Biology: Plant Kingdom

DIVISON – Thallophyta

(1)      Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic (both fresh water and marine) organisms.

(2)      They occur in a variety of other habitats: moist stones, soils and wood. Some of them also occur in association with fungi (lichen) and animals (e.g., on sloth bear).

(3)       The form and size of algae is highly variable, ranging from colonial forms like Volvox and the filamentous forms like Ulothrix and Spirogyra (Figure 3.1).

(4)       A few of the marine forms such as kelps, form massive plant bodies. The algae reproduce by vegetative, asexual and sexual methods.

(5)       Vegetative reproduction is by fragmentation.

(6)      Each fragment develops into a thallus.

(7)       Asexual reproduction is by the production of different types of spores, the most common being the zoospores.

(8)      They are flagellated (motile) and on germination gives rise to new plants. Sexual reproduction takes place through fusion of two gametes.

(9)      These gametes can be flagellated and similar in size (as in Ulothrix) or non-flagellated (non-motile) but similar in size (as in Spirogyra).

(10)   Such reproduction is called isogamous. Fusion of two gametes dissimilar in size, as in species of Eudorina is termed as anisogamous.

Fusion between one large, nonmotile (static) female gamete and a smaller, motile male gamete is termed oogamous, e.g., Volvox, Fucus.

Sexual reproduction is of three types:

Solved Examples:

1. Algae have cell wall made up of
(A) Cellulose, galactans and mannans
(B) Hemicellulose, pectins and proteins
(C) Pectins, cellulose and proteins
(D) Cellulose, hemicellulose and pectins

The correct answer is option (A)

2. In the five kingdom classification, Chlamydomonas and Chlorella have been included in
(A) Algae
(B) Plantae
(C) Monera
(D) Protista

The correct answer is option (B)

BRYOPHYTA:

1         The study of Bryophytes is known as Bryology.

2        The term 'Bryophyta' was proposed by 'Robert Braun'.

3        Hedwig is considered to be the father of Bryology. But some scientists consider Cavers as the father of Bryology.

4        Father of Indian Bryology is Prof. Shiv Ram Kashyap.

General Features :

1.       Bryophytes include the various mosses and liverworts that are found commonly growing in moist shaded areas in the hills.

2.       Bryophytes are also called amphibians of the plant kingdom because these plants can live in soil but are dependent on water for sexual reproduction.

3.       They usually occur in damp, humid and shaded localities. They play an important role in plant succession on bare rocks/soil.

4.       The plant body of bryophytes is more differentiated than that of algae. It is thallus-like and prostrate or erect, and attached to the substratum by unicellular or multicellular rhizoids.

5.       They lack true roots, stem or leaves.

6.       They may possess root-like, leaf-like or stem-like structures.

7.       The main plant body of the bryophyte is haploid.

8.       It produces gametes, hence is called a gametophyte.

9.       The sex organs in bryophytes are multicellular.

10.   The male sex organ is called antheridium. They produce biflagellate antherozoids.

11.   The female sex organ called archegonium is flask-shaped and produces a single egg.

12.   The antherozoids are released into water where they come in contact with archegonium. An antherozoid fuses with the egg to produce the zygote. Zygotes do not undergo reduction division immediately. They produce a multicellular body called a sporophyte. The sporophyte is not free-living but attached to the photosynthetic gametophyte and derives nourishment from it. Some cells of the sporophyte undergo reduction division (meiosis) to produce haploid spores. These spores germinate to produce gametophyte.

13.   Bryophytes in general are of little economic importance but some mosses provide food for herbaceous mammals, birds and other animals.

14.   Species of Sphagnum, a moss, provide peat that have long been used as fuel, and as packing material for trans-shipment of living material because of their capacity to hold water.

15.    Mosses along with lichens are the first organisms to colonise rocks and hence, are of great ecological importance.

16.   They decompose rocks making the substrate suitable for the growth of higher plants. Since mosses form dense mats on the soil, they reduce the impact of falling rain and prevent soil erosion.

17. The bryophytes are divided into liverworts and mosses.

Liverworts:

Hepaticopsida - Liverworts

1         All the bryophytes included in this class have shape like liver, so they are known as liverworts.

2        Plant body of this group is thallus-like. Thallus has rhizoid and scales. Rhizoids are unicellular and unbranched while the scales are multicellular.

3        The sporophyte of Liverworts completely depends on gametophyte for food, water and habitat.

4        Foot, seta and capsule are the parts of sporophytes of liverworts. (Except Riccia consists of capsule only)

5        In this class, the formation of spores and nurse cells takes place by the cells of endothecium. Cells of amphithecium form only wall of sporophyte.
Amphithecium = Wall of sporophyte
Endothecium= sporogenous cells = spore mother cells + nurse cells

6        Elaters are present in sporophyte of some members of liverworts. (For example: Marchantia - In Marchantia nurse cells are modified into elaters). Elaters help in the dispersal of spores and they are hygroscopic.
Example: Liverworts
Riccia, Marchantia, Cryptothallus, Riella, Pellia, Porella

Mosses:

Bryopsida or Musci - Mosses

1         All the Mosses are included in this class. The plant body of mosses is made up of stem, leaves and rhizoids. The rhizoids present in the plants of this class are multicellular and branched. These rhizoids have oblique septa.

Notes

The presence of leaves in gametophyte is one of the unique characters of Moss. In plant kingdom, not a single gametophyte has leaves.

2        The sporophyte of moss in bryophyta is highly developed while the sporophyte of liverwort is the simplest. The sporophyte of moss is divided into foot, seta and capsule.

3        The sporophyte of moss is also semi-parasite like, that of Hornworts, i.e., it is photosynthetic. The sporophyte absorbs the water from gametophyte with the help of foot.

4        Capsule is the fertile part of the sporophyte, i.e., the formation of spores takes place in it. Foot and seta are the sterile part of the sporophyte.

5        Seta helps the capsule to remain in the air, due to which the dispersal of spores can take place conveniently.

6        Elaters and nurse cells are absent in bryopsida.

Endothecium - Spores

Life Cycle Of Bryophytes :-

Pteridophytes :

1.       The Pteridophytes include horsetails and ferns.

2.        Pteridophytes are used for medicinal purposes and as soil-binders.

3.       They are also frequently grown as ornamentals. Evolutionarily, they are the first terrestrial plants to possess vascular tissues – xylem and phloem.

4.        The pteridophytes are found in cool, damp, shady places though some may flourish well in sandy-soil conditions.

5.       In pteridophytes, the main plant body is a sporophyte which is differentiated into true root, stem and leaves . These organs possess well-differentiated vascular tissues.

6.        The leaves in pteridophyta are small (microphylls) as in Selaginella or large (macrophylls) as in ferns.

7.        The sporophytes bear sporangia that are subtended by leaf-like appendages called sporophylls. In some cases sporophylls may form distinct compact structures called strobili or cones (Selaginella, Equisetum).

8.       The sporangia produce spores by meiosis in spore mother cells. The spores germinate to give rise to inconspicuous, small but multicellular, free-living, mostly photosynthetic thalloid gametophytes called prothallus.

9.       These gametophytes require cool, damp, shady places to grow. Because of this specific restricted requirement and the need for water for fertilisation, the spread of living pteridophytes is limited and restricted to narrow geographical regions.

10.   The gametophytes bear male and female sex organs called antheridia and archegonia, respectively.

11.   Water is required for transfer of antherozoids – the male gametes released from the antheridia, to the mouth of archegonium.

12.   Fusion of male gamete with the egg present in the archegonium result in the formation of zygote.

13.   Zygote thereafter produces a multicellular well-differentiated sporophyte which is the dominant phase of the pteridophytes.

14.   In majority of the pteridophytes all the spores are of similar kinds; such plants are called homosporous. Genera like Selaginella and Salvinia which produce two kinds of spores, macro (large) and micro (small) spores, are known as heterosporous.

15.   The megaspores and microspores germinate and give rise to female and male gametophytes, respectively. The female gametophytes in these plants are retained on the parent sporophytes for variable periods.

16.   The development of the zygotes into young embryos take place within the female gametophytes. This event is a precursor to the seed habit considered an important step in evolution.

17. The pteridophytes are further classified into four classes: Psilopsida (Psilotum); Lycopsida (Selaginella, Lycopodium), Sphenopsida (Equisetum) and Pteropsida (Dryopteris, Pteris, Adiantum).

Life Cycle Of Pteridophytes :

4. Gymnosperms :

(1) The gymnosperms (gymnos : naked, sperma : seeds) are plants in which the ovules are not enclosed by any ovary wall and remain exposed, both before and after fertilisation.

(2) The seeds that develop post-fertilisation, are not covered, i.e., are naked. Gymnosperms include medium-sized trees or tall trees and shrubs.

(3) One of the gymnosperms, the giant redwood tree Sequoia is one of the tallest tree species.

(4) The roots are generally tap roots.

(5) Roots in some genera have fungal association in the form of mycorrhiza (Pinus), while in some others (Cycas) small specialised roots called coralloid roots are associated with N2- fixing cyanobacteria.

(6) The stems are unbranched (Cycas) or branched (Pinus, Cedrus). The leaves may be simple or compound.

(7) In Cycas the pinnate leaves persist for a few years.

(8) The leaves in gymnosperms are well-adapted to withstand extremes of temperature, humidity and wind.

(9) In conifers, the needle-like leaves reduce the surface area. Their thick cuticle and sunken stomata also help to reduce water loss.

(10) The gymnosperms are heterosporous; they produce haploid microspores and megaspores. The two kinds of spores are produced within sporangia that are borne on sporophylls which are arranged spirally along an axis to form lax or compact strobili or cones.

(11) The strobili bearing microsporophylls and microsporangia are called microsporangiate or male strobili.

(12) The microspores develop into a male gametophytic generation which is highly reduced and is confined to only a limited number of cells. This reduced gametophyte is called a pollen grain. The development of pollen grains take place within the microsporangia.

(13) The cones bearing megasporophylls with ovules or megasporangia are called macrosporangiate or female strobili.

(14) The male or female cones or strobili may be borne on the same tree (Pinus). However, in cycas male cones and megasporophylls are borne on different trees.

(15) The megaspore mother cell is differentiated from one of the cells of the nucellus. The nucellus is protected by envelopes and the composite structure is called an ovule. The ovules are borne on megasporophylls which may be clustered to form the female cones. The megaspore mother cell divides meiotically to form four megaspores.

(16) One of the megaspores enclosed within the megasporangium develops into a multicellular female gametophyte that bears two or more archegonia or female sex organs.

(17) The multicellular female gametophyte is also retained within megasporangium.

(18) Unlike bryophytes and pteridophytes, in gymnosperms the male and the female gametophytes do not have an independent free-living existence.

(19) They remain within the sporangia retained on the sporophytes.

(20) The pollen grain is released from the microsporangium. They are carried in air currents and come in contact with the opening of the ovules borne on megasporophylls.

(21) The pollen tube carrying the male gametes grows towards archegonia in the ovules and discharge their contents near the mouth of the archegonia.

(22) Following fertilisation, zygote develops into an embryo and the ovules into seeds. These seeds are not covered.

Life Cycle Of Gymnosperms :

Angiosperms :

1.      Unlike the gymnosperms where the ovules are naked, in the angiosperms or flowering plants, the pollen grains and ovules are developed in specialised structures called flowers.

2.      In angiosperms, the seeds are enclosed in fruits.

3.      The angiosperms are an exceptionally large group of plants occurring in wide range of habitats.

4.      They range in size from the smallest Wolffia to tall trees of Eucalyptus (over 100 metres).

5.      They provide us with food, fodder, fuel, medicines and several other commercially important products. They are divided into two classes : the dicotyledons and the monocotyledons.