In this article we will discuss about the reproduction in Porella with the help of diagrams.
Vegetative Reproduction in Porella:
Porella reproduces vegetatively by the following two methods:
(a) By Progressive Death and Decay of the Gametophyte:
The apical growth of the thallus is accompanied by the progressive death and decay of the older parts of the thallus and consequent separation of the younger parts at the point of dichotomy of the thallus. The separated branches develop to form new plants.
(b) By Gemmae:
In some species (P. rotundifolia) discoid multicellular gemmae ace produced on the lower surface of the leaves. Gemmae germinate to produce new plants.
Sexual Reproduction in Porella:
Porella is dioecious. Male gametophytes are comparatively smaller than the female gametophyte.
Antheridia are borne on specialised lateral antheridial branches which project out at right angles to the main axis (Fig. 6.23A). The dorsal leaves, called bracts, are smaller than those on the main branch and are closely imbricated. The ventral leaves (amphigastria) of the antheridial branch are known as bracteoles. A single antheridium is borne in the axil of each leaf (Fig. 6.23B).
Development of Antheridium:
An antheridial initial, situated at the base of the young bract, divides transversely to produce an outer cell and a basal cell (Fig. 6.24A, B). The basal cell does not divide further and forms the embedded part of the stalk. The outer cell functions as antheridial mother cell which, by transverse division, forms an upper primary antheridial cell and a lower primary stalk cell (Fig. 6.24C-D).
A two-celled thick long stalk is developed from the primary stalk cell, following repeated transverse and vertical divisions. The primary antheridial cell forms the main body of the antheridium. It forms two identical antheridial cells by a vertical division.
Each of these cells, by a periclinal division, forms two unequal cells, the outer smaller first jacket initial and the inner larger primary androgonial cell (Fig. 6.24E). The latter again divides to form a second jacket initial. Both the jacket initials form a single layered jacket of the antheridium following periclinal divisions (Fig. 6.24F-G).
The primary androgonial cell forms a large number of rectangular androgonial cells following several divisions in all possible planes.
Androgonial cells further transformed to androcyte mother cell. Each androcyte mother cell following a diagonal division forms two androcytes, each of which metamorphoses into a biflagellate antherozoid.
Structure of Mature Antheridium:
The mature antheridium is differentiated into a globose body and a long stalk (Fig. 6.24G). The jacket is single-layered at the upper part, while it becomes 2-3 layered at the lower region through periclinal divisions (Fig. 6.24G). The jacket contains inside a mass of androcytes which ultimately metamorphose into biflagellate antherozoids.
The distal part of antheridial jacket is single- layered and thin. This part of the jacket breaks up into many irregular lobes. This allows the antherozoids to release into the water.
Archegonia are produced at the apex of archegonial branch on the female plant. The archegonial branch is much smaller than the vegetative branch, bearing a number of large perichaetial leaves (bracts). The lower bracts form involucre, while the two upper bracts coalesce to form a perianth (Fig. 6.23C). Ten to fifteen archegonia develop within the perianth.
Development of Achegonium:
The archegonia develop in acropetal succession. Each archegonium develops from a single superficial cell, the archegonial initial which increases in size and appears as a papillate outgrowth.
The pattern of development of archegonia is identical with that of Riccia and Marchantia.
Structure of Mature Archegonium:
A mature archegonium differentiates into a neck and a venter (Fig. 6.25). The neck is long, comprises of five vertical rows of neck cells enclosing 6-8 neck canal cells. The venter is 2-layered, consists of a small ventral canal cell and a large egg. A rosette of 4 cover cells is present at the top of the archegonial neck.
Fertilisation of Archegonium:
Like other bryophytes, water is essential for release of sperms and eventual fertilisation in Porella. The process of fertilisation is found to be similar with that of other bryophytes.
The zygote increases in size and secretes a wall around itself. Like other bryophytes; zygote divides transversely to form an epibasal cell and a hypobasal cell (Fig. 6.26A). The hypobasal cell does not divide further, it forms a suspensor. The epibasal cell divides transversely to form two daughter cells.
These daughter cells undergo repeated transverse as well as vertical divisions in regular sequence to form an irregular mass of cells (Fig. 6.26B). Later, the peripheral amphithecium and inner endothecium are differentiated by the periclinal divisions in the upper part of the embryo.
The amphithecium gives rise to the capsule wall, while the entire endothecium functions as archesporium. The archesporium forms the sporogenous tissue through the repeated divisions in all possible planes.
The sporogenous tissue differentiates into spore mother cells and elater mother cells. The elater mother cells become elongated, endowed with two spiral thickening and form the sterile elaters. The spore mother cells divide meiotically to produce haploid spores. The foot and seta develop from the lower part of the embryo (Fig. 6.26C).
Structure of Mature Sporophyte:
The mature sporophyte of porella differentiates into three parts, viz., foot, seta and capsule (Fig. 6.26C). The young sporophyte is enclosed by three protective coverings — calyptra, perianth and involucre (Fig. 6.23C and Fig. 6.26C).
It is the expanded bulbous mass of cells at the base of the sporogonium.
It is an elongated structure which connects -the foot with the capsule, made up of parenchymatous cells.
It is a globose structure containing numerous spores and elaters. The jacket is 3-4 cells thick, made up of thick-walled parenchymatous cells, except for four vertical rows of thin-walled cells that demarcates the vertical lines of dehiscence.
Dehiscence of Capsule:
At maturity, the seta elongates suddenly, pushing the capsule out of the calyptra and perianth (Fig. 6.26D). As the capsule wall dries up, the capsule now splits into four valves along the line of dehiscence. The hygroscopic movement of elaters help in discharging of spores.
The spore is the first stage of gametophytic generation. A spore has two concentric walls: the outer ornamented exine and the inner thin- walled intine. Sometimes a third layer, called perinium, is found outside the exine.
The spore germinates immediately after the fall on the suitable substratum. It differentiates into an apical cell. Then a multicellular thailoid structure is formed (Fig. 6.27).
The rhizoids develop from the lower side and leaves are produced on the upper side.
The germination of spores may take place while the spores are still within the capsule.
Fig. 6.28 shows the life cycle of Porella.