Riccia thallus structure and reproduction

RICCIA
Prepared by:
Dr. E. Gayathiri
Department of Plant Biology and Plant Biotechnology
Guru Nanak College, Chennai

OVERVIEW
• HABITAT
• EXTERNAL AND INTERNAL STRUCTURE OF THALLUS
• REPRODUCTION IN THALLUS
• ALTERNATION OF GENERATION

Classification (Systematic position)
• Kingdom: Plantae
• Subking.: Cryptogams
• Division : Bryophyta
• Class : Hepaticopsida
• Order : Marchantiales
• Family : Ricciaceae

Habitat
•
1. The genus Riccia comprises about 138 species.
2. 33 species have been recorded so far R. crystallina, R. kashyapii,
R. pandel are endemic in India.
3. Riccia mainly grows on damp soil and shady places and other similar
terrestrial habitats.

External Structure of Thallus
1. Due to dichotomous branching a circular patch of plants is formed which is
called rosette.
2. Plant body represents haploid, the gametophyte generation. R. fluitans, the
aquatic species, is partially submerged or it floats in water.

Riccia thallus-Rhizoid
• Rhizoids are unicellular and unbranched.
• Two types of rhizoids, - smooth walled and tuberculated.
• Rhizoids - fixation and absorption.
• The scales in young plants are arranged in one transverse row near the
apex.
• When the thallus matures, they form two rows near the margin of the
thallus.
Source: http://selfstudy.co/sp/medical/botany/plant-kingdom/bryophyta/5acd971ba0a9b5775fdb62be

Internal structure of thallus
A vertical section of thallus has
depression in the centre - dorsal groove.
Internal differentiation.
The thallus is formed of two distinct
regions, viz.
a. the lower or ventral storage region
and
b. an upper or dorsal photosynthetic
or assimilatory region
Source: http://premabotany.blogspot.com/2018/12/riccia-classification-structure-of.html

Storage region
In the ventral surface - closely packed
parenchymatous tissue are present
The chloroplasts are absent in this region.
The region acts as storage tissue, storing
water and reserved food material (starch).
From the lower epidermal cells, develop
rhizoids and amphigastria.
The rhizoids help in absorption while the
amphigastria give protection.

Photosynthetic region
Present on the dorsal surface
It consists of chlorenchymatous cells with one cell
thickness.
These vertical rows of cells are called
photosynthetic filaments.
Between the chlorenchymatous rows - air
chambers, air-canals are air-clefts.
The air chamber lacks photosynthetic filaments.
Around the air cleft six to eight vertical rows of
chlorenchymatous cells, terminal bigger colorless
cell Source: http://premabotany.blogspot.com/2018/12/riccia-classification-structure-of.html

Photosynthetic region
Terminal cells together form an interrupted
upper epidermis.
The air-cleft communicate with the outer
environment through the gaps called air-pores.
In the case of the aquatic species, the epidermis
is continuous and air-pores are absent.
The thallus shows large air-cavities which store
air and give buoyancy to the plant.
The epidermal cells also show presence of
chloroplasts.

Reproduction
A.Vegetative
B.Sexual
1. Fragmentation
2. Adventitious branches
3. Tubers
4. Persistent apices

Reproduction
A. Vegetative
1. Fragmentation –Older cell
on decay the older
dichotomy separate -
favourable conditions, - new
thallus.
2. Adventitious branches –
arise from the mid-ventral
surface of the thallus -
detached - new plants.
Source: http://Slideplayer

Reproduction
A. Vegetative
1. Tubers - The tips of branches store food
material and become swollen - tubers. In
favourable - new plants.
2. Persistent apices – prolonged dry
conditions, the plant dies except the apical
part. This apical part however grows deep
into soil and becomes thick. It resumes
active growth in next season and develops
into a new thallus.

Reproduction–
B. Sexual:Oogamous type.
The sex organs are well developed, multicellular and
separate.
The male reproductive organ - Antheridium (gametes,
antherozoid).
The female reproductive- Archegonium (gamete ,ovum)
Both the sex organs - on the same plant- monoecious.
on different plants- dioecious or unisexual.
The sex organs are in the dorsal surface ,deeply sunk in
the mid-furrow of the thallus. Source: http://Madanacademy, Self study.co

Antheridium :
• multicellular and elongated structure enclosed in an
antheridial cavity
 consists of a stalk and a body
 body is ovoid or pear-shaped
 outermost sterile layer called the jacket layer or the
antheridial wall - protective
 encloses a mass of fertile, cubical cells called androcyte
mother cells
 Each mother cell divides to form two androcytes or
spermatids
 numerous antherozoids or spermatozoids are formed in
an antheridium.

Antherozoid :
 spermatozoid is a minute,
slender, curved and flagellate
structure.
 Flagella are two in number
 The body possesses elongated
blepharoplast, a nucleus and a
little cytoplasm.
 The unused cytoplasm remains
attached to the posterior end
forming a vesicle.

Archegonium
 Present in a cavity
(Archegonial cavity).
 Archegonium is flask
shaped.
 Archegonium formed of
three parts - stalk,
ventre and neck
 Stalk is small and few
celled
Archegonium

Archegonium
 Venter is broad & venter wall is one cell in
thickness
 Venter encloses a cavity - venter cavity
 two unequal cells
 smaller - ventral canal cell
 larger, posterior cell - ovum or the egg cell
 Neck consists of six vertical rows of cells
 At the tip - four specialized cells - lid cells or
cover cells.

Dehiscence of Sex organs water or even moisture is necessary
 spermatozoids lie in the antheridial cavity because the walls of
spermatids are already dissolved
 water finds its way to the antheridial cavity through the ostiole of
the cavity
 antheridium bursts liberating antherozoids due to pressure created
 archegonium reaches maturity, the ventral canal cell and the neck
canal cells disintegrate or degenerate to form mucilage
 mucilage absorbs more water and pressure is created separating the
lid cells
 Sperms reach to the egg cell.

Fertilization:
 Certain chemical substances are exuded along with the
mucilage
 These chemicals probably attract the spermatozoids.
 Only one of the antherozoids surrounding the egg succeeds in
uniting with the egg
 Gametophytic stage ends with the process of fertilization.
 The zygote, the first cell of the sporophytic generation is
embedded in venter

Sporophyte
 Zygote is the first cell of
diploid or sporophytic
generation
 The mature sporophyte of
Riccia is the simplest
among the sporophytes
 It consists of only capsule
or the spore sac, while
foot and seta are absent

Sporophyte
 The sporophyte is embedded in the
storage tissue of the gametophyte
and enclosed in the venter cavity of
the archegonium.
 zygote divides mitotically to form
sporogonium
 The sporophytes are last cells of
diploid generation
 The sporophytes divide meiotically
to produce haploid cells-spores

Sporophyte
 sporophyte is completely
dependent for nutrition on the
gametophyte - complete
 sporogonium of Riccia never
dehisces
 surrounding calyptra decay or
disintegrate, the spores remain
behind on the soil.
 favourable conditions, they
germinate to from new plants

Spores
 spore is tetrahedral
 spore wall or sporoderm is formed of
two layers, the outer exine
(exosporium), and the inner intine
(endosporium)
 exine is variously ornamented
 exine is cuticularised, while the intine
is thin walled

Spore germination
 presence of light and the presence
of moisture
 spore absorbs water and the
pressure created ruptures the exine.
 intine comes out forming a short
germ tube
 contents of the spore migrate to
the tip of the germ-tube and a
partition is formed
 division and redivision of the apical
region results into a new
gametophyte

Riccia thallus structure and reproduction

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