riccia(liverworts):
Classification:
Division: Bryophyta
Class: hepaticopsida (liverworts)
Order: marchantiales
Genus: Riccia
Species: Fluitans (only floating sp.) natans, discolor.
Habitat
·
Riccia is cosmopolitan (worldwide) genus.
·
~130 species known, 22 found in India.
·
Only R.fluitans is floating sp. Rest all are
terrestrial.
·
It grows on damp soil-rocks, under shade
(mostly).
· Habit/structure.
1.
Gametophyte
2. Sporophyte
1 1)Gametophyte:
§
Plant body is thallus.
§
Prostate
§
Dorsiventral, ribbon like
§
Dichotomously branched
§
May from rosette form
§
Thickened midrib present
·
Midrib found on dorsal surface formed by `median
longitudinal furrow’.
·
Growing point is `apical notch’.
·
Ventral surface has rhizoids & scales.
è Rhizoids:
elongated hair like structure act as root, unicellular.
Function: 1. Attach thallus to
substratum.
2.
Absorb water & food from soil.
Types: 1)
smooth walled
2 2) Tuberculate
è : scales:violet color on ventral side present on both edge of thallus multicellular work
as anchor. Covers/protects growing apex of thallus at tip.
Internal structure (anatomy)
·
Thallus shows internal tissue differention.
·
In transverse section, it appears elliptical
shape.
·
Storage zone: lower/ventral region of
thallus-has compact colorless, parenchymatous tissue. No intercellular spaces.
Chlorophyll absent, starch present.
·
Photosynthetic zone: upper/dorsal region of
thallus has vertical rows of chlorophyll us cells. Narrow vertical air chambers
present.
§
It is photosynthetic zone.
§
Rhizoids are present on ventral side.
§
Scales are also present in two rows on ventral
side.
§
Apical growth: The tips of thallus contain 3-5
apical cells.
§
Thallus hence grows apically.
Reproduction
è vegetative
reproduction
·
By progressive death and decay of old parts of
thallus, the growing dichotomous branch splits into two, hence two new growing
thallus is made.
·
Adventitious branch arise from ventral surface
of thallus, separation of these branches results in formation of new thalli.
·
Apex at the end of growing season grows down
into the soil and became thick. At favorable season it starts growing back.
·
New thallus may be produced from the apex of a
rhizoid.
·
At end of growing season, thalli may produce
tubers.
·
Tubers will grow new thallus at end of
unfavorable season.
è sexual reproduction
·
Antheridia (male sex organ) and archegonia
(female sex organ) may grow on some thallus (homo thallic/monoencious) or on
separate thalli (heterothallic, dioecious).
·
No distinct morphological difference between
male and female spores.
·
Sex organ are develop singly on the dorsal
surface of the thallus within median longitudinal furrow extending backwards
from the growing point.
·
Sex organs arise superficially on the floor of
dorsal furrow of the thallus. At it matures, up growth of surrounding
vegetative cells surrounds it.
·
Hence it becomes embedded in tree.
·
Each sex organ comes to lie at the
bottom/ventral of the individual cavity.
·
Cavity is known as `anthredial chamber’.
the mature antheridium:
stalk:short,few celled,attached to the base of the antheridial chamber.
oval body:
- flat base,rounded/conical apex,covered by sterile jacket layer cell.
- inside jacket is single layer of cell which produce androcytes.
- androcytes produce two antherozoids(male sperm).
Archegonium:
- Chamber is surrounded by narrow cylindrical canal,archegonium chamber in cylindrical canal,distal portion of neck projects beyond archegonial chamber.
- The mature archegonium
- Archegonium is flask shaped,with long neck.
- Attached to the tissue of thallus by short stalk.
- Basal swollen portion=venter.
- Elongated slender=neck.
- Neck is single cell layer tube.has 6-9 tiers of elongated cells arranged in 6 vertical rows=makes narrow canal(tube).
- Canal is occupied by one raw of 4 neck canal cells.
- Upper part of neck is made of 4 enlarged neck cover cells.
- Venter is one cell layer cover,which enclose naked egg and a small venter canal cell.
- When archegonium is fully mature neck canal and venter canal cell dissolves and makes mucilaginous mass.
- Fertilization(syngamy)
- By help of water,jacket layer of antheridium breaks and release all anthrozoids in water droplets.
- By help of water,4 neck cover cells swell up and breaks neck is hence free for anthrozoids entry.
- anthrozoids get attracted by sugars in mucilaginous mass in neck and reaches to egg.
- egg(n) gets fertilized by one single anthrozoids(n).
- it produces diploid(2n) zygote.
sporophytes
- protect growing sporogenium.stage of life cycle from zygote to formation of spores is known as sporophyte(2n).
- zygote secreates its own cell wall.
- venter becomes layers of many cells known as embryo.
- at stage 20-40 cell spherical mass is formed.
- from this stage outer layer cells:amphithecium.
- and inner cell mass is:endothecium.
- amphithecium:makes sterile jacket.
- endothecium:archesporium(fertile).
- means first cell generation of the sporogenous tissue.
- they divide several times to make mass of sporogenous cells:spore mother cells.
- some of these cells die to give nutrition to developing spore,known as 'nutritive cells'.
- each of spore mother cells(2n) divide by meiosis(reduction division) and produce 4 cells.
- known as spore tetrad(4 spores)
- each of these 4 spore will be dispersed when sporophyte breaks.
- no special mechanism adapted by riccia for dispersal.
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