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Read this article to learn about Bryophytes. After reading this article you will learn about:- 1. Reproduction in Bryophytes 2. Affinities of Bryophytes 3. Affinities of Bryophytes.
Reproduction in Bryophytes:
Bryophytes reproduce by vegetative and sexual methods.
Vegetative Reproduction in Bryophytes:
Bryophytes posses a characteristic feature and that is their tendency towards extensive vegetative reproduction. The vegetative reproduction takes place in favourable season for vegetative growth. Majority of the Bryophytes propagate vegetatively and it is brought about in many ways.
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Some important methods of vegetative reproduction are as follows:
1. By death and decay of the older portion of the thallus or fragmentation.
2. By persistent apices.
3. By tubers.
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4. By gemmae.
5. By adventitious branches.
6. By Regeneration.
7. By innovation.
8. By primary protonema.
9. By secondary protonema.
10. By bulbils.
11. By apospory.
12. By cladia.
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13. By separation of whole shoots.
14. By separation of shoot tips.
15. By rhizoidal tips.
1. By Death and Decay of the Older Portion of Thallus or by Fragmentation:
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In Bryophytes the growing point is situated at the tip of the thallus. The basal, posterior or older portion of the thallus starts rotting or disintegrating due to ageing or drought. When this process of disintegration or decay reaches up to the place of dichotomy, the lobes of the thallus get separated.
These detached lobes or fragments develop into independent plants by apical growth. This is the most common method of vegetative reproduction in Riccia, Marchantia, Anthoceros and some mosses like Sphagnum (Fig. 1 A-C).
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2. By Persistent Apices:
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Due to prolonged dry or summer or towards the end of growing season the whole thallus in some Bryophytes (e.g., Riccia, Anthoceros, Cyathodium) dries and get destroyed except the growing point. Later, it grows deep into the soil and becomes thick. Under favourable conditions it develops into a new thallus.
3. By Tubers:
Tubers are formed in those species which are exposed to dessication (drying effect of the air). Towards the end of the growing season, the subterranean branches get swollen at their tips to from the underground tubers. On the periphery of a tuber are two to three layers of water proof corky, hyaline cells develop.
These layers surround the inner cells which contain starch, oil globules and albuminous layers. During the unfavorable conditions the thallus dies out but the dormant tubers remain unaffected. On the return of the favourable conditions each tuber germinates to form a new plant e.g., Riccia, Anthoceros, Conocephalum, Conicum, Fossombronia etc. Thus, tubers also serve as organ of perennation (Fig. 1 D).
4. By Gemmae:
Gemmae are green, multicellular reproductive bodies of various shapes. These are produced in gemma cups, on the surface of the leaves, on stem apex or even inside the cells. They get detached from the parent plant and after falling on a suitable substratum gemmae give rise to a new individual directly (e.g., Marchantia) or indirectly (e.g., Mosses).
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Some common forms of gemmae produced in different Bryophytes are:
Class I. Hepaticopsida:
(A) Multicellular, discoid, gemmae:
(i) Produced in gemma cup on dorsal surface e.g., Marchantia, Lunularia (Fig. 1 E, F).
(ii) Produced on leaves e.g., Radula (Fig. 1 G).
(iii) Produced on erect gemmiferous branches e.g., Metzgeria uncigera (Fig. 1 H).
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(B) One to four celled gemmae:
(i) One to three celled gemmae on stem apex e.g., Lophozia heterocolpa.
(ii) One to three celled gemmae on leaves e.g., : Marsupella emarginata, Lophozia barbata.
(iii) Two celled gemmae produced within any external cell of the thallus e.g., Riccardia multifida (Fig. 1 I, J).
(iv) Three to four celled gemmae produced in the axils of the leaves e.g., Treubia.
(C) Sub spherical gemmae:
Produced in abundance in flask shaped gemma receptacle e.g., Blasia.
(D) Star shaped gemmae:
Produced on the dorsal surface of the thallus e.g., Blasia.
Class II. Anthocerotopsida:
Multicellular gemmae produced along the margins of the dorsal surface of the thallus e.g., Anthoceros.
Class III. Bryopsida:
(A) Articulated gemmae:
Produced on the leaves e.g., Ulota phyllantha, Orthotrichum lyelli etc. (Fig. 1 K).
(B) Multicellular gemmae:
(i) Stalked, green, lenticular gemmae produced at the tip of shoot e.g., Tetraphis pellucida (Fig. 1 L).
(ii) Globular, produced at the base of the stem e.g., Bryum rubens, B. erythrocarpum (Fig. 1M).
(iii) Fusiform, produced at the ends of distinct leafless terminal stalk e.g., Aulacomnium androgynum (Fig. IN).
(iv) Produced on the rhizoids of leafy shoots e.g., Tortula stanfordensis, Ditrichum cylindric. – Bryum erythrocarpum etc.
5. By Adventitious Branches:
The adventitious branches develop from the ventral surface the thallus e.g., Riccia fluitans, Anthoceros. On being detached from the parent plant these branches develop into new thalli. In Marchantia, Dumortiera these branches develop from archegoniophore while in Pellia these branches arise from the dorsal surface or margins of the thallus (Fig. 1 O ).
6. By Regeneration:
The liverworts possess an amazing power of regeneration. Part of the plant or any living cell of the thallus (e.g., rhizoid, scales).are capable of regenerating the entire plant f e.g., Riccia, Marchantia etc.
7. By Innovation:
In Sphagnum one of the branches in the apical cluster instead of forming drooping branches or divergent branches, develop more vigorously than the others and continues the growth upwards.
This long upright branch has all the characteristics of main axis. It is called innovation. Due to progressive death and decay of the parent plant these innovation become separated from the parent plant and establish themselves as parent plants.
8. By Primary Protonema:
Primary protonema is the filament like stage produced by the developing spores of the mosses. It produces the leafy gametophores. It breaks into short filament of cells by the death of cells at intervals. Each detached fragment grows into a new protonema which bears a crown of leafy gametophores e.g., Funaria.
9. By Secondary Protonema:
The protonema formed by other methods than from the germination of spores is called secondary protonema. It may develop from any living cells of the leafy gainetophore i.e., from leaf, stem, rhizome, injured portion of the leafy gametophore, antheridium, paraphysis or archegonium. From this arise the leafy gametophores or lateral buds in the same manner as in primary protonema e.g., Funaria, Sphagnum (Fig. 1 P).
10. By Bulbils:
These are small resting buds develop on rhizoids. Bulbils are devoid of chlorophyll but full of starch. On germination bulbils produce a protonema which bears leafy gametophores (Fig. 1 P).
11. By Apospory:
The production of diploid gametophyte from the unspecialized sporophyte without meiosis is known as apospory e.g., Anthoceros. In Funaria green protonemal filaments may arise from the unspecialized cells of the various parts of sporogonium. These protonemal filaments bear lateral buds which develop into leafy gametophores.
12. By Cladia:
These are the small or broad detachable branches which help in vegetative reproduction. These are of two types:
(i) Leaf cladia:
Arising from the individual cell of the leaf e.g., Plagiochila, Bazzania, Frullania fragilifolia etc.
(ii) Stem cladia:
These cladia arise from the stem and occupy the same position as sexual branches e.g., Bryopteris.
13. By Separation of Whole Shoots:
A number of catkins like deciduous branches develop over the entire surface of the gametophytic plant. On separation these branches develop into new plant e.g., Pohlia nutans.
14. By Separation of Shoot Tips:
It occurs in Campylopus piriformis. The separated shoot tips develop into new plant.
15. By Rhizoidal Tips:
The apical part of the young rhizoids divide and re-divide to form a gemma like mass of cells e.g., Riccia glauca. These cells contain chloroplast and are capable to develop into new thallus. (Fellncr, 1975; Campbell, 1915).
Methods of Perennation:
Perennation is the survival from season to season, generally with a period of reduced activity between each season.
The chief methods of perennation are:
1. By persistent apices.
2. By tubers.
3. By bulbils.
The spore and gemmae can also perennate and in some, instances even the protonema does. The perennial mosses perennate as gametophytes.
Sexual Reproduction:
1. Sexual reproduction is highly oogamous.
2. Male and female sex organs are known as antheridia (Sing, antheridium) and archegonia (Sing, archegonium), respectively.
3. Sex organs are jacketed and multilayered.
4. Antheridium is stalked, pear shaped or oblong and has an outer one cell thick jacket which encloses a mass of fertile cells called androcytes. Each androcyte metamorphoses into biflagellate antherozoid.
5. Archegonium is stalked, flask shaped structure. It has a basal swollen portion called venter and an elongated neck. The neck is filled with many neck canal cells whereas venter has a large egg cell and a small venter canal cell,
6. Antherozoids are attracted towards the neck of the archegonium chemotactically by certain substances (like sugars, malic acid, proteins, inorganic salts of potassium etc.) present in the mucilaginous substance formed by the degeneration of neck canal cells and venter canal cell.
7. Water is essential for fertilization.
8. The fertilized egg or zygote is the beginning of the sporophytic phase. Il is retained within the venter of the archegonium.
Sporophyte:
1. Without resting period, the zygote undergoes repeated divisions to form a multicellular structure called the embryo.
2. The first division of the zygote is always transverse and the outer cell develops into embryo. Such an embryogeny is called exoscopic.
3. Embryo develops into a sporophyte or sporogonium.
4. The sporophyte is usually differentiated into foot, seta and capsule. In certain cases it is represented only by capsule (e.g., Riccia) or by foot and capsule (e.g., Corsinia).
5. Sporophyte is attached to parent gametophytic plant body throughout its life. It partially or completely depends on it for nutrition.
6. Foot is basal, bulbous structure. It is embedded in the tissue of parent gametophyte. Its main function is to absorb the food material from the parent gametophyte.
7. Seta is present between the foot and capsule. It elongates and pushes the capsule through protective layers. It also conducts the food to the capsule absorbed by foot.
8. Capsule is the terminal part of the sporogonium and its function is to produce spores
9. All Bryophytes are hoinosporous i.e., all spores are similar in shape, size and structure
10. Capsule produces sporogenous tissue which develops entirely into spore mother cells it e.g., Riccia) or differentiated into spore mother cells and elater mother cells (e.g., March, Anthoceros).
11. Spore mother cells divide diagonally to produce asexually four haploid spores which are arranged in tetrahedral tetrads.
12. Elater mother cells develop into elaters (e.g., Marchantia) or pseudo elaters (e.g., Anthoceros which are hygroscopic in nature. Elaters are present in liverworts and absent in mosses.
13. Venter wall enlarges with the developing sporogonium and forms a protective multicellular layer called calyptra (gametophytic tissue enclosing the sporophyte).
Young Gametophyte:
1. The meiospore (spore formed after meiosis) is the first cell of the gametophytic phase.
2. Each spore is unicellular, haploid and germinates into young gametophytic plant (e.g., Riccia or Marchantia) or first germinates into a filamentous protoenma on which buds are produced to give rise to a young gametophytic plant, (e.g., Funaria).
Affinities of Bryophytes:
From evolutionary point of view Bryophytes occupy an intermediate position between the Algae and the Pteridophytes. They show affinities with both Algae and Pteridophytes.
Resemblance of Bryophytes with Algae:
1. Plant body simple, thalloid and gametophytic.
2. Autotrophic.
3. Gametophytic phase is dominant.
4. Roots are absent.
5. Cell wall is made up of cellulose.
6. Pigments (chlorophyll a, chlorophyll b, α and β carotene, Lutin, Violaxanthes and Xeoxanthin) are similar in chloroplast.
7. Vascular tissue is absent.
8. Antherozoids are motile (bi-flagellated).
9. Flagella are whiplash type.
10. Water is essential for fertilization.
11. A filamentous protonema is produced by Bryophytes (juvenile stage in mosses) which resembles with the filamentous green algae.
12. In order Anthocerotales of Bryophytes, plastids are with pyrenoids which is a characteristic of Chlorophyceae (Green algae).
Resemblance of Bryophytes with Pteridophytes:
1. Plants are terrestrial.
2. Primitive simple leafless and rootless sporophytes of Pteridophytes (members of order Psilophytales) can be compared with the sporophytes of Bryophytes.
3. Sexual reproduction is oogamous.
4. Androcytes are enclosed by sterile jacket layer.
5. Antherozoids are flagellated.
6. Water is essential for fertilization.
7. Permanent retention of zygote within the archegonium.
8. Zygote forms the embryo.
9. Moss capsule is similar to terminal sporangium and columella of Psilophytales.
10. Both Bryophytes and Pteridophytes are characterised by heteromorphic alternation, of generation.
Origin of Bryophytes:
Nothing definite is known about the origin of Bryophytes because of the very little fossil record. There are two views regarding the origin of Bryophytes.
These are:
(1) Algal hypothesis of the origin of Bryophytes.
(2) Pteridophytean hypothesis of the origin of Bryophytes.
Algal Hypothesis of Origin:
There is no fossil evidence of origin of Bryophytes from algae but Bryophytes resemble with algae in characters like-amphibious nature, presence of flagellated antherozoids and necessity of water for fertilization.
This hypothesis was supported by Lignier (1903), Bower (1908), Fritsch (1945) and Smith (1955) etc. According to Fritsch (1945) and Smith (1955) Bryophytes have been originated from the heterotrichous green algae belonging to the order Chaetophorales for e.g., Fritschiella, Coleochaete and Draparnaldiopsis.
Pteridophytean Hypothesis of Origin:
According to this hypothesis Bryophytes are descendent of Pteridophytes. They are evolved from Pteridophytes by progressive simplification or reduction.
This hypothesis is based on certain characters like-presence of type of stomata on the sporogonium of Anthoceros and apophysis of mosses similar to the vascular land plants, similarly in the sporophytes of some Bryophytes (e.g., Anthoceros, Sphagnum, Andreaea) with some members of Psilophytales of Pteridophytes (e.g., Rhynia, Hormophyton etc.)
This hypothesis was supported by Scot (1911), Kashyap (1919), Kidston and Lang (1917-21), Haskell (1914) Christensen (1954), Proskaner (1961), Mehra (1968) etc.
