CHAPTER - 3 PLANT KINGDOM

 

Topic Covered ✏

3.1 Algae 
3.2 Bryophytes 
3.3 Pteridophytes 
3.4 Gymnosperms 
3.5 Angiosperms

Introduction

Artificial systems (earliest systems) of classification

• Given by Linnaeus
• Used only gross superficial morphological characters such as habit, colour, number and shape of leaves, etc. 
• They were based mainly on vegetative characters or on the androecium structure. 

• Limitations
• They separated the closely related species since they were based on a few characteristics. 
• Gave equal weightage to vegetative and sexual characteristics; this is not acceptable since we know that often the vegetative characters are more easily affected by environment. 

Natural classification systems

• Given by George Bentham and Joseph Dalton Hooker. 
• Based on natural affinities among the organisms 
• Consider, not only the external features, but also internal features, like ultrastructure, anatomy, embryology and phytochemistry. 

Phylogenetic classification systems 

• Based on evolutionary relationships between the various organisms are acceptable. 
• This assumes that organisms belonging to the same taxa have a common ancestor. 

Numerical Taxonomy 

• Easily carried out using computers is based on all observable characteristics. 
• Number and codes are assigned to all the characters and the data are then processed. 
• Each character is given equal importance and at the same time hundreds of characters can be considered. 

Cytotaxonomy 

• Based on cytological information like chromosome number, structure, behaviour 

Chemotaxonomy

• Uses the chemical constituents of the plant to resolve confusions, are also used by taxonomists these days.

• These become more important when there is no supporting fossil evidence.

3.1 Algae 

• Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic.
• They occur in a variety of other habitats: moist stones, soils and wood. 
• Some of them also occur in association 
• with fungi (lichen) and 
• with animals (e.g., on sloth bear). 

• The form and size of algae is highly variable
• Colonial forms - Volvox
• Filamentous forms - Ulothrix and Spirogyra

• A few of the marine forms (kelps), form massive plant bodies. 

Reproduction in Algae

• Vegetative reproduction

• By fragmentation. 
• Each fragment develops into a thallus. 

• Asexual reproduction

• By the production of different types of spores, the most common being the zoospores. 
• They are flagellated (motile) and on germination gives rise to new plants. 

• Sexual reproduction

• Takes place through fusion of two gametes. 

• Isogamous - These gametes can be flagellated and similar in size or non-flagellated (non-motile) but similar in size .

• Anisogamous - Fusion of two gametes dissimilar in size.

• Oogamous - Fusion between one large, non-motile (static) female gamete and a smaller, motile male gamete.

Economic importance

• Half of the total carbon dioxide fixation on earth  by algae through photosynthesis. 
• Being photosynthetic they increase the level of dissolved oxygen in their immediate environment. 
• They are primary producers of energy-rich compounds which form the basis of the food cycles of all aquatic animals. 
• Many species of Porphyra, Laminaria and Sargassum are among the 70 species of marine algae used as food. 
• Certain marine brown and red algae produce large amounts of hydrocolloids (water holding substances), which are used commercially.
• Algin from brown algae
• Carrageen from red algae. 

• Agar, one of the commercial products obtained from Gelidium and Gracilaria are used to grow microbes and in preparations of ice-creams and jellies. 
• Chlorella, a unicellular alga rich in proteins, is used as food supplement even by space travellers. 

The algae are divided into three main classes: -

1. Chlorophyceae, 
2. Phaeophyceae and 
3. Rhodophyceae

3.1.1 Chlorophyceae

• Commonly called green algae.
• Plant body may be unicellular, colonial or filamentous. 
• Usually grass green due to the dominance of pigments chlorophyll a and b. 
• Pigments are localised in definite chloroplasts. 
• Chloroplasts - may be discoid, plate-like, reticulate, cup-shaped, spiral or ribbon-shaped in different species. 
• Have one or more storage bodies called pyrenoids located in the chloroplasts. 
• Pyrenoids contain protein besides starch. 
• Some algae may store food in the form of oil droplets. 
• Have a rigid cell wall made of an inner layer of cellulose and an outer layer of pectose. 

   Reproduction in Chlorophyceae

• Vegetative reproduction - usually takes place by fragmentation or by formation of different types of spores. 
• Asexual reproduction - By flagellated zoospores produced in zoosporangia. 
• Sexual reproduction - shows considerable variation in the type and formation of sex cells and it may be isogamous, anisogamous or oogamous. 

    Example- 

• Chlamydomonas, 
• Volvox, 
• Ulothrix, 
• Spirogyra and 
• Chara 

 

3.1.2 Phaeophyceae 

• Commonly called brown algae.
• Primarily in marine habitats. 
• Show great variation in size and form. 
• Simple branched, 
• Filamentous forms - Ectocarpus
• Profusely branched forms - kelps, (height of 100 metres) 

• They possess chlorophyll a, c, carotenoids and xanthophylls. 
• They vary in colour from olive green to various shades of brown depending upon the amount of the xanthophyll pigment, fucoxanthin present in them. 

• Food is stored as complex carbohydrates (laminarin or mannitol) 
• The vegetative cells have a cellulosic wall usually covered on the outside by a gelatinous coating of algin. 
• The plant body is usually attached to the substratum by a holdfast, and has a stalk, the stipe and leaf like photosynthetic organ – the frond. 

    Reproduction in Phaeophyceae

• Vegetative reproduction - takes place by fragmentation. 
• Asexual reproduction - in most brown algae is by biflagellate zoospores that are pear-shaped and have two unequal laterally attached flagella. 
• Sexual reproduction - may be isogamous, anisogamous or oogamous. 

• Union of gametes may take place in water or within the oogonium (oogamous species). 
• The gametes are pyriform (pear-shaped) and bear two laterally attached flagella.

 

    Example -

• Ectocarpus, 
• Dictyota, 
• Laminaria, 
• Sargassum 
• Fucus

3.1.3 Rhodophyceae 

• Commonly called red algae due to predominance of the red pigment (r-phycoerythrin).
• Majority of the red algae are marine 
• Mostly found in the warmer areas. 
• Occur in both well-lighted regions close to the surface of water and also at great depths in oceans where relatively little light penetrates. 
• Red thalli of most of the red algae are multicellular. 
• Some of them have complex body organisation. 
• The food is stored as floridean starch which is very similar to amylopectin and glycogen in structure. 

    Reproduction in Rhodophyceae

• Vegetatively - by fragmentation. 
• Asexually - by non-motile spores and 
• Sexually - by non-motile gametes. 

• Sexual reproduction is oogamous and accompanied by complex post fertilisation developments. 

• Example -

• Polysiphonia, 
• Porphyra, 
• Gracilaria and 
• Gelidium.

3.2 BRYOPHYTES 

• Bryophytes include the various mosses and liverworts
• Commonly growing in moist shaded areas in the hills.
• Bryophytes are also called amphibians of the plant kingdom because these plants can live in soil but are dependent on water for sexual reproduction. 
• They usually occur in damp, humid and shaded localities. 

    Body organization

• 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. 
• They lack true roots, stem or leaves. 
• They may possess root-like, leaf-like or stem-like structures. 
• The main plant body of the bryophyte is haploid. It produces gametes, hence is called a gametophyte. 

    Reproduction in bryophytes 

• The sex organs in bryophytes are multicellular. 
• Male sex organ - called antheridium (produce biflagellate antherozoids). 
• Female sex organ - called archegonium is flask-shaped and produces a single egg. 

    Life cycle of bryophytes 

• Antherozoids are released into water where they come in contact with archegonium and fuses with the egg to produce the zygote. 

• Zygotes do not undergo reduction division immediately. They produce a multicellular body called a sporophyte. 

• Sporophyte - It is  not free-living but attached to the photosynthetic gametophyte and derives nourishment from it. 

• Gametophyte - Some cells of the sporophyte undergo reduction division (meiosis) to produce haploid spores. These spores germinate to produce gametophyte. 

    Economic importance

• Bryophytes in general are of little economic importance but great ecological importance
• Some mosses provide food for herbaceous mammals, birds and other animals. 
• They play an important role in plant succession on bare rocks/soil. 

• Species of Sphagnum (a moss) - provide peat 
• That have long been used as fuel, 
• As a packing material for trans-shipment of living material because of their capacity to hold water. 

• Mosses along with lichens

• They are first organisms to colonise rocks and hence, are of great ecological importance. 
• 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. 

Bryophytes are divided into 

1. Liverworts and 
2. Mosses

3.2.1 Liverworts 

• Grow usually in moist, shady habitats such as banks of streams, marshy ground, damp soil, bark of trees and deep in the woods. 
• The plant body of a liverwort is thalloid, e.g., Marchantia. 
• Thallus - is dorsiventral and closely appressed to the substrate. 
• The leafy members have tiny leaf-like appendages in two rows. 

    Reproduction in liverworts

• Asexual reproduction 

• Takes place by fragmentation of thalli, or by the formation of specialised structures called gemmae . 
• Gemmae - are green, multicellular, asexual buds, which develop in small receptacles called gemma cups located on the thalli. 
• The gemmae become detached from the parent body and germinate to form new individuals. 

• Sexual reproduction, 

• Male and female sex organs are produced either on the same or on different thalli. 
• The sporophyte is differentiated into a foot, seta and capsule. 
• After meiosis, spores are produced within the capsule. These spores germinate to form free-living gametophytes.

Marchantia

3.2.2 Mosses 

• The predominant stage of moss is the gametophyte which consists of two stages. 

1. Protonema (first stage) 
2. Leafy stage (second stage)

• Protonema stage - 
• Which develops directly from a spore. 
• It is a creeping, green, branched and frequently filamentous stage. 

• Leafy stage
• Which develops from the secondary protonema as a lateral bud. 
• They consist of upright, slender axes bearing spirally arranged leaves. 
• They are attached to the soil through multicellular and branched rhizoids. 
• This stage bears the sex organs. 

    Reproduction in mosses

• Vegetative reproduction - 
• By fragmentation and budding in the secondary protonema.

• Sexual reproduction, 
• Sex organs antheridia and archegonia are produced at the apex of the leafy shoots. 
• After fertilisation, the zygote develops into a sporophyte, consisting of a foot, seta and capsule. 

• Sporophyte - More elaborate than that in liverworts. 

• Capsule - contains spores. Spores are formed after meiosis. 
• The mosses have an elaborate mechanism of spore dispersal. 

• Examples

• Funaria, 
• Polytrichum
• Sphagnum

3.3 PTERIDOPHYTES 

• Pteridophytes include horsetails and ferns. 
• Pteridophytes are used for medicinal purposes, as soil-binders and also frequently grown as ornamentals. 

• Pteridophytes are found in cool, damp, shady places though some may flourish well in sandy-soil conditions.
• First terrestrial plants to possess vascular tissues – xylem and phloem. 

    Body organization

• 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. 

• Leaves in pteridophyta 
• Small (microphylls) - Selaginella
• Large (macrophylls) - ferns. 

    Life cycle of Pteridophytes

• Sporophytes - bear sporangia that are subtended by leaf-like appendages called sporophylls. 
• Sporophylls - may form distinct compact structures called strobili or cones (Selaginella, Equisetum). 

• Prothallus
• Inconspicuous, 
• Small but multicellular,
• Free-living, 
• Photosynthetic thalloid gametophytes.

• Gamatophytes 

• Require cool, damp, shady places to grow. 
• Because of this specific restricted requirement and the need for water for fertilisation,They spread of living pteridophytes is limited and restricted to narrow geographical regions. 
• The gametophytes bear male and female sex organs called antheridia and archegonia, respectively. 

    Zygote development

• In Homosporous - 

• Zygote thereafter produces a multicellular well-differentiated sporophyte
• Majority of the pteridophytes - homosporous .

• In heterosporous - 

• Zygote develops into young embryos within female gametophytes. 
• Selaginella and Salvinia - heterosporous. 

• Megaspores - germinate and give rise to female gametophyte
• Microspores - germinate and give rise to Male gametophyte

• Female gametophytes in these plants are retained on the parent sporophytes for variable periods. 
• 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. 

    Pteridophytes are further classified into four classes: 

• Psilopsida - Psilotum
• Lycopsida - Selaginella, Lycopodium
• Sphenopsida - Equisetum
• Pteropsida - Dryopteris, Pteris, Adiantum

3.4 GYMNOSPERMS 

• Gymnos : naked, sperma : seeds
• Ovules - Not enclosed by any ovary wall (remain exposed, both before and after fertilisation) 
• Seeds - Develop post-fertilisation, are not covered, i.e., are naked. 
• Gymnosperms include medium-sized trees or tall trees and shrubs
• Sequoia - Giant redwood tree  (one of the tallest tree species)

    Roots 

• Generally tap roots. 
• Pinus - Have fungal association in the form of mycorrhiza
• Cycas - Have coralloid roots (small specialised roots) are associated with N2 - fixing cyanobacteria. 

    Stems

• Unbranched - Cycas
• Branched - Pinus, Cedrus
• Long and Dwarf shoot - Pinus and Ginkgo. 

    Leaves 

• May be Simple or compound
• Leaves in gymnosperms are well-adapted to withstand extremes of temperature, humidity and wind. 
• In Cycas - Pinnate leaves (compound) persist for a few years. 
• In conifers 
• The needle-like leaves reduce the surface area. 
• Their thick cuticle and sunken stomata also help to reduce water loss.

    Life cycle of Gymnosperms

• The gymnosperms are heterosporous
• They produce haploid microspores and megaspores. 
• The two kinds of spores are produced within sporangia that are borne on sporophylls .
• Sporophylls - which are arranged spirally along an axis to form lax or compact strobili or cones.

• Strobili / Cones

• Bearing microsporophylls and microsporangia are called microsporangiate or male strobili. 
• Bearing megasporophylls with ovules or megasporangia are called macrosporangiate or female strobili. 

• Microsporangiate or male strobili

• The microspores develop into a male gametophytic generation .
• Male gametophytic - 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. 

• Macrosporangiate or female strobili. 

• 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. 
• Ovules - Borne on megasporophylls which may be clustered to form the female cones. 
• The megaspore mother cell divides meiotically to form four megaspores within the megasporangium.

                                                  

• One of the megaspores - Develops into a multicellular female gametophyte that bears two or more archegonia or female sex organs. 
• The multicellular female gametophyte is also retained within megasporangium. 

• Pinus is monoecious, i.e., male and female cone or strobili are borne on same tree. 
• Cycas is dioecious, i.e., male cone and megasporophylls are borne on different trees. (Female cone is absent in Cycas).

• In gymnosperms the male and the female gametophytes do not have an independent free-living existence. They remain within the sporangia retained on the sporophytes.

• Seed formation

• 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. 
• The pollen tube carrying the male gametes grows towards archegonia in the ovules and discharge their contents near the mouth of the archegonia. 
• Following fertilisation, zygote develops into an embryo and the ovules into seeds. 
• These seeds are not covered. 

3.5 ANGIOSPERMS 

• Pollen grains and ovules - are developed in specialised structures called flowers. 
• In angiosperms, the seeds are enclosed in fruits. 
• The angiosperms are an exceptionally large group of plants occurring in wide range of habitats. 

• Range in size from the 

• Smallest plant - Wolffia
• Tall trees - Eucalyptus (over 100 metres).

• They provide us with food, fodder, fuel, medicines and several other commercially important products. 

• They are divided into two classes : 

• Dicotyledons and 
• Monocotyledons