Doc: Riccia - Class 11

Kingdom    : Plantae

Sub kingdom   : Embryophyta

Division : Bryophyta

Class    : Hepaticopsida

Order   : Marchantiales

Family : Ricciaceae

Genus  : Riccia

Habit And Habitat 

Riccia was discovered by F.F. Ricci.

Species of Riccia are generally found in the moist land.

The common Indian species are as follows :

i.    Riccia discolor/R.himalayensis

ii.   R. glauca

iii.  R. pathankotensis

iv.  R. robusta

v.   R. crystallina

Riccia discolor found in Rajasthan.

Some species of Riccia are aquatic.

Such as - Riccia fluitans, R.abuensis, R.riella etc.

STRUCTURE 

 The main plant of Riccia is gametophyte in the form of thallus.

It is green, flat, dorsiventral and dichotomously branched. It grows prostrate on the soil.

At the growing point of thallus a notch is present, at the apical end. It is known as Apical notch.

At the base of apical notch a apical cell is present. It is pyramidal shape. The thallus grows by the activity of this apical cell.

Each lobe of the thallus is thick in the middle and thin at their margins.

On the upper (dorsal) surface of each lobe vein is present in the middle which extends from anterior to posterior. It is known as longitudinal median vein.

A long groove is present in the region of median vein, is called Median groove.

In this groove, sex organs are developed in acropetal order (New at apical notch old away from it) on the dorsal surface of thallus.

Note : The dorsal surface of the Marchantia thallus (Liver shaped) bears gemma cups along the mid rib.

(a) Rhizoid : 

• Rhizoids are found in the midrib region on the ventral/Adaxial surface of the Riccia thallus. (In scattered form)
•  Rhizoids are unicellular, unbranched, colourless and tubular.

Two type of rhizoids are found in Riccia

1.   Smooth-walled :

• Outer and inner walls of rhizoids are smooth & straight.

2.   Tuberculated :

• Peg like ingrowths are present on the surface of inner wall.
• Note : Smooth walled & tuberculated rthizoids are also found in Marchantia & Plagiochasma.
• Rhizoids are absent in the aquatic species of Riccia. Such as R.fluitans.

Function : The main function of the rhizoids are is to fix the plant with the substratum and it absorbs some amount of water and minerals.

• The whole ventral surface also absorbs water and minerals.

(b) Scales :

• Triangular, multicellular and violet colour scales are present on the margin of ventral surface of thallus.
• Violet (Purple) colour of the scales is due to presence of anthocyanin pigment in their cell sap.
• They are ligulate. The scales are single celled thick. Maximum scales are found in apical notch.
• Scales are absent or less developed in aquatic species of Riccia.
• Plant growing moist terrestrial habitats usually have small and ephemeral scales, whereas those of dry habitats have large and persistent Scales.

Note : In Marchantia scales are appendiculate & ligulate.

• In Riccia the half scales are found at margin of the thallus. Complete scales are found in the apical notch.

Function :

(1)  The main function of the scales is the protection of growing points.

(2)  Some amount of water also absorbed through the scales.

• Scales are formed by the activity of the apical cell and arranged in a single row, but as the thallus grows and widen, each scale splits into two halves along the median line, so that two rows of scales are seen, one row near each margin of the thallus.
• In terrestrial species e.g. R.sanguinea, Riccia robusta etc. scales are completely absent. 
• Interally, the thallus is divided into two distinct zones or regions.

(i)   Upper or dorsal : Photosynthetic or assimilatory region.

(ii)  Lower or Ventral : - Storage region.

(i)   Assimilatory Region or Photosynthetic region : -

• The cells of this region are loosely arranged and parenchymatous.
• Chloroplasts (Discoid type) are found in the cells of this region. So this region is able to carry the process of photosynthesis. Therefore it is known as photosynthetic region.
• The chlorophyllous cells are arranged in vertical rows.
• A narrow, deep space is found in between the vertical rows, called airspace or air canal.
• Upper cell of each vertical row is colourless, larger in size & differentiated to form upper epidermis.
• Air canal opens at upper epidermis through the pore called air pore.
• Air canals are schizogenous in origin. It means it is formed by the separation of cells.
• The presence of air canal or air chamber is the ancestral feature. They show aquatic ancestral Character.
• Food material synthesizes in this region through the photosynthesis.

(ii)  Storage Region : -

• The cells of this region are colourless, compact, parenchymatous and without intercellular spaces.
• The starch is stored by the cells of this region as food.
• The lower most layer of cells are arranged systematically to form a lower epidermis. 

Reproduction  

It reproduces by two different ways.

(i)   Vegetative Reproduction   (ii)   Sexual Reproduction

• It is responsible for the gregarious (Growing close together but not matted) habit of the plant. It takes place by following methods:

(a) By progressive death and decay of older parts of thallus.

• It is the normal method of vegetative reproduction. It takes place in favourable growth season (period).
• The older part of the plant body gradually becomes aged, die and ultimately decay.
• When this process of decay reaches up to dichotomous point, two branches get separated and each branch form a new thallus through the apical growth.

(b)  By Rhizoidal tips.

(c)  By Adventitious branches.

(d)  By tuber formation.

• In many species like R.discolor, R.perennis growing in dry habitat.
• When the dry season starts, the apical cell divides to form a multicellular mass of cells on the apex of each branch. This mass of cells of the lobes becomes thick due to storage of food material and develop a thick protective layer around it after degeneration of marginal cells. This thick structure is known as tuber.
• The tubers remains dormant in dry season. On the returns of favourable condition or moist season, each tuber resumes growth and give rise to new thallus. Tubers shows "perennation".
• By persistent apices/ By death of thallus in drought condition except growing region.
• This method found in region having prolonged dry season as in Punjab and Rajasthan. eg., R.discolor.
• In the beginning of dry season entire body of the thallus becomes dry except the apices.
• The apices very often grow down into the soil and becomes thick due to the storage of food material.
• On the advent of favourable condition. (ie., rainy season) all the apices grow and form a new plant. 
• The main plant of Riccia is gametophyte. It reproduces by gamete formation.
• It is oogamous type.
• Male sex organ called as antheridium and female sex organ called as archegonium.
• The sex organs of Riccia are borne on dorsal surface of the thallus in the median groove.
• The sex organs are developed singly, usually deep in the tissues in acropetal order. (Youngest sex organ near the apex and oldest sex organ away from the apical notch.)
• The species of Riccia are Monoecious and dioecious. But most of the species of Riccia are monoecious.

(a)  Monoecious species : Male and female sex organs lies on the same thallus. Such as: R.robusta, R.glauca, R.pathankotensis, R.gangetica etc.

(b)  Dioecious species : Male and female sex organs are present on separate thallus such as R.discolor  / R.himalayensis.

[i] Antheridium :

• Each mature antheridium is some what elongated oval or pear shaped and stalked structure.
• Each antheridium enclosed in a cavity or chamber, known as antheridial-Chamber.
• Each antheridium has a short, few celled stalk by which it is attached with the base of antheridial chamber.
• The jacket of the antheridium is made up of sterile cells.
• Jacket is single celled thick and multicellular.
• Antheridium has large number of small cells, called Androgonial cells.
• An oblique or diagonal cell division takes place in each androgonial cell. Resulting, two androcytes or antherozoid mother cells (Sperm mother cells) are formed.
• Each androcytes converts into a male gamete (elongated nucleus present) or antherozoids by metamorphosis.
• These antherozoids are motile male gametes.
• The are comma like or curve shaped and biflagellate. (In Marchantia rod like and biflagellate.)

Note : Most of the species of Marchantia are dioecious or heterothallic.

• At the maturation of antheridium cell wall of the androcytes dissolves during metamorphosis and mucilage is formed.
• It is hygroscopic in nature.
• The apical cells of Antheridial Jacket absorb water by imbibition from the antheridial chamber Resulting, a pressure increase on the apical region of antheridium and the Jacket bursts at the apex then antherozoids come out with mucilage.
• On coming in contact with water, the antherozoids swim freely with the help of their flagella.

Dehiscence of antheridium takes place in the presence of high amount of water, is called Hydrochasy.

[ii] Archegonium :

• A mature archegonium is flask shaped structure and enclosed in archegonial chamber.
• It is attached with the base of archegonial chamber with the stalk.
• The basal swollen portion of archegonium is called Venter.
• The archegonium has a single layered jacket.
• The jacket in the neck region is composed of six vertical rows of cells. In each vertical row 6 to 9 (mainly 6) neck cells are present.
• Venter region contains two cells. A large cell which is termed egg or oosphere and above the oosphere is a small venter canal cell.
• The neck region has 4(mainly) – 6 neck canal cells.  (In Marchantia 4-8 N.C.C.)
• The four terminal cells of jacket of the neck of archegonium function as cover-cells.

When archegonium is fully mature, the neck canal cells and venter canal cells of archegonium degenerate.

Only one cell remains which is called egg cell.

Disintegration of the cells gives rise to mucilaginous substance.

Mucilage is hygroscopic in nature. It absorbs water, results increase a pressure in archegonium and consequently the cover cells of archegonium separate from each other and forming funnel shaped opening and the  mucilage oozes out.

Inorganic potassium salts and soluble protein are present in mucilage which attracts the antherozoids.

Antherozoid shows chemotactic movement.

Due to this chemotactic response many antherozoids enter into the neck of archegonium. But eventually, only one antherozoid fuses with the egg cell and other perish.

Now fertilized egg cell is called Zygote which is diploid structure.

Development Of Sporophyte 

Development of zygote takes place only inside archegonium.

Zygote is the first cell of sporophytic generation.

The first division is transverse in zygote and second one is perpendicular to the first one while the third division is vertical which give rise to 8-celled-embryo [octant].

Subsequent division occur in all possible planes until a mass of 20-40 cells is formed.

The superficial cells of this mass now undergo periclinal division, resulting in an outer amphithecium layer and a central mass of cells, the endothecium.

The cells amphithecium divides again and again anticlinically to form the wall of sporogonium. It is single layered.

Endothecium acts as a archesporium.

The cells of endothecium divides in all planes, resulting, a mass of cells (sporogenous tissue) is formed. It has two types of cells.

• Sporocytes or spore mother cells and
• Nutritive cells or Nurse cells. (Prestructure of elaters)

Note : In Marchantia nurse cells are transform into elaters (2N). Elaters are hygroscopic in nature and help in dispersal of spores.

The nurse cells provide nutrition to the sporophytes. (In Riccia)

One periclinal division takes place in the cells of venter region during the development.

Due to this bilayered covering is formed around the sporogonium. It is termed calyptra.

Calyptra is haploid because, it is formed by venter region of archegonium.

The cell wall of sporogonium and sporocytes or spore mother cells are diploid.

Now, meiotic cell division takes place in spore mother cells, results, haploid spores are formed.

Now wall of sporogonium disintegrates, to provide nutrition to the spores.

 Later innermost layer (wall) of Calyptra also breaks down to provide additional nourishment.

 In mature sporophyte spore tetrads lie in a cavity enclosed by a single layered calyptra.

• All the structures in capsule are Diploidexcept spore.
• The sporophyte of Riccia is simplest in Bryophyta. It is made up of only capsule.
• The sporophyte is devoid of foot and seta.
• In Marchantia sporophyte is differentiated into Foot, Seta and Capsule. 
• There is no special means of dehiscence of capsule of Riccia. 
• The liberation of the spores is brought about by the death and decay of capsule and thallus.

Structure Of Spore 

Spore is the first cell of the gametophytic generation.

The spores are arranged in tetrahedral tetrad.

The spores of Riccia are uninucleate, pyramidal and rough. Their diameter is 0.05 mm to 0.12 mm.

 The spore wall is tri-layered.

(i)   Outer layer or Exospore or Exosporium : It is thick and spiny. It is made up of callose and cutin.

(ii)  Middle layer or Mesospore or Mesosporium : It is thick and solid. It is composed of Cutin.

(iii) Innermost layer of Endospore or Endosporium : It is thin and elastic.

 It is composed of pecto-cellulose. These layer are developed in centripetal order.

• The spores germinate when there is enough water in the soil and new thallus is formed.

Life Cycle

• Haplo-diplotic type alternation of generation is found in Riccia.
• Sporophyte of riccia completely depend upon gametophyte for nutrition and shelter. 

Note : [In haplo-diplontic alternation of generation a dominant, independent,  photosynthetic, thalloid or erect phase is represented by a haploid gametophyte and it alternates with the short, lived multicellular sporophyte which is totally (eg., Riccia) or partially dependent (eg., Funaria) on the gametophyte for its anchorage and nutrition.]

• Haplo-diplontic type of alternation of generation is found Bryophytes.