Bryophytes: Amphibians of the Plant Kingdom

Sanjeet

Bryophytes occupy a position just intermediate between the green thallophytes and the vascular cryptogams. They are plants of amphibious zone. During the dry period they become almost brittle in texture. With the onset of rainy season the apparently dried, brittle thalli turn green and become active to carry out the normal life functions.

The group Bryophyta includes the simplest and most primitive land plants. About 960 genera and 24,000 species have been reported in Bryophyta. Most of the Bryophytes are land dwellers which inhibit damp, shaded and humid localities. They are essentially terrestrial but they fail to complete their life cycle without water. Thus due to peculiar type of their habitats, they are neither treated as perfect land plants nor aquatic. They are therefore, most appropriately called as Amphibians of the plant kingdom. However a few grow under diverse habitat such as aquatic submerged in water, as epiphytes on tree trunks and branches, epiphyllous or even in desert.

Characters of Bryophytes

Habitat

The plants usually grow in moist and shady places. They are terrestrial but require presence of water to complete their life cycle. The group Bryophyta, therefore regarded as amphibians of the plant kingdom.

Gametophytic plant body

1.      The plant body is gametophyte which is haploid and bears gametes.

2.      The plants are small and inconspicuous ranging from 1 mm to 30-40 cm or more.

3.      The plant body may be thallus like i.e., not differentiated into true-roots, stem and leaves or leafy shoots. The leafy shoots may be dorsi-ventral or erect, differentiated into stem like central axis and leaf-like appendages.

4.      The plant body is attached to the substratum by means of branched, multicellular rhizomes apparently resembling the roots. True roots are completely absent.

5.      The plants are green and possess chloroplasts. They are autotrophic. However Cryptothallus mirabilis are saprophytic and lead a heterotrophic mode of nutrition.

6.      The vascular tissues are completely absent. However, in few mosses the xylem like hydroids, which conduct water and phloem like leptoids, which conduct the assimilates have been reported.

7.      The gametophyte which bears gametes is conducted with sexual reproduction and constitutes the most conspicuous, nutritionally independent phase in the life cycle.

Reproduction

1.      Sexual reproduction is always oogamous type. It means the male gametes are small, motile antherozoids and the female gametes are large and non motile eggs.

2.      The gametes are produced in complex multicellular jacketed sex organs.

3.      Both kinds of sex organs may be developed on the same individual or on different plants. The former are called Monoicous and the latter as dioeciously.

4.      The male reproductive organs are antheridia and female reproductive organs are archegonia.

5.      The antheridium is ellipsoidal or club shaped sometimes spherical in form. It is differentiated into a stalk and a body. The stalk attaches itself to the gametophyte tissue. The body of antheridium has wall of single layer of sterile cells. It surrounds a mass of small squarish or cubical cells called the androcytes.

6.      The androcytes produce the biflagellate male gametes called as sperms. Several sperms are produced in each antheridium. Each sperm usually consists of a minute, slender, spirally-curved body with two long, terminal whiplash type flagella.

7.      The archegonia are flask-shaped stalked organ. The slender and elongated upper portion is called neck and the lower sac-like, swollen portion, the venter. The neck encloses variable numbers of neck canal, whereas venter encloses venter canal cell and a large egg.

Fertilization

Water is essential for fertilization. The mature antheridium ruptures at its apex liberating the sperm. At the same time the axial row of neck canal cells including the ventral canal cell in the mature archegonium disorganize and the tip of archegonia also opens. The liberated sperms swimming in thin film of water reach the archegonia and completable one reach there and fuse with eggs. The fertilized egg is retained within the venter and undergoes repeated division to form an embryo. The gametes are the last structures of the gametophyte generation.

Saprophyte

1.      The diploid zygote is the first cell of saprophyte generation. Within venter of the archegonium, the zygote undergoes segmentation and develops without a resting period into a multicellular, undifferentiated embryo. It obtains its nourishment directly from the thallus.

2.      The embryo by further segmentation and differentiation finally develop into a full-fledged saprophyte which is called as sporogonium.

3.      The sporogoniu in most of the cases gets differentiated into foot, seta and capsule. However the foot and seta are absent in Riccia and the seta is absent in Corsinia and Anthoceros.

4.      The sporogonium is completely dependent on the gametophyte for water and mineral supply and in most of the cases, partly or wholly for organic nutrition. The saprophytes remains attached to the gametophyte plant body throughout its life.

5.      The foot is embedded in the tissue of the parent gametophyte. It absorbs nutrition for the sporogonium. The seta conducts the food absorbed by the foot to the capsule. The terminal capsule is mainly concerned in the production of spores which are non-motile and wind disseminated.

6.      The spores are morphologically similar in size and shape. However in Marchantia out of 4 spores produced from one spore mother cell, two produce male thalli and the two female thalli.

The young gametophyte

1.      The spores, produced from sporogonium are haploid. They are the first cells of gametophyte generation.

2.      Each spore while falling on a suitable soil germinates to give raises the gametophyte plant either directly or indirectly as a lateral bud from the protonema.

3.      The occurrence of heterologous or heteromorphic type of alternation of generation is a constant feature of the life cycle of Bryophytes in which alternating individual are dissimilar. The distinct phases are:

a)      The gametophyte phases and

b)      The saprophytic phase

The haploid spore produced from the diploid saprophyte on germination produce gametophyte plant body. The haploid gametophyte is mainly concerned with the production of haploid male and female gametes. These gametes after fertilization form diploid zygote which is the first cell of saprophytic generation. It is retained within the archegonium and multiplies to produce the embryo, which later develops into sporophytic plant body, the sporogonium. The diploid spore mother cell of sporognium after meiosis produces haploid spores which is the first cell of gametophyte generation.

Classification of Bryophytes

Braun was the first person who introduces the term Bryophyta and called it acotyledonae in 1864. He included Algae, Fungai, Lichen and Mosses in it. Schimper gave Bryophyta as the rank of a division in 1879. Eichler divided Bryophyta intw two groups Hepaticae and Musci in 1883. Engler divided the division Bryophyta into classes Hepticae and Musci and further divided each of the two classes into 3 orders as follow:

Hepaticae (Liverworts)

a)      Marchnatiable

b)      Jungermanniales

c)      Anthocerotales

Musci (Mosses)

a)      Sphagnales

b)      Andreales

c)      Bryales

A large numbers of botanists like Bessey, Fritsch, Wettstein, Buch, Evans and Verdoom (1938), Evans, Engler , Melchior and Werdermann followed this traditional system of classification. Anthocerotales usually listed as an order of Hepaticae was given an isolated position by Underwood (1894), gayet (1897) gave anthocerotales the rank of a class. Howe (1899), Campbell (1918, 1940), Smith (1938, 1955), Takhtajan (1953), Schuster (1953, 1958) distinguished three classes in Bryophyta-Hepaticae, Anthocerotae and Musci.

Cavers omitted the words Hepaticae and Musci and divided Bryophyta into 10 orders:

1.      Sphaerocarpales

2.      Marchantiales

3.      Jungermanniales

4.      Anthocerotales

5.      Sphagnales

6.      Andreaeales

7.      Tetraphidales

8.      Polytrichales

9.      Buxbaumiales

10.  Eubryales

Finally, Rothmaler in 1951 changed the nomenclature of the three classes of Bryophyta to Hepaticopsida, Anthoceropsida and bryopsida which are in accordance with the latest recommendations of the International Code of Botanical Nomencluture, Utrecht, 1956. Proskauer has changed the name Anthoceropsida to Anthocerotopsida. The modern bryoligists , thus classify Bryophyta into the following classes:

Class 1. Hepaticapsida

Class 2. Anthocerotopsida

Class 3. Bryopsida

Hepaticospida

The Latin word Hepatica means liver, thus the Hepaticospida are popularly known as Liverworts. The plants of Hepaticospida have following characteristics features.

1.      The vegetative plant-body of the gametophyte is usually dorsi-ventral and is either a thallus or a leafy axis, when foliose the leaves are without mid-rib arranged on axis in 2-3 rows.

2.      Anatomically the gametophyte is either simple or composed of many tissues. A photosynthetic tissue on the dorsal surface of gametophyte is always present and the cells forming it have numerous small chloroplasts without pyrenoids.

3.      The sex organs are formed from a single superficial cell.

4.      The sporogonium is small and generally without any chlorophyllous tissue or stomata.

5.      The sporogonium is determinate in development and usually differentiated into foot, seta and capsule.

6.      The foot and seta are absent in Riccia and the seta is absent in Corsinia and Anthoceros.

7.      The archesporium originates from the endothecium of an embryo.

8.      The sporogenous tissue of the capsule either becomes differentiated into spore mother cells and sterile cell called the elaters.

9.      Columella is absent in capsule.

The class Hepaticopsida consists 280 genera and about 9,500 species. This class is divided into five orders by incorporating the suggestions of Caveri’s , Campbell and Schuster.

1.       Order-I : Takakiales

2.      Order-II : Calobryales

3.      Order-III : Hungermanniales

4.      Order-IV : Marchantiales

5.      Order-V : Sphaerocarpales

Takakiales

The plant body is differentiated into an erect, radial leafy shoot or gametophores and brancjed subterraneae, cylindrical rhizome devoid of rhizoids, entire plant body is covered with copious mucilage hairs.

Calobryales

Gametophyte rhizomatous, erect leafy axis, leaves arranged in three vertical rows and all the leaves are similar, globose stalked antheridia, developing in a distnict manner, archegonia with a neck of four vertical rows of cells, elongate sporophyte differentiated into foot, seta and capsule, unistratose capsule wall which becomes multistratose at the apex.

Jungermanniales

Gametphyte simple thallus or leafy axis, with little internal differentiation, scales and tuberculate rhizodes absent, rhizodes only smooth-walled, antheridia spherical or sub-globose, archegonial neck consists of 5 verticle rows of cells, sporogonium which gets differentiated into foot, seta and capsule. Capsule wall 2 or more cell-layered thick. Elaters present. Capsule dehisces by 4 valves.

It is divided into two sub-orders :

a)      Metzgerineae

b)      Jungermannineae

Metzgerineae

Gametophyte thalloid, dorsiventral, prostrate, sex organs scattered on dorsal surface, the apical cells not used up in archegonia formation, capsule wall 2-5 cell layered thick. This sub-order includes 9 families.  Example:

Pelliaceae : Gametophyte thalloid, thallus lobed by irregular incisions, sex organs scattered on dorsal surface, capsule spherical or oval elaterophore present at the base in capsule.Genera are Pellia, Noteroclada, Calycularia.

Jungermannineae

Gametophyte bears leafy axis, with three rows of leaves-two lateral and one smaller ventral, branching never dichotomous, antheridia borne in axil of perigonial bracts; archegonia borne in groups at the apex of axis or branch, apical cell is used up in the formation of last archegonium, sporogonia terminal on stem or branches. This sub-orders includes about 17 families. Example

Porellaceae :  leaves usually incubous, rhizoid form tufts at the base of amphigastria, the amphigastria large, perianth large, bilabiate and inflated mouth compressed; capsule valves split half way down. Genus: Porella

Order Marchantiales

Gametophyte usually a prostrate, green, dorsiventral, dichotomously branched thallus with a more or less marked midrib: Internally, thallus is differentiated into dorsal air chambers, air pores and usually green photosynthetic tissues and ventral storage zone; ventral surface bears scales and two types of rhizoids- smooth –walled and tuberculate; sex organs are sessile or stalked, scattered along with midrib or grouped in receptacles, the sporogonium simple with or without seta, capsule wall one cell layer thick, capsule dehisces by various ways but never by four regular valves.

Campbell divided the order into five families, these are:

1.      Ricciaceae

2.      Corsiniaceae

3.      Marchantiaceae

4.      Targioniaceae

5.      Monocleaceae

Example :

Ricciaceae :

Thallus simple, differentiated into upper photosynthetic zone and lower storage zone; distinct air pores absent; sex organs borne single on the dorsal of thallus along the midrib; apex od archegonial neck projected; foot and seta absent in sporogonium, capsule sac-like, embedded in gametophytic thallus; spores dispersal by disintegration of capsule wall; elaters absent.

Marchantiaceae:

Thallus shows rhomboidal area, each with central pore on dorsal surface, air pore well developed; thallus differentiated into upper photosynthetic zone and lower storage zone; sex organs borne in groups on receptacles; female receptacle called archegoniphore always stalked, sporogonium differentiated into foot, seta and capsule; sporogenous from forms spore mother cells and sterile elater.

Order Sphaerocarpales

Gametophyte small, orbiculate to cuneate plate of green tissue which may be simple or dichotomously-branched thallus, without internal differentiation, all vegetative cells except the rhizoids contain chloroplasts, sex organs enclosed within involcres, borne singly; archegonial neck consists of six vertical rows of cells; capsule wall one cell layer thick. The order is divided into two families:

1.      Sphaerocarpaceae : with bilaterally thallose gametophyte consisting of two genera Sphaerocarps and Geothallus

2.      Riellaceae : with asymmetrical gametophytes consisting of the aquatic genus Riella

Class Anthocerotopsida

Gametophyte simple, lobed, dorsiventral thallus devoid of any midrib, internally homogenous without any differentiation of tissues, air pores and air chambers absent but mucilage cavities may be present, rhizoids only smooth walled, tuberculate rhizoids and scales absent, each cell possesses single large chloroplast with central pyrenoid, oil bodies absent, sex organs lie sunken in the thallus, antheridia borne singly or in group in antheridial chambers beneath the upper surface of the thallus, archegonia embedded on dorsal surface of thallus, sporogonium differentiated into foot, merisematic zone and capsule, seta absent, capsule has got central sterile columella which is arched over by a dome shaped archesporium derived from amphitthecium, archesporium differentiates into spores and pseudoelaters, capsule dehisces basipetally by two valves, showing hygroscopic twisting.

In this class there is single order consisting two families:

1.      Anthocerotaceae: capsule long; cylindrical and vertically placed on thallus, capsule-wall multilayered, cells contain chloroplast, stomata present in epidermis, columella endothecial in origin, pseudo-elaters simple or branched.

2.      Notothylaceae: capsule short; compact and horizontally-placed on margins of fertile lobes, capsule cylindrical and tapers at both ends, capsule wall 3-4 layered, cells lack chlorophyll, stomata absent, columella may be present or absent, pseudoelaters simple with spiral or oblique thickening bands.

Class Bryopsida

Gametophyte differentiated into two stages: prostrate protonema and erect, radial, persistent leafy shoot, the gemetophore. The rhizoids, both on the protonema and gametophores are multicellular, branched and with diagonal cross walls. The gametophores have spirally arranged leaves on stem. Sex organs, elevated on long stalks on the gamethophore, develop from superficial cells, sporogonium differentiated into foot, seta and capsule, wall of capsule several layers with stomata on epidermis, archesporium encloses central columella, forms only spore mother cells, elaters absent.

The class Bryopsida has been divided into three sub-classes:

a)      Sphagnidae

b)      Andreaeidae

c)      Bryidae

Sub-class : Sphagnidae

Most primitive sub-class of Bryopsida. The protonema is broadly thallose, one-celled thick and produces one gamtophore. The rhizoids are multicellular and obliquely septate. Leaves of gametophores unistatose, lack midrib, composed of narrow, living, green cells and large hyaline, dead cells, branches arise laterally in the axil of leaves, antheridia develop in the axils of leaves in antheridial branch while archegonia are terminal on archegonial branches fromamphithecium, seta short, operculum present but peristome absent, apex of archegonial branch elongates to form pseudopodium.

Sub-class : Andreaeidae

Protonema thallose, adult gametophores small, dark brown or reddish and brittle, internally stem shown uniform cells, perchaetial leaves large, erect and convolute, archesporium and columella originate from endothecium, wall of capsule without spongy photosynthetic tissue, seta short , replaced functionally by pseusopodium.

Sub- class : Bryidae

Protonema usually filamentous, adult gametophores leafy axis; leaves with distinct midrib; zygote divides by transverse division forming hypobasal and epibasal halves which form apical cells of their own, endothecium forms columella and archesporium, columella continues uptonapex of capsule, spore sac separated from wall by air space traversed by trabeculae, seta long, pseudopodium not formed. Peristome usually present.

Oogamous: Characterized by or having small motile male gametes and large non-motile female gametes.

Peristome: A fringe of toothlike appendages surrounding the mouth of the spore capsule of some mosses. The teeth unfold under damp conditions and curl up under dry conditions to disperse spores gradually.

Seta: The stalk of the sporophyte that bears the capsule.

Thallus: A plant body undifferentiated into stem, root, or leaf.

Heterotrophic: An organism that cannot synthesize its own food and is dependent on complex organic substances for nutrition.

Mosses: Any of various green, usually small, nonvascular plants of the class Musci of the division Bryophyta.

Monoicous: Having archegonia and antheridia on the same plant; bisexual.

Antheridia: A sperm-producing organ occurring in seedless plants, fungi, and algae.

Archegonia: A multicellular, often flask-shaped, egg-producing organ occurring in mosses, ferns, and most gymnosperms.

Squarish: Somewhat or almost square.

Venter: The swollen lower portion of an archegonium containing the egg

Capsule: The thin-walled, spore-containing structure of mosses and related plants.

Haploid: Having the same number of sets of chromosomes as a germ cell or half as many as a somatic cell.

Columella : Any small columnlike structure in various plants and animals, often forming the central axis of development for the organism or an anatomical structure

Trabeculae: any of various rod-shaped cells or structures that bridge a cavity, as within the capsule of a moss or across the lumen of a cell.

Septate: divided by septa a septate plant ovary.