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POGONATUM : morphology, anatomy, reproduction etc.

Pogonatum, commonly known as spike moss, is a genus of approximately 70 species of mosses primarily found in tropical Asian regions. Its morphology includes a perennial gametophyte with a basal rhizomatous portion and aerial upright shoots, while its anatomy features a complex stem structure with specialized cells for transport and storage. The reproductive methods include vegetative propagation through gemmae and secondary protonema, alongside sexual reproduction involving dioecious structures, culminating in the development of sporophytes that eventually release spores for the next gametophytic generation.

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POGONATUM
Kingdom Division Class Order Family Genus : Plantae : Bryophyta : Bryopsida : Polytrichales : Polytrichaceae : Pogonatum POGONATUM “The spike moss”
AN OVERVIEW ● HABIT ● MORPHOLOGY ● ANATOMY ● REPRODUCTION ● THE SPOROPHYTE ● DEHISCENCE OF CAPSULE ● NEW GAMETOPHYTE
HABIT Pogonatum is a genus of mosses — commonly called spike moss — which contains approximately 70 species that cover a cosmopolitan distribution. ● It can be seen mostly in Asian countries with a tropical climate.
Morphology ● The gametophyte is perennial, differentiates into a rhizomatous basal portion and aerial upright shoots (Fig. 6.54A). ● The basal portion bears numerous rhizoids. ● The rhizoids are multicellular, elongated, thick-walled with oblique septa and are twisted together in a rope-like manner to form strong cable-like strings.
● The aerial shoot may be unbranched or branched, composed of a central axis, covered with spirally arranged leaves. ● In the lower part of the upright stem, the leaves are very small, scaly and much paler in colour. ● While, in the upper part of the stem, the leaves are comparatively larger with broad sheathing bases. ● The upper part of these large leaves extend into a narrow brown or deep red coloured limb having serrated margins. ● The leaves are provided with a thick midrib and two laterally extended thin and narrow wing-like lamina.
Anatomy Stem A T.S. of the stem shows an epidermis, the cortex and the central cylinder consisting of elongated leptoids and hadrom cylinder . ● The epidermis is the outermost covering of the stem, composed of thick- walled cells. ● The epidermis is broken up giving an irregular outline of the stem by the interruption of a number of leaf bases.
The cortex is differentiated into two zones, the outer cortex and the inner cortex. ● The outer cortex is composed of deeply coloured, 3-5 layered chlorenchymatous cells. ● The inner cortex is made up of thin-walled parenchymatous cells, full of starch.
Many leaf-traces are present in the outer part of the inner cortex, comprising of a small group of thick-walled cells. ● Next to cortex, a zone called leptoid is present which is comprised of elongated living cells, devoid of starch. ● This zone is comparable to the sieve tubes of the higher plants. ● There is a zone of leptoid cells interspersed with starch-containing cells, called leptoid mantle.
The central zone of the stem is called hadrom cylinder, comprised of two types of cells: ● the elongated thick- walled cells with protoplasts, called stereides, ● and thin-walled cells, devoid of protoplasts, called hydroids.
Leaf ● The upper surface of the midrib is completely covered by parallel vertical plates of cells, called lamellae . ● This is comparable to the gills of Agaricus fungi. ● Each lamella consists of 4-8 cells . ● All the cells, except the terminal one contain chloroplasts.
● The terminal cell is larger than the remaining cells of the lamellae. ● In some species (viz. P. microstromium), terminal cells split into two conical cells. The lower epidermis forms a distinct layer, comprised of thin-walled cells.
Reproduction Vegetative propagation Vegetative propagation of Pogonatum takes place by the following methods: ● Gemmae: ● Secondary Protonema: 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 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. ● They are surrounded by the perichaetial leaves which are red or orange in colour . ● The antheridial head forms a compound cluster cup structure or inflorescence. ● At the base of each perichaetial leaf, clusters of antheridia and hairs like multicellular paraphyses are present .
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 . ● 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 .
Archegonial Head: ● Archegonia also develop in clusters of 3-6 at the top of the female shoot . ● 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 . ● 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.
The sporophyte
Structure of the Mature Sporophyte: The mature sporophyte of Pogonatum is differentiated into a foot, a long seta and a capsule . 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 .
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 . It is connected with the theca by a ring-like diaphragm, but there is no organised annulus. (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 stomata.
(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 is 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 near the peristome teeth breaks forming minute hole. ● The spores are released through the minute holes that are controlled by the upward and downward movement of the peristome teeth.
New Gametophyte ● The spore is the first cell of the gametophytic generation. ● The spores are small and spherical. ● 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. 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 . ● 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.
LIFE CYCLE