Morphology, Life Cycle, and Classification of Bryophytes

Bryophytes are considered to be the first land plants among embryophytes. Exactly how this happened is not clear because the fossil records are not complete. When there was a shift from an aquatic mode of life to land habit the species had to face many challenges.

How could water and minerals be taken from the soil and transported to parts that are not in contact with the soil? How could the soft bodies keep from drying out? To meet these challenges there was a need to develop certain structural modifications.

The land plants belonging to various groups have continued to exist approximately from the Devonian period. This demonstrates that they are well adapted to their particular niche on land. It is the nature of these adaptations that is of interest to us in this unit.

In this article, we will discuss the characteristic features of each group and describe a few genera belonging to these groups. You will study how these genera differ from each other and also from the majority of flowering plants which are so commonly growing around you.

General Characteristics and Life Cycle of Bryophytes

The Division Bryophyta includes the simplest and the most primitive members of land plants that lack roots, and do not have a vascular system. There are some mosses that have a primitive system of tubes that conduct water and food.

The water-conducting tubes are called hydroids. They have elongated, thick, dead cells. But they are not lignified like tracheids and vessels. The food-conducting tubes are called peptoids, and they are connected through plasmodesmata.

A single plant is very small, hardly a few cm in size. It seldom grows large because of lack of supporting tissues. Thousands of tiny moss plants often grow together and give a thick, green carpet-like appearance.

Bryophytes show two distinct and well-defined phases of the life cycle, sexual and asexual, which follow each other. The gametophyte is haploid and produces gametes. The sporophyte is diploid and produces spores.

The haploid generation alternates with diploid .generation (recall the alternation of generations in algae). Both the gametophyte and sporophyte may be several centimeters in length but the gametophyte is the long-lived phase of life cycle.

You may note that in other land plants the sporophyte is the dominant generation. The gametophyte may be thalloid or has an axis differentiated into stem-like and leaf-like structures which lack xylem and phloem.

You may note that these leaf-like structures are part of gametophyte, whereas in vascular plants the leaves strictly develop on sporophyte. The gametophyte is green, photosynthetic and nutritionally independent, and anchors to the soil by unicellular or multicellular filaments called rhizoids.

Rhizoids appear like roots but unlike roots they lack vascular tissues and are much simpler in structure.

Now try to list a few points that distinguish bryophytes from algae.

Bryophytes are most abundant in moist tropical areas. But they also grow in deserts, mountains and are observed in parts of Antarctica. In dry areas their growth and activity is restricted to wet seasons only. Some mosses grow in fresh water streams but they are not found in sea flora.

Life Cycle of Bryophytes

We are illustrating here the life cycle of bryophytes taking Funaria as an example. The gametophyte of Funaria bears two types of specialised multicellular reproductive organs called the gametangia (gamete holders) which protect egg and sperm during development.

The male gametangia, called antheridia (sing, antheridium), produce sperms. The female gametangia, called archegonia (sing, archegonium), produce eggs. The gametangiá have outer sterile layer of cells forming protective jacket.

Bryophytes are oogamous i.e. the egg is larger, non-flagellated and non-motile, and the sperm is smaller and motile.

Read Also: Types of Fungi Reproduction

What types of sexual reproduction occur in algae?

You may recall that besides oogamy some algae show isogamy and anisogamy.

After fertilization, the sporophyte starts developing inside the archegonium. It may grow several centimetres in length, become photosynthetically sufficient but it draws minerals and water from gametophyte.

However, in contrast to the sporophyte of all other land plants it never becomes independent of gametophyte. It remains permanently attached to it, until maturity and senescence. It is wholly or partially dependent on it for nutrition.

Mature sporophyte is differentiated into a haustorial foot, a stem- like seta and a terminal spore producing capsule. In Riccia both foot and seta are absent. While in others like Sphagnum seta is absent. Within the capsule spores are produced by reduction division of spore mother cells.

The bryophytes are homosporous i.e. spores of any given species are all alike. Which some pteridophytes are heterosporous (they produce two types of spores – microspores a megaspores).

A spore represents the first stage of gametophytic generation. It is unicellular haploid and germinates to produce a short-lived green protonema.

The adult gametophore develops on this protonema. Protonema may be thalloid, globular or filamentous. The protonema and the adult gametophore are strikingly different from each other.

An adult gametophyte bears gametangia which produce haploid male and female gametes. The gametes represent the last stage of gametophytic generation and the zygote represents the first stage of sporophytic generation, whereas the spore mother cells (diploid) represent the last.

The spore mother cells undergo reduction division to form haploid spores. So, any stage in the life cycle which is haploid, belongs to gametophytic generation, whereas the diploid stage belong to sporophytic generation.

Now let us sum up the distinguishing features of bryophytes.

They lack vascular system. In some of the mosses a primitive conducting system is present that transports food and water.

The gametophyte is dominant generation and sporophyte remains attached to it. In other land plants the sporophyte is dominant and independent

There are strong reasons to believe that green algae served as ancestors of bryophytes.

The move from water to land offers an organism some distinct competitive advantages as well as challenges. What could be the advantages of the terrestrial habitat over the aquatic? Some of the advantages are as follows:

• Greater availability of sunlight for photosynthesis,
• Increased level of carbon dioxide, and
• Decreased vulnerability to predation.
• If some more points cross your mind, add to this list.
• Can you now think what are the challenges of land environment? Try to list them below.

Compare your points with the following:

Plants on land are exposed to direct sunlight and air. Hence there is danger of drying out or desiccation because of evaporation. Gametes and zygotes are also susceptible to desiccation.

The aquatic plants are supported by the buoyancy of water, but on land, plants need some anchor to fix to the ground and also require support to stand erect.

Absorption of minerals and water, and their transportation to the parts which are not in contact with soil. In other words, land plants need supply lines for the distribution of water and nutrients.

Effective dispersal of spores at right time and at right place for the survival of progeny, with the help of hygroscopic structures like elaters and peristome teeth.

Morphology of Bryophytes

So far you have studied the general characteristics of bryophytes. You may recall that the Division Bryophyla is divided into three classes (a) Hepaticopsida (liverworts) (b) Anthocerotopsida (hornworts) and (c) Bryopsida (mosses).

Let us now study the representative genera from each class below;

1. Hepaticopsida

The gametophyte of liverworts usually lies close to the ground. There are two forms of liverworts. In some, the gametophyte is dorsi-ventral, thalloid in form with obvious upper and lower surfaces.

These are thalloid liverworts. While in others it is differentiated into leaf-like and stem-like structures like those of mosses. The latter are known as leafy liverworts. The leaves of leafy liverworts are without midrib, whereas midrib is present in the leaves of mosses.

Internally, the gametophytes of liverworts may be homogenous or composed of different types of tissues.

Liverworts grow on moist ground or rocks that are always wet. They can be found in muddy areas near streams. In greenhouses you may find them growing in flower pots.

In this course you will study two representatives of the order Marchantiales (Riccia and Marchantia) and one of the order Jungermanniales (Pellia).

Take a look at this classification:

• Division – Bryophyta
• Class – Hepaticopsida
• Order – Marchantiales
• Family – Ricciaccae

The gametophytes of Marchantiales are exclusively thalloid. The order Marchantiales consists of about 35 genera and approximately 420 species.

We will first study in detail the genus Riccia and then Marchantia.

1i. Riccia

Riccia belongs to the family Ricciaceae which is the most primitive and the smallest family of the order Marchantiales. Riccia has more than 130 species and is very widely distributed. Most of the species are terrestrial and grow mainly on moist soil and rocks. Riccia fluitans is an aquatic species.

In structure, Riccia represents the simplest of the bryophytes. Its gametophyte is small green fleshy, thalloid. It grows prostrate on the ground and branches freely by dichotomy. Several Riccia plants grow together and take the form of circular patches, which are typically resette-like.

The branches of the thallus are called thallus-lobes. According to the species, thallus lobes are linear to wedge-shaped. The dorsal surface of the thallus has a prominent midrib, represented by a shallow groove called the dorsal groove. At its apex there is a depression termed as apical-notch.

The sporophytes are sunk deeply, in the dorsal groove, each in a separate cavity. Both male and female sex organs may develop on the same tha]lus (monoecious) or on different thalli (dioecious).

On its ventral surface, there are a number of slender, colourless, unicellular. unbrandhed processes called rhizoids that help to attach the thallus to the substratum. The rhizoids are of two types: (a) smooth walled – these have smooth walls and (b) tuberculate – these have peg-like ingrowths of wall projecting into the lumen.

On the ventral surface towards the apex and along the margins of thallus small plate like structures are also present. These are scales which are arranged in a single row and are single cell in thickness.

These scales project forward and overlap the growing point to protect it from desiccation. The growing point is located in the notch and consists of a transverse row of 3 to 5 cells.

The growth of the thallus occurs in length as well as in width by the divisions of these cells. Each thallus branches dichotomously and several dichotomies lie close to one another forming a typical rosette.

Again take a look at this simple classification of the bryophyte;

• Division – Bryophyta
• CIass – Hepaticopsida
• Order – Marchantiales
• Family – Marchantiaceae

1ii. Marchantia

The family Marchantiaceae, to which Marchantia belongs, includes about 23 genera and approximately 200 species.

The special feature of this family is that in all the genera the gametophyte bears archegonia on vertical stalked receptacles called archegoniophore (carpocephala).

In Marchantia anthoridia are also produced in stalked receptacles known as antheridiophores. The type- genus Marchantia is placed among the most advanced members with about 65 species, of which Marchantia polymorpha is the most widely distributed.

Marchantia usually grows in cool moist places along with mosses and in areas of burnt ground. It is deep green in colour. Like Riccia its gametophyte is flat, prostrate, dorsi-ventral and dichotomously branched thallus.

There is a prominent midrib low ridge covered with rhizoids. Along the midrib there are a number of cuplike structures with frilled margins. These are called gemma cups which contain numerous vegetative reproductive bodies called gemmae (sing. gemma).

In mature thalli, antheridiophores and archegoniophores, which bear antheridia and archegonia respectively, are also present at the growing apices of certain branches. Marchantia is dioecious. Like Riccia the apex of each branch is notched and a growing point is situated in it.

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You will note that on dorsal surface the thallus is marked into hexagonal areas which are visible to the naked eye. If we examine with a hand lens we can see a pore at the centre of each hexagon.

Like Riccia the thallus of Marchantia is anchored to the surface by rhizoids which are of smooth-walled as well as tuberculate type. Scales are also present on the ventral surface, but in Marchantia they are arranged on both side of the midrib.

2. Anthocerotopsida

Classification:

• Division – Bryophyta
• Class – Anthocerotopsida
• Order – Anthocerotales
• Family – Anthocerotaceae

The genus Anthoceros has about 200 species.

The class Anthocerotopsida contains the single order Anthocerotales. We will study Anthoceros as the representative of this class.

2i. Anthoceros

It grows principally in moist shady places on the sides of ditches, or in moist cracks of rocks. The gametophytes of Anthoceros are dorsi-ventral, thallose, somewhat lobed or dissected, and sometimes have a tendency toward dichotomous branching (Fig.1.5A).

The thallus of Anthoceros is dark green, velvety on the upper surface and variously lobed. The midrib is either indistinct or absent. It also lacks tuberculate rhizoids and scales. Only smooth walled rhizoids are present.

3. Bryopsida

Classification:

• Division – Bryophyta
• Class – Bryopisida
• Order – Sphagnales
• Family – Sphagnaceae

This is the largest class of bryophytes and includes about 600 genera and 14,500 species. Bryopsida is divided into three subclasses: Sphagnidae (peat mosses), Andreaeidae (rock mosses) and Bryidae (true mosses). Bryidae include about 14,000 species.

You will study the genus Funaria as a representative of this order. Order Sphgnales is represented by a single genus Sphagnum which includes about 300 species. Let us first study Sphagnum.

3a. Sphagnum

Sphagnum forms peat bogs in northern parts of the world. In some countries peat is burnt as fuel. Sphagnum is also used in plant nurseries as packing material. Mats of this moss hold moisture and help the seeds of other plants to germinate and grow.

Sphagnum is confined to acidic, water-logged habitat. It is the principal component of peat bogs where it forms a more or less continuous spongy layer. The adult gametophyte develops as an upright leafy-shoot, called gametophores from a simple thallose, one-cell thick protonema.

The gametophore is differentiated into stem and leaves. The terminal growth of the stem is due to an apical cell. The axis is attached to the soil by means of multicellular, branched rhizoids with oblique cross walls.

Rhizoids are present only in young gametophores and disappear when it matures. Afterwards, the gametophore absorbs water directly.

The mature gametophores consists of an upright stem bearing leaves. Every fourth leaf of the stem bears a group of three to eight lateral branches in its axis. These branches are of two types: (i) divergent and (ii) lying next to the stem.

Sometimes, one of the branches in a tuft continues upward growth to the same height as the main axis and resembles it in structure.

These strongly developed branches are called innovations and they ultimately get detached and become independent plants. The branches near the apex of a stem are short and densely crowded in a compact head called coma.

The leaves lack midrib. They are small and arranged in three vertical rows on the stem. In the surface view of a leaf one can observe two types of cells:

(i) Narrow, living, chlorophyll containing cells and

(ii) Large dead, empty, rhomboidal, hyaline (glass-like, transparent) cells with pores and s spiral as well as annular wall thickenings.

In transverse section, leaf shows beaded appearance, with large, dead hyaline cells regularly alternating with the small, green, chlorophyllous cells. The spiral thickenings provide mechanical support and keep the hyaline cells from collapsing when they are empty.

The pores help in rapid intake of water and also in exchange of cations for H⁺ions which are the metabolic products of Sphagnum. Hence, they create acidic environment in their immediate surroundings.

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The hyaline cells take up and hold large quantities of water, sometimes as much as twenty times the weight of the plant. The narrow chloroplast containing cells carry on photosynthesis.

In a mature leaf these two types of cells are arranged in a reticulate manner. This peculiar leaf structure accounts for the ability of the Sphagnum plant to absorb and retain large quantities of water and consequently for its outstanding bog-building properties. Because of their water absorbing quality they are used in gardening.

3b. Funaria

Classification:

• Division – Bryophyta
• Class – Bryopsida
• Sub-class – Bryidae
• Order – Funariales
• Family – Funariacea

Funaria is a very common moss. It is very widely distributed throughout the world. One species, Funaria hygrometrica is cosmopolitan and it the best known of all the mosses.

Like other bryophytes that you have studied, the most conspicuous form of the moss plant is the adult gametophyte. This consists of a main erect axis bearing leaves which are arranged spirally.

This adult gametophyte is called gametophores. It is small, about 1-3 cm high. The leaves do not have a stalk but show a distinct midrib. The gametophore is attached to the substratum by means of rhizoids which are multicellular, branched and have oblique septae.

The gametophyte bears sporophyte which has foot, seta and capsule. The gametophores develops from a filamentous, green short-lived protonema. The protonema produces buds at certain stage of development, which initiate the development of upright leafy green axis the gametophore.

In summary, the Bryophyta includes the simplest and the most primitive member of land plants. They lack roots based on morphological and anatomical features, they can be grouped into three major classes-Hepaticopsida, Anthocerotopsida and Bryopsida.

There is alternation of generations between the gametophyte and the sporophyte. Members include Riccia, Marchantia, Anthoceros, Sphgnum, and Funaria.

Bryophytes are the simplest, primitive non-vascular land plants among embryophytes. Because of several common characteristics, it is believed that they evolved from green algae.

There is alternation of generations between green independent gametophyte and sporophyte which is wholly or partially dependent on it. Sporophyte is generally a small capsule with or without foot and seta. The gametophyte develops from protonema and bears sex organs – archegonia and antheridia. Bryophytes are homosporous.

The challenges of land environment for a plant are fixation to the ground, desiccation, conduction of water and dispersal of sperms and spores.

These are taken care of by developing land adaptations such as epidermis, cuticle, stomata, airpores, rhizoids, multicellular jacket of cells for the protection of developing gametes, and retention of zygote in the archegonium.

In some bryophytes the primitive conducting tissues – hydroids and leptoids have also developed. The gametophyte of liverworts – Riccia and Marchantia is dorsi-ventral, thalloid structure and is internally differentiated. The pores on the dorsal surface allow exchange of gases and are much advanced in Marchantia.

The leafy liverworts have leaf-like and stem-like appendages. The gametophyte of Anthoceros, is also dorsiventral, but not differentiated internally.

Blue green algae Nostoc live in mucilage cavities of the thallus. Bryophytes are classified into liverworts (Hepaticopsida) hornworts (Anthocerotopsida) and Mosses (Bryopsida).

Mosses – Sphagnum and Funaria have erect axis and bear leaf-like structures.

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