Riccia is cosmopolitan in distribution. The Riccia thallus structure reveals that it is photosynthetic and exhibits internal tissue differentiation.
Classification of Riccia
Division: Bryophyta
Class: Hepaticae
Order: Marchantiales
Family: Ricciaceae
Genus: Riccia
Morphology of Riccia Thallus
Riccia thallus grows prostrate or horizontally and prefers moist soil to grow on. It grows in clusters, giving a rosette known as the Riccia rosette. The thallus is flat and dorsiventral. It is dichotomously branched and fleshy. It is green and has an apical notch as well as a median groove.
Riccia Thallus Structure
Branching of the thallus is different in each species.
Riccia cruciata has a cross-shaped thallus.
R. pathancotensii looks like a circular rosette cluster.
R. melanospora has hairs on the upperside.
Rhizoids
The ventral surface of the Riccia thallus contains rhizoids on the median groove. The rhizoids are unicellular elongations of epidermal cells. There are two types of rhizoids.
Smooth-walled rhizoids have smooth inner walls without any projections. These are living and hyaline.
Tuberculated rhizoids have peg-like projections on the inner walls. These are living when young but are dead when mature.
Scales
Scales are arranged in transverse rows on the ventral side of the Riccia thallus.
They are one cell thick, simple, and multicellular.
Scales are seen at the apex and protect the growing point.
They appear red, pink, or violet because of anthocyanin.
Moreover, they are more prominent in dry habitats.
Depending on the Riccia species, the rhizoids and scales are different.
R. frostii does not have tuberculated rhizoids.
R. crystalline lacks rhizoids.
R. natans and R. fluitans do not have scales or rhizoids.
Anatomy of Riccia Thallus
Due to the dorsal groove on the Riccia thallus, it appears in the shape of lips on a cross-section. Internally, Riccia thallus is divided into an upper photosynthetic zone and a lower storage region. The upper region appears green due to the presence of chloroplasts, and the lower storage region is colorless.
Photosynthetic Zone
The photosynthetic region contains the chloroplast and appears green.
The photosynthetic region has vertical rows of unbranched filaments.
The cells have chloroplasts, except in the apical cells, which are colorless.
These apical cells are bulbous.
Apical cells of all filaments form an epidermal layer-like structure.
Vertical filaments have spaces in between them, which form vertical air chambers.
These air chambers open to the outside through an apical pore-like opening by the apical cells.
Depending on the arrangement of the filament, there are 3 subgroups of Riccia.
The first group has compactly arranged filaments with a narrow space between them. Eg., R. melanospora and R. pathankotensis.
In the second group, the filaments are loosely packed, so the space between them is wider. Eg., R.frostii and R.plana.
The third group is an intermediate between the first two. Here, the filaments in the middle are arranged compactly while those on the margin are loosely packed. Eg. R.pandei.
In some species, these assimilatory filaments feature thick bands and perforated walls.
Storage Region
The storage region at the lower part of the thallus is made of compactly arranged parenchymatous tissue. These cells store starch produced by the assimilatory filaments.
The cells on the lower portion have a layer of small cells like an epidermis.
Unicellular outgrowths or rhizoids appear in the middle portion of these epidermal cells.
Rhizoids help with fixation and absorption.
Scales on the central side are formed towards the apex and function as protection and water retention.
Reproduction of Riccia
Riccia shows vegetative and sexual reproduction modes.
Vegetative Reproduction
Fragmentation: The progressive death and decay of older parts of the thallus separates the lobes into independent thalli.
Adventitious branches: These branches develop from the ventral surface of the thallus and form new plants.
Persistent apices: Under unfavourable conditions, the apices become thickened and perform perennation.
Tubes are rounded perennating structures that develop at the tips of branches and, by germination, develop into new plants.
Rhizoid proliferation: In certain species, like Riccia glauca, the rhizoid tip may proliferate to form a new thallus.
Sexual Reproduction
Sexual reproduction in Riccia is oogamous. They may be monoecious like R. billardieri, R. gangetica, or dioecious like R. discolor, R. pearsonii. Their sex organs are produced individually in sunken chambers, below the mid-dorsal groove, and have pores for communication.
Antheridium
The antheridial chamber has a single oval or pear-shaped antheridium.
It is produced on a short multicellular stalk.
It has a single-layered jacket that encloses a mass of androcytes (sperm mother cells).
The tip of the jacket breaks open to release sperm.
The sperm or antherozoids are elongated, curved, and biflagellated.
Archegonium
The archegonium is enclosed in the archegonial chamber.
It is a flask-shaped structure with a basal swollen venter and tubular neck.
The neck is formed of six vertical rows of cells with four lid cells.
Inside the neck, there are 4 – 6 neck canal cells.
On maturity, the neck canal cells and the ventral canal cell disintegrate to form mucilage.
Fertilization
The antherozoids show chemotactic movement towards the archegonium.
It enters the archgonium and reaches the egg cell in the venter.
Only one antherozoid fuses with the egg to form the zygote (oospore).
The oospore undergoes mitotic divisions to develop into the diploid sporophyte.
Venter wall forms the two-layered calyptra around the sporophyte.
The sporophyte remains attached to the archegonium.
Sporophyte of Riccia
Riccia has the simplest sporophyte among all bryophytes. Riccia sporophyte has just the sporogonium, and there are no foot or seta.
The sporophyte is diploid and is developed from the zygote. The sporophyte develops a calyptra from the ventral cells.
The zygote is first divided into two, then into 4, and later into an octant state.
Cells in the octant state divide irregularly to form a mass of cells.
Cells in the peripheral region undergo periclinal division to form an amphithecium, and the remaining cells form an endothecium.
The cells of an amphithecium undergo anticlinal division, which grows the sterile jacket of the sporogonium or capsule.
The sporogonium is the only diploid part of Riccia when it matures.
As the sporophyte matures, the capsule wall disintegrates, which marks the end of the diploid phase.
Germination of Spore in Riccia
The germination of spores in Riccia starts only after a resting period of 8-10 days. Germination occurs only if the environmental conditions, such as moisture, temperature, light, etc., are favorable.
The spores form a germ pore through which the germ tube emerges.
The germ tube develops into a club-shaped form.
Protoplasm in the spore rushes into this tube-like structure, and later chloroplast is developed.
Later, a cross wall separates the tube into a terminal part and the germ tube.
The terminal cell contains albumin as well as oil granules.
Eventually, this terminal cell divides multiple times to form 8 cells in the upper and lower tiers.
One of the four upper cells functions as the apical cell, having two cutting faces.
It divides to form a dorsiventral flat thallus.
Meanwhile, rhizoids develop from the base of the germ tube.
Secondary rhizoids arise from the thallus to make it an independent thallus.
Life Cycle of Riccia
The life cycle of Riccia consists of two distinct individuals. They are an independent, thalloid main gametophyte plant and the colourless, spherical, radially constricted sporophyte. These two individuals occur one after the other, generation after generation. This phenomenon is expressed as Alternation of Generation.
References
Abraham P C. Bryophytes, Pteridophytes, Gymnosperms & Paleobotany. 2000. St. Mary’s Books & Publications.
Soni, N. K. (2010). Fundamentals of Botany (Vol. 1). Jodhpur: Vishal Publishing Co.
Parihar, N. S. (1963). An Introduction to Embryophyta (Vol 1), Bryophyta.
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Have you ever thought about creating an e-book or guest authoring on other sites? I have a blog centered on the same topics you discuss and would really like to have you share some stories/information. I know my viewers would enjoy your work. If you're even remotely interested, feel free to send me an e-mail.