Test: Riccia - Class 11 MCQ

Riccia is a genus of liverworts in the order Marchantiales.
These plants are small and thalloid, that is not differentiated into root, stem and leaf. Depending on species, the thallus may be strap-shaped and about 0.5 to 4 mm wide with dichotomous branches or may form rosettes or hemirosettes up to 3 cm in diameter, that may be gregarious and form intricate mats.

In Riccia, the dehiscence of the capsule is absent. The spores simply get free from sporogonium on death and decay of the thallus. This is a long and simple process. The sporophyte of Riccia lacks complexities and therefore is the simplest among the bryophytes.

In Riccia,membranous scales arise in the margin and project beyond . In some species they are absent or are rudimentary. In merchantia the scales are arranged in 2 or 3 rows on either side of midrib. They are not placed in the margins,these ventral scales help them to retain moisture below the thallus  acropetal - of leaves or flowers; developing or opening in succession from base to apex. phytology, botany - the branch of biology that studies plants. basipetal - of leaves or flowers; developing or opening in succession from apex to base.

The scales in Riccia are ligulate. The scales are multicelled thick. Maximum scales are found in apical notch. Scales are absent or less developed in aquatic species of Riccia.

The nurse cells refer to those cells which help in providing food and also help other cells .
It is also great in providing stability to its neighbouring cells.
Reproduction in Riccia takes place in different ways:
(1) Vegetative reproduction
(2) Adventitious branches
(3) through tubers
(4) by rhizoids.

Rhizoids are nearly lacking in aquatic forms, but there are usually numerous unicellular rhizoids of two types on the ventral surface. One type is called smooth and the other type is the pegged or tuberculated rhizoids; these help in anchorage and absorption.

Gametophytic Phase: The plant body of Riccia is gametophytic and gametophytes are fleshy, prostrate and dichotomously branched. Repeated dichotomy results into a typically rosette like appearance.
In Riccia cruciata only two dichotomy result in a cruciate form (Fig. 4). Each branch of the thallus is linear, wedge-shaped or obcordate. Thallus is 5-7 mm long and 1.2 mm broad in all the terrestrial species. However, in Riccia. fluitans it is 30-50 mm long and 1-2 mm broad.

The layer forming the roof is called the "epidermis," and the small opening left leading into the chamber is bounded by a special ring of cells and forms the "stoma" or air-pore. In most species of Riccia the air-chambers are only narrow passages, but in the other Marchantiales they are more extended.

External Structures of Riccia: The gametophytic plant body of Riccia is a dichotomously branched dorsiventral prostrate thallus. In terrestrial habits, the plants usually occur in a rosette form due to the presence of a number of dichotomies that grow together from one place.

Oogamy is the familiar form of sexual reproduction. It is a form of anisogamy (heterogamy) in which the female gamete (e.g. egg cell) is significantly larger than the male gamete and is non-motile.

The gametophytic generation of Riccia consists of the green thallus plant, the sex organs and the gametes. This generation starts with the spores and ends with the fertilisation. The gametes are the last structures produced during the phase. The fusion of gametes leads to the formation of zygote.

A free-floating barrel is a specific design technology used in highly accurate rifles, particularly match grade rifles, to increase the accuracy of the weapon. With conventional rifles, the barrel rests in contact with the fore-end of the stock.

Under the Koppen climate classification, for tropical climates, a wet season month is defined as a month where average precipitation is 60 millimetres (2.4 in) or more. In contrast to areas with savanna climates and monsoon regimes, Mediterranean climates have wet winters and dry summers.

In bryophytes, the calyptra is an enlarged archegonial venter that protects the capsule containing the embryonic sporophyte. The calyptra is usually lost before the spores are released from the capsule that the reason they are haploid.
Within a spore capsule sporogenous cells give rise to the spores but not all sporogenous cells need give rise to spores. The sporogenous cells that will produce spores first differentiate into spore mother cells. Bryophytes spores typically develop as tetrads, which is a group of four. Each such group of 4 develops from one spore mother cell and the four spores are closely packed in a tetrahedral formation. 

The sex organs of Riccia are borne on dorsal surface of the thallus in the median groove. The sex organs are developed singly, usually deep in the tissues in acropetal order. (Youngest sex organ near the apex and oldest sex organ away from the apical notch.) The species of Riccia are Monoecious and dioecious.

The sporophyte of Riccia is the simplest amongst bryophytes. It consists of only a capsule, missing both foot and seta, and does not perform photosynthesis.
The thallus of Riccia is flat, prostrate, dark green coloured and dichotomously branched. On the ventral surface of the thallus are found rhizoids and scales. Rhizoids are of two types- the vegetative reproduction occurs by means of progressive death and decay of the thallus, by means of tubers and by rhizoids.
In sporogenesis, at the time of meiosis, the spore mother cells lie free within the cavity of sporogonium. Spore mother cells undergo meiosis. Nucleus of each spore mother cell divides by two successive divisions to form 4 haploid nuclei. In the first division the chromosome number is reduced to half of the somatic number. The four nuclei migrate to the periphery and lie at equal distance and make it a six to eight cells at axis.
 

Under unfavorable or dry weather condition Riccia can survive through tubers, persistent apices, and spores. Tubers escape from desiccation with the formation of a resistant layer of cork cells. Tubers store food and function as a perennating organ which germinates into new gametophytes on the return of favorable environmental conditions.
In case of persistent apices, thalli of some species become completely dry except the apical region with a little of adjacent tissue. These apical parts remain dormant during the dry season and form new thalli when favorable conditions return.
In the case of spores, a thick covering known as heterocyst is formed by the spores around them to save them.

Because in diagram there is gemma cup which is the reproductive structure of bryophyta.

An anticlinal division leads to the formation of anticlinal walls between daughter cells. Such a division allows the tissue to increase its circumference, thus increasing the girth of the organ. Periclinal is parallel to the surface. Such a division results in an increase in girth of the organ thus adding length.

Bryophytes are not vascular plants as the conducting tissues are not xylem and phloem rather they are hydroid and leptoid. All mosses have a sporic (diplohaplontic) life cycle that is oogamous.

A sporophyte develops from an egg, held within a flask-like archegonium, that has been fertilized and there's more about that process in the SEXUAL REPRODUCTION The fertilized egg grows by the formation of additional cells. In the great majority of species the embryonic sporophyte elongates and one part becomes a foot that penetrates the gametophyte and anchors the embryonic sporophyte to the gametophyte. The opposite end will develop into the spore-bearing capsule (and also the supporting stalk, or seta, in species in which the mature capsule is stalked). The embryonic sporophyte is often protected by a calyptra, a covering that develops from the wall of the archegonium. The calyptra is therefore a very close covering over the embryonic sporophyte. This photo click for photo shows an intact, reddish calyptra over an embryonic sporophyte of the moss Dawsonia longiseta. In addition to the calyptra there is often some additional protective covering though in a number of genera the tissue of the calyptra fuses with other tissue to form a composite protective covering.

Each androgonial mother cell divide diagonally by mitotic division to form two rectangular cells called as androcytes,which further metamorphosis to form antherozoid.So from 1 androgonial mother cell 2 antherozoid will form and hence from 20 androgonial mother cell 40 antherozoid will form.

Scales are seen in the thalloid bryophytes like Marchantia and Riccia. They are present on the ventral surface of thallus. The scales are protective in nature and they provide protection to the young growing tip of the thallus. They overgrow the apical notch of the thallus to protect the growing point.

The mature spores show three wall layers: the outermost exosporium, the middle mesosporium and the innermost endosporium. Structure of the Mature Sporophyte: The sporophyte of Riccia is represented by only a single globular capsule which is found to be the simplest organisation among the bryophytes.

Option B is correct because they all are haploid.

Bryophytes also need a moist environment to reproduce. Their flagellated sperm must swim through water to reach the egg. So mosses and liverworts are restricted to moist habitats. There are no mosses in the desert.