Q.3. Describe the different types of reproduction in bryophytes studied by you. (2013)
Related Questions -
Q. Describe the vegetative reproduction in Bryophytes studied by you. (2010, 12)
Ans. There are two types of reproduction in bryophytes :
(i) Vegetative reproduction (ii) sexual reproduction.
(i) Vegetative reproduction: - The most common method of multiplication in bryophytes is the vegetative reproduction occurring by variety of means. The vegetative multiplication is fundamentally a process of division or a sort of detachment of some part of the plant body which subsequently develops into a complete plant. Bryophytes are characterized due to their ability to reproduce quickly by vegetative methods. There are three different ways of vegetative reproduction.
· By progressive death and decay of older parts resulting in the separation of two or more parts of a plant from branching.
· By the separation of an organ and regeneration of a new plant from it
· By means of specialized propagating units such as gemmae, tubers etc.
(1) By Progressive Death and Decay: -
The growth and branching following progressive death and decay of older portions of plants is the common method of vegetative reproduction in Hepaticopsida, Anthcerotopsida, and in member of Bryopsida.
(2) By Separation of Organs: -
In some cases the separation of a complete organ served the effective means of vegetative reproduction. In leafy liverworts certain deciduous branches and leaves play an important role in spreading. Frullania fragilifolia propogates by its antical lobe of leaf which sheds a propagule. The postical lobes remain intact along the stem. These are called leaf cladia. Stem cladia arising from the stem and on detachment giving rise to new plants are formed in Leptolejeunea and Bryopteris.It is commonly believed that almost every cell of Marchantia and mosses is capable of regenerating the entire plant.
(3) By Formation of Adventive Branches: -
In the thallose Marchantiales and Anthocerotales the production of adventive branches from the underside of mid rib and their subsequent breaking and developing into new plants is commonly seen eg. Marchantia and Anthoceros.
(4) By Formation of Tubers: -
The formation of tubers has been widely reported in Ricciaceae and in anthocertoceae. In Anthoceros the tubers with or without stalks arise from growing points and remain buried in the soil. The food material is stored in them. They persist while rest of the thallus gets dried.
(5) By Gemmae: -
Gemmae constitute an important method of vegetative propagation in Marchantia, Anthoceros etc. Each gemma is a multicellular discoid body with one cell elongated hyaline stalk. It has two prominent lateral notches marking the growing point. A gemma is several layer thick in middle while only one cell thick at margin. In multicellular body of gemma majority of cells contain chloroplasts and a few cells contain oil bodies. Few isolated colourless cells are the rhizoidal cells.
(6) By Formation of Innovation: -
The formation of innovation is seen in Sphagnum. One of the tufts of branches becomes dominant and erect like main axis and is called innovation. It is a common method of vegetative propagation and is a cause of dense growth of sphagna.
(7) By Formation of Protonema: -
The formation of secondary protonema as seen in Sphagnum takes place from the marginal cells of primary protonema. Secondary protonema as found in several other cases of Bryopsida develop from the rhizoids of gametophore, primary protonema, wounded portion of leafy shoot or sporogonium etc. The breaking of filamentous protonema into smaller pieces each developing into a new protonema is a common method of vegetative propagation in members of bryopsida.
(i) Sexual reproduction: - The life cycle of bryophytes consists of two distinct phases or generations:
(i) the gametophytic generations (the gametophyte)
(ii) the sporophytic generation (the sporophyte).
The gametophytic phase is mainly concerned with the production of gametes (or sexual reproduction). Each cell of the gametophyte has a single set of chromosome number i.e., haploid (n). It begins with the productions of haploid spores. The spores fall on substratum and germinate to produce gametophytic plant body. The gametophyte is an independent plant body which may be thalloid or leafy axis. Reaching to certain stage of maturity, it develops sex organs. The male sex organ is antheridium, which produces biflagellate motile male gametes called antherozoids. The female sex organ is archegonium, which produces non-motile female gamete called egg. The sexual reproduction is oogamous.
The haploid, motile antherozoid fuses with the haploid, non-motile egg, and forms diploid zygote. The gametophytic generation ends with the formation of diploid (2n) zygote.
The zygote is the first cell of sporophyte generation. It is retained within the archegonium and multiplies to produce the embryo, which later develops into sporophytic plant body the sporogonium. The sporophyte is dependent on gametophyte and it remains attached to it throughout its life.
The spore mother cells are produced from the sporogenous tissue in sporophyte divide by meiosis. Each diploid spore mother cell divides by meiosis and forms 4 haploid spores. The sporophytic generation ends with the formation of spores and the gametophytic generation begins.
Q.4. Explain the structure and significance of elaterophore and protenema. (2012)
Ans. Elaterophore: -
A group of 20-100 stout elaters remain attached by their lower ends at the mid base of capsule called elaterophore eg. in Pellia elaters are narrow, long spindle shaped cells. They are tapering at their ends and possess 2-3 spiral bands of thickenings in their inner walls. They are hygroscopic and as a result of their coiling and uncoiling under different moist conditions the spores are separated and scattered. They are provided with chlorophyll.
Protonema: - A protonema (plural: protonemata) is a thread-like chain of cells that forms the earliest stage (the haploid phase) of a bryophyte life cycle. When a moss or liverwort first grows from the spore, it grows as a protonema which develops into a leafy gametophore.
Moss spores germinate to form an alga-like filamentous structure called the protonema. It represents the juvenile gametophyte. While the protonema is growing by apical cell division, at some stage, under the influence of the phytohormone cytokinin, buds are induced which grow by three-faced apical cells. These give raise to gametophores, stems and leaf like structures (bryophytes do not have true leaves (megaphyll). These gametophores are the adult form of the gametophyte. Protonema are characteristic of all mosses and some liverworts but are absent from hornworts. The protonema is also the photosynthetic part of a germinating fern spore.
Q.5. Write note on alternation of generation in Bryophytes.
Ans. Alternation of generations: -
The life cycle of bryophytes consists of two distinct phases-(i) the gametophytic phase, and (ii) the sporophytic phase. The two phases come one after the other in an alternating manner. The gametophytic phase is followed by sporophytic phase and the later is followed by gametophytic phase. Occurrence of these two generations one after the other in the life cycle is called alternation of generations. The plant bodies of the two generations differ in form and morphology. Thus, the bryophytes exhibit heterologous type of alternation of generations.