Riccia Structure, Life Cycle, Sexual and Asexual Reproduction

Riccia belongs to the family Ricciaceae of the order Marchantiales. Majority of the species of this genus are terrestrial but some species are aquatic as these are found growing actually in the water. 

The following seven species have been commonly reported from the North Western Himalayas and the Punjab plains in Pakistan:

Riccia robusta;R. melanospora; R, cruciata; R. sanguinea,’ R. himalayansis; R.pathankotensis, Riccia robusta; R sanguinea and R. pathankotensis are very common in the Punjab plains found growing on the moist soil in places like river beds, banks of rivers and canals, and along the streams. 

RICCIA VEGETATIVE STRUCTURE 

Riccia is a thallose liverwort in which the vegetative plant, which is a gametophyte, generally forms rosettes due to the crowded growth of the thallus lobes this crowded growth of the various lobes is due to the repeated dichotomies of the thallus The thallus lobes arc flat growing horizontally on the soil and each lobe has a small not that the apex where the growing point is situated. 

The lobes are thicker in the middle and gradually become thinner towards the margins. A deep cleft or furrow is commonly present in the middle of each lobe on the dorsal side; the sex organs are found embedded in this furrow. The lobes in Riccia fluitans, which is an aquatic species, are narrow and ribbon-like having repeated dichotomous branches which do not form any rosetts, Numerous unicellular rhizoids are found attached to the ventral side of the thallus, these rhizoids are generally crowded in the middle part of the thallus and are of two types i.e., smooth and tuberculate as in Marchantia. 

The ventral side also bears scales which are formed of a single layer of cells and are arranged in two rows in the older parts of the thallus while in the younger parts these scales are in one row and in the apical region are crowded below the notch for the protection of the growing point. 

INTERNAL STRUCTURE OF THE THALLUS 

The thallus of Riccia has a simple structure and is formed of parenchymatous cells which are bounded on both sides by the upper and lower epidermis. Internally the thallus is differentiated into two regions, upper assimilatory region and the lower storage region. 

The storage region is formed of compactly arranged parenchymatous cells which are either colourless or contain only few chloroplasts, but these cells are rich in starch grains. Some of the cells of the lower epidermis grow out and elongate forming the rhizoids, while certain cells divide and produce the single layered scales. 

In the majority of species the assimilatory region is formed of vertical rows of cells which are about six to eight cells in height and are separated by large air spaces, the cells of these filaments are rich in chloroplasts as these form the main photosynthetic tissue. 

The cells of the upper epidermis are also loosely placed having spaces in between them which function as pores through which the underlying air spaces communicate out for the exchange of gases. Some species the assimilatory region is spongy being formed of irregularly placed air spaces which are separated from each other by single layered partitions whose cells are rich in chloroplasts. 

REPRODUCTION IN RICCIA 

Riccia plants, like other liverworts, multiply vegetatively by the decay or death of the Older parts which results in the separation of younger branches, each of which grows into an independent plant. In certain species (R. fluitans) small adventitious lateral branches may be produced from the ventral side of the thallus, these adventitious branches on detachment give rise to new plants, Formation of vegetative reproductive gemma-like multicellular bodies at the apices of rhizoids have also been reported in certain species. 

SEXUAL REPRODUCTION 

Riccia plants may be monoecious or dioecious depending upon the species. Both the male (antheridia) and female (archegonia) reproductive organs are produced in acropetalous order on the dorsal side of the thallus in the median furrow. In the monoecious species the antheridia are generally produced earlier than the archegonia. 

ANTHERIDIA 

Antheridia are found in a linear row on the dorsal side of the thallus embedded in the median furrow.Each antheridium is a globular structure having a very short insignificant stalk. The wall of the antheridium consists of a single layer of cells and encloses a mass of androcytes or antherozoid mother cells. 

At maturity the contents of each androcyte are transformed into a typical coiled biflagellate antherozoid. When the antherozoids are produced the original walls of the androcytes become gelatinous thus producing a mucilaginous mass in which the antherozoids float freely. The antherozoids are released in the surrounding water by the bursting of the antheridial wall and then these swim freely in the water. 

DEVELOPMENT OF THE ANTHERIDIUM 

Each antheridium develops near the apex from a superficial cell which divides transversely into a lower basal cell and an outer cell. The basal cell divides further producing the basal part of the antheridial stalk which remains embedded in the thallus tissue. The outer cell divides transversely producing a row of four cells, The two lower cells of this row by further transverse and vertical divisions produce a short stalk of the antheridium. 

In the upper two cells of the row two vertical divisions take place at right angle to each other producing a group of eight ceils or octants. Periclinal divisions then take place in these octants producing outer jacket initials and inner androgonial initials. The jacket initials by further anticlinal divisions only produce the single layered wall of the antheridium; while the androgonial initials by further transverse and vertical divisions produce a mass of androgonial cells which are enclosed by the single layered wall. 

The last division in each androgonial cell is diagonal thus producing two androcytes (antherozoid mother cells). The contents of each androcyte are transformed into an antherozoid. As the development of the antheridia is going on the surrounding vegetative tissue grows up and the antheridia become embedded in the dorsal furrow. The thallus continues its growth at the apex while the development of the antheridia is going on, therefore, the mature antheridia are lying some distance back from the apex. 

ARCHEGONIA 

The archegonia, like the antheridia, are also embedded in the mid-dorsal furrow of the thallus and are arranged in acropetalous order i.e., the younge archegonia lie towards the growing point while the older ones are towards the hinder region. Each archegonium is a flask-shaped structure having a multi-cellular stalk. 

The basal swollen portion of the archegonium is known as the venter and the upper elongated portion as the neck. The wall of the neck consists of a single layer of cells which are arranged in six vertical rows. The canal of the neck contains a row of four neck canal cells and is closed at the tip by four lid cells. The wall of the venter is also formed of a single layer of cells which are arranged in several vertical rows. 

The venter contains a lower larger cell, the egg or oosphere, and a smaller upper cell known as the ventral canal cell. The necks of the mature archegonia protrude out above the general surface of the thallus, When the archegonia are mature the ventral canal cell and the neck canal cells disorganise forming mucilaginous mass which swells by absorbing water and comes out the neck by pushing apart the lid cells thus forming an opening for the entry of antherozoids. 

DEVELOPMENT OF THE ARCHEGONIUM 

Each archegonium develops from a superficial cell situated on the dorsal side about three or four cells away from the growing point. This archegoniai initial divides by a transverse division into an upper primary cell and a lower primary stalk cell. The primary stalk cell by further transverse and longitudinal divisions produces a short multicellular stalk. 

ln the primary archegonial cell three excentric vertical divisions take place resulting in the production of three peripheral cells around an axial cell. A vertical division takes place in each peripheral cell resulting in the production of six jacket initials. The axial cell cuts of a small cover cell at the tip.A transverse division takes place in all the cells resulting in the production of a lower and an upper group of cells. 

The upper group of Peripheral cells divides by transverse divisions only producing the single layered wall of the neck in which the cells remain arranged in six vertical rows. The lower group of Peripheral cells by further anticlinal divisions produces a single layered wall of the venter. In the meantime the axial cell by a transverse division has divided into an upper neck canal initial and a lower central cell. 

The neck canal initials produce a row of four neck canal cells. The central cell divides Producing an egg or oosphere towards the base and a small ventral canal cell towards the upper side. 

FERTILIZATION 

Fertilization takes place in the presence of water the antherozoids liberated in the surrounding water swim towards the archegonia, being attracted chemotactically by the mucilage coming out of the archegonia. Several antherozoids may enter a single archegonium but only one of them fuses with the egg for fertilization. 

The male and female nuclei fuse together producing a single diploid nucleus. The fertilized oosphere secretes a wall and becomes the oospore which develops into the sporogonium. The mature sporogonium in Riccia is of very simple structure. it is globular or rounded in shape and is enclosed in the enlarged venter of the archegonium. 

This sporogonium is not differentiated into foot, seta and capsule as in other liverworts. The of the mature capsule consists of a single layer of cells surrounding a mass of spore mother cells. The mature sporogonia are embedded in the tissue of the thallus and are visible to the naked eye as small black dots on the thallus. 

The nucleus of each spore mother cell undergoes meiosis (reduction division) producing four haploid nuclei, and each spore mother cell then divides into four uninucleate parts, each of which is transformed into a single haploid spore. The four spores produced from each spore mother cell remain held together in tetrads for a long time. 

The wall of each mature spore is very unevenly thickened and it consists Of three layers, the outer exosporium which is thin, hard and cutinised; the middle mesosporium which is very thick and soft; and the inner endosporium which is membranous. All the spore mother cells in a sporogonium are generally transformed into spores there are no sterile cells or elaters. 

In certain species few sport mother cells do not produce spores but disorganise producing a mucilaginous fluid, The original wall of the sporogonium disorganises producing a mucilaginous fluid before the maturation of spores. The mature spores are actually enclosed by the outer layer of the enlarged venter which is Often mistaken as the sporogonium wall. 

The spores are released by the bursting of the wall of the enlarged venter which is comparable to the calyptra of other liverworts The release of spores is delayed for a long time till the decay Of the vegetative tissue which may take even a year or so after the maturation of spores. 

DEVELOPMENT OF THE SPOROGONIUM 

The oospore increases very much in size filling the whole cavity of the venter. enlarged oospore then divides by two vertical and one transverse division, all at right angle to each other, producing a group of eight cells or octants. These octants divide further producing a group of about 20-30 cells. Then the periclinal divisions take place in. These ceils resulting in the production of an outer layer of jacket initials or amphithecium and an inner mass of archesporial initials or endothecium. 

The cells of the amphithecium by further anticline! divisions give rise to a single layered wall of the sporogonium; while the cells cf the endothecium by repeated divisions produce a mass of archesporial cells. When the development of the sporogonium is initiated periclinal divisions take place in all the wall cells of the venter making the venter wall double layered. 

The venter goes on increasing in size along with the developing sporogonium, and in this way the mature sporogoniurn remains enclosed in the double layered wall of venter. The cells of inner layer of the venter wall the sporogonial wall along with some spore mother cells disorganize producing a viscous liquid.

Each spore mother cell produces the four spore the unsual method after the reduction division of its nucleus, spore mass is enclosed in the outer layer and not by the sporogonium wall which has disorganised. As all stated the mature spores are released by the bursting of this wall after a long time when the tissue of the thallus has decayed.  

GERMINATION OF SPORES 

When a spore falls on a suitable place, it germinates by the production of a long tube. This germ tube is produced by the rupture of the outer hard exosporium and mesosporium, and then the endosporium forming the germ tube. The protoplasmic of the flow into the tip of this germ tube which becomes separated from rest of the by the formation of a septum.

Riccia Life Cycle

ALTERNATION OF GENERATION 

The vegetative plant or thallus in Riccia, is a gametophyte, whose cells are haploid. This gamete producesmale(antherozoids) and female (oosphere) gamete surrounding vegetative tissue. The apical growth of the thallus continues and the mature archegonia come to lie near the hinder portion. 

Sometimes all the developmental stages can be traced in the single thallus due to the production of archegonia in an acropetalous succession, antheridia and archegonia respectively. The union of these gametes or fertilization results in the production of an oospore which is diploid. The oospore develops into the sporogonium or the sporophyte which represents the asexual stage as it produces the spores asexually. 

The spores are haploid as these have been produced after the reduction division in the spore mother cells. The spores germination give rise to the gametophyte. For the completion of the whole life cycle the gametophyte produces the sporophyte, this sporophyte produces the gametophyte again, and these two generation regularly alternate with each other.