form an upper epibasal cell and a lower
hypobasal cell. Both of these cells divide by two successive diagonal divisions, so that a wedge- shaped apical cell is differentiated both in the epibasal and hypobasal cells.

Thus, two terminal growing points are established at the two oppo­site ends, each with two cutting faces (Fig. 6.58A). The apical cells are further divided to form a young embryo, though the epibasal (upper) apical cell divides in a more regular way than the hypobasal (lower) apical cell.

The cap­sule and the upper part of the seta are formed from the epibasal part. The basal part of the seti and the foot are derived from the hypobasal part. The archegonial wall enlarges simultaneously and becomes the calyptra.

During the development of embryo, the epibasal part, just below the apical cell, is diffe­rentiated into two fundamental embryonic lay­ers, the outer amphithecium and the inner endothecium. The amphithecium forms multi- layered jacket of the capsule, the trabeculae in the outer air space and the outer wall of the spore sac.

The endothecium forms the archespo­rium, the central columella and the trabeculae in the inner air space. The operculum region of the capsule develops from both the amphithecial and endothecial cells.

Structure of the Mature Sporophyte:

The mature sporophyte of Pogonatum is differentiated into a foot, a long seta and a cap­sule (Fig. 6.58C).

1. Foot:

It is a dagger-shaped structure, pene­trating the tip of the gametophyte. The foot is made up of thin-walled parenchymatous cells.

2. Seta:

It is green in colour when young and becomes reddish to deep-brown at maturity. Anatomically, it is differentiated into an epider­mis of one layer, hypodermis of several layers of thick-walled cells, a cortex of loosely arranged parenchymatous cells and a central cylinder of very thin-walled cells (Fig. 6.58B).

3. Capsule:

The capsule is differentiated into the following three parts:

(a) Opercular Region:

It is a conical apical part of the capsule which is a beaked cup-like structure (Fig. 6.58E). It is connected with the theca by a ring-like diaphragm, but there is no organised annulus.

The peristome is situated just below the operculum with a ring of 32 (viz. P. microstromium, P. stevensii) or 16 (viz. P. perichaetiale) peristome teeth. The peristome teeth are solid, pyramidal structure, made up of longitudinal fibres derived from concentric layers of cells of the capsule (Fig. 6.58D).

Hence they are included under the Nematodonteae type. The peristome teeth are attached to the margin of the epiphragm which is a mem­branous extension of the distal end of columella.

(b) Theca:

It is the middle part of the cap­sule. It is urn-shaped fertile part of the capsule which appears oval or elliptical in outline when viewed in a transverse section. The jacket or wall of the theca is 4-5 layered thick. The outermost layer is the epidermis which is devoid of stoma- ta (Fig. 6.58E).

An outer air space is present inner to the jacket layers, which is traversed by many radially extended trabeculae. The outer air space is followed by cylindrical spore-sac bounded on either side by a two-celled thick wall. The young spore-sac contains archesporium, while a mature sac contains numerous spore tetrads.

The spore-sac is further surrounded on the inner side by an inner air space which is again traversed by many trabeculae. The central part of the theca is occupied by the sterile tissue known as colu­mella (Fig. 6.58E) which provides mechanical support to the capsule and also helps in the conduction of water and nutrients.

(c) Apophysis:

It is the basal part of the capsule, mostly comprised of parenchymatous cells. It has a thick-walled epidermis. The central part is occupied by the conducting strand which js in continuity with the columella and seta.

Dehiscence of Capsule:

At maturity, the capsule dries up, the calyp­tra falls off and the operculum is broken loose by the pressure of the columella. Subsequently, the operculum drops off exposing the peristome teeth.

The cells between the epiphragm and the peristome teeth break forming minute hole. The spores are released through the minute holes that are controlled by the upward and down­ward movement of the epiphragm and peris­tome teeth.

New Gametophyte:

The spore is the first cell of the gametophy­tic generation. The spores are small and spheri­cal (Fig. 6.59A). The spore wall is differentiated into an outer smooth-walled exine and an inner delicate intine. The spores contain a conside­rable amount of chloroplasts and oil bodies in their cytoplasm.

The spore germinates under favourable conditions. The exine ruptures and the intine comes out in the form of one or more green algal filament (Fig. 6.59B). The filament cuts off cells, becomes septate and forms filamentous primary protonema. The protonema branches freely by means of an apical cell and sub­sequently, forms two types of branches, viz. chloronemal branches and rhizoidal branches (Fig. 6.59C).

The non-green rhizoidal branches develop below the substratum and are meant for anchoring the protonema in the substratum. The chloronemal branches are green and aerial being rich in chloroplasts and bear many late­ral buds. Each bud grows into an adult leafy gametophore.