Riccia – Morphology, Anatomy and Reproduction

Riccia is a simple thalloid liverwort belonging to Division Bryophyta. It is commonly studied to understand the basic structure and reproduction of liverworts. This blog post is about Riccia – Morphology, Anatomy & Reproduction Guide, You can download this note as PDF. Link provided at the end of the post. Happy learning…

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1. Morphology of the Gametophyte Thallus

External Features (Dorsal and Ventral View)

• The plant body is a flat, ribbon-like thallus that grows prostrate on moist soil.
• The thallus shows clear dorsiventral differentiation.
• Repeated dichotomous branching produces a rosette-like arrangement.
• Each branch is linear to wedge-shaped with a notch at the apex.
• A thick midrib runs along the center; a shallow groove is visible on the dorsal side.
• The ventral surface bears rhizoids and scales.
• Scales occur near the margins and appear violet in color.
• Rhizoids arise from the midrib region.

Rhizoids

Two types are present:

1. Smooth-walled rhizoids – inner walls are smooth.
2. Tuberculate rhizoids – inner walls form peg-like projections extending into the lumen.

Scales

• Violet colored
• Multicellular
• One cell thick

Sex Organs

• Embedded in the mid-dorsal groove.
• Mature sporophytes appear as black dots under a dissecting microscope.

2. Anatomy of the Riccia Thallus (Gametophyte)

A transverse section (T.S.) shows the internal organization.

General Structure

• The thallus appears boat-shaped in cross-section.
• It is thickest at the midrib and thinner at the edges.
• Two distinct regions are visible:
  • Upper photosynthetic region
  • Lower storage region

Storage Region

• Composed of compact parenchyma cells.
• Cells store starch.
• Bounded below by a lower epidermis.
• Rhizoids originate from this region.

Photosynthetic Region

• Made of vertical, unbranched assimilatory filaments.
• Filaments are separated by narrow air chambers.
• Cells are barrel-shaped and contain many chloroplasts.
• Air chambers open outside through simple air pores.
• The uppermost cells lack chloroplasts and form an indistinct epidermis.
• Violet scales are visible near the margins in cross-section.

3. Riccia Antheridium (Male Reproductive Organ)

Key Features

• The plant is monoecious; both sex organs occur on the same thallus.
• Antheridia develop in the mid-dorsal groove.
• Each antheridium lies inside an antheridial chamber.
• The chamber opens to the exterior through a pore.
• The structure is partly embedded in both photosynthetic and storage regions.

Structure of Mature Antheridium

• Short multicellular stalk
• Globular or club-shaped body
• Central mass of androcytes (antherozoids)
• Surrounded by a single sterile jacket layer
• Jacket cells are tangentially elongated

4. Riccia Archegonium (Female Reproductive Organ)

Key Features

• Located in the mid-dorsal groove.
• Flask-shaped at maturity.
• Short stalk present.

Structure

• Broad basal venter
• Long neck

Venter contains:

• One egg cell
• One venter canal cell

Neck contains:

• Six vertical rows of cells
• 6–9 cells in height
• Four neck canal cells
• Four cover cells at the tip

Before Fertilization

• All axial cells except the egg disintegrate.
• Cover cells separate.
• Passage is created for antherozoids to enter.

5. Riccia Sporophyte (Sporogonium)

General Characteristics

• Develops inside the fertilized archegonium.
• Remains embedded in the gametophyte tissue.
• Consists only of a capsule.
• Foot and seta are absent.

Capsule Development

• Young capsule has:
  • A jacket layer
  • Two-layered calyptra
• At maturity:
  • Jacket and inner calyptra layer disintegrate
  • Only outer calyptra remains
• Spores are released after the thallus decays.

Spores

• Arranged in tetrahedral tetrads initially.
• Each spore measures about 0.05–0.12 mm in diameter.
• Contains dense cytoplasm and a nucleus.

Spore Wall Layers:

1. Exosporium – thin and cutinized
2. Mesosporium – thick
3. Endosporium – thin and homogeneous

• The wall surface is irregular and folded.

6. Systematic Position of Riccia

Division: Bryophyta

• True roots absent
• Vascular tissues absent

Class: Hepaticopsida

• Mostly thalloid forms
• Rhizoids non-septate
• Chloroplasts lack pyrenoids
• Capsule lacks columella

Order: Marchantiales

• Scales present
• Two types of rhizoids
• Air chambers and pores present

Family: Ricciaceae

• Simple air pores
• Sex organs in mid-dorsal groove
• Sporophyte reduced to capsule only

Genus: Riccia

• Scales at margins
• Assimilatory filaments vertical and unbranched

7. Habitat and Collection

• Common in both plains and hilly regions.
• Grows on damp soil and rocks.
• Often appears after heavy rainfall.
• Frequently found in unused soil or brick crevices.

8. Riccia fluitans (Aquatic Species)

• Only free-floating aquatic species of the genus.
• Thallus highly dichotomously branched.
• Ribbon-like, elongated and thin.
• Rhizoids and scales absent.
• Reproduces vegetatively through adventitious branches.
• Remains sterile while floating.
• Produces sex organs when water levels drop and it becomes terrestrial.

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