Riccia

 Riccia 

Kingdom:	Plantae
Division:	Marchantiophyta
Class:	Marchantiopsida
Order:	Marchantiales
Family:	Ricciaceae
Genus:	Riccia

External Morphology

 • Dorsiventrally differentiated, prostrate, fleshy thallus showing dichotomous branching to form rosettes 

• Each branch of the thallus is either wedge-shaped or linear 

• The dorsal surface of the green, fleshy thallus is generally thick in the centre and gradually becomes thinner towards the margin

 • Middle portion has a median groove or furrow on the dorsal surface running through the entire length of the thallus and is termed the midrib

 • At the distal end of the dorsal groove, lies an apical notch or a depression which protects the growing point 

Rhizoids

• The ventral surface of the thallus shows numerous, unicellular rhizoids that not only help in attaching the thallus to the substratum but also help in the uptake of water and minerals

 • Rhizoids are of two types: smooth-walled and tuberculate. Smooth-walled rhizoids are, smooth-walled, simple and slightly wider. Tuberculate rhizoids are thinner and have peglike projections in the lumen. 

Scales

• Ventral surface also bears violet, membranous scales which are multicellular, one-cell thick and increase the surface area for absorption 

• They are ephemeral in hygrophilous species but persistent in xerophytic species; and even absent in some species like R. crystallina 

• They generally overlap each other in the apical portion of the thallus but are arranged in two rows

Anatomy of Riccia

  

 (a)Photosynthetic region, and

(b) Storage region.

Photosynthetic Region:

• It is situated on the upper or dorsal surface of the thallus and consists of loose green tissue. It is also known as assimilatory region.

• It consists of a layer of epidermis, many air pores, air spaces or air chambers and many one-celled thick vertical rows of chlorophyll-containing cells.

• In the cells of vertical rows are present many discoid chloroplasts.

• In between these vertical rows are present air spaces or air chambers.

 •Uppermost cells of these vertical rows of chlorophyll-containing cells remain devoid of chloroplast and thus form a hyaline discontinuous layer of upper epidermis.

 •Continuity of the upper epidermis is broken by many air pores.

Storage Region:

• It is situated on the lower or ventral surface of the thallus and consists of colourless cells.

• Cells are closely packed and parenchymatous.

External Features:

• It is the only aquatic species of Riccia. It remains floating on or under the water surface.

• Thallus is long, ribbon-like  and shows a clear dichotomous type of branching.

• Scales and rhizoids are absent.

 • develop only when it comes in contact with mud of the pond.

 •Formation of adventitious branching is the means of vegetative reproduction.

Anatomy

• The upper epidermis is a continuous layer.

 •Air spaces are large and formed by irregularly ar­ranged lamellae 

• Lamella is single-layered.

 •Cells of upper epidermis and lamella contain chlo­roplasts.

• Rhizoids and scales are absent.

Sex Organs of Riccia

•Sexual reproduction is oogamous.

 •Sex organs are antheridia and oogonia. They are present in the region of mid-dorsal groove on the dorsal surface of the thallus.

• Sex organs develop in acropetal succession.

 •Most of the species are homothallic but a few spe­cies are heterothallic, e.g., R. himalayensis.

• Monoecious species are generally protandrous, i.e., antheridia develop first and oogonia later on.

Antheridium:

• Antheridium is present in a cavity on the dorsal surface called antheridial chamber.

• Antheridial chamber has a narrow opening or pore on the apical side.

• A mature antheridium is a stalked, club-shaped or pear-shaped body.

• Antheridial stalk is multicellular.

• It remains surrounded by an outermost layer of one- celled thick sterile jacket.

 •Inside the jacket layer are present many small, cubi­cal androcyte mother cells.

• Each androcyte mother cell contains dense cyto­plasm and large nucleus. It divides diagonally into two androcytes.

• Each androcyte metamorphoses into a single struc­ture, variously called antherozoid, spermatozoid or sperm.

• Each antherozoid is a minute, uninucleate body con­taining two long flagella at its anterior end.

•Lower flagellum is slightly larger than upper one.

•Dehiscence of antheridium takes place in the pres­ence of water.

Archegonium:

• The archegonium remains embedded in the archegonial cavity on the dorsal surface of the gameto­phyte.

 •Upper part of the neck of archegonium generally protrudes out of the cavity.

• An archegonium is a flask-shaped structure made up of a long, elongated neck and a globular venter.

• Venter is sessile and surrounded by a one-celled thick layer, made up of 12 to 20 cells.

• Neck consists of 4 to 6 neck canal cells, and re­mains surrounded by six vertical rows of cells.

• Each longitudinal row consists of 6 to 9 cells.

 •At the tip of the neck are present four cover cells or lid cells.

 •Venter contains an upper, small ventral canal cell and a lower, large egg cell.

 •At the time of fertilization all the cells, except the egg, disintegrate and form a mucilaginous liquid, which gives entry to the spermatozoids. The ultimate product of the fertilization is zygote.

 •The ultimate product of the fertilization is zygote.

Sporogonium:

• It is simple and made up of only capsule or spore-sac .

 •Foot and seta are absent.

•It remains embedded in the gametophyte, and it is a non-green structure, thus depending entirely on the gametophyte for food.

• Inside sporogonium are present many spore mother cells which remain surrounded by a capsule wall and two-layered calyptra.

•Spore mother cells divide reductionally, and each of them thus forms four haploid spores, arranged tetrahedrally.

 •Prior to the formation of spores, the wall of the sporogonium as well as the inner layer of calyptra dissolve, and thus only a single-layered calyptra is present outside the spores.

• Elaters are absent .

• Neck of the long archegonium may remain outside for some time but it ultimately withers.

Spore:

• It is the first cell of the gametophytic generation.

 •Shape of the spore is rounded or pyramidal.

 •Each spore is surrounded by a thick, black or sculp­tured wall.

 •Wall of the spore consists of three layers: outer­most exine or exosporium, which is thick and sculp­tured; middle thin mesosporium and: innermost, thin intineorendosporium.

• Spores are unicellular and uninucleate structures.

 •In the cytoplasm are present many oil globules.

• Spores germinate into gametophyte.