plant kingdom

• PLANT KINGDOM
• In the previous chapter, we looked at the broad classification of living organisms under the system proposed by Whittaker (1969) wherein he suggested the Five Kingdom classification viz. Monera, Protista, Fungi, Animalia and Plantae. In this chapter, we will deal in detail with further classification within Kingdom Plantae popularly known as the 'plant kingdom'.
• Fungi, and members of the Monera and Protista having cell walls have now been excluded from Plantae though earlier classifications placed them in the same kingdom. So, the cyanobacteria that are also referred to as blue green algae are not 'algae' any more.
• Organisms described in this chapter under Plantae

  • Algae
  • Bryophytes
  • Pteridophytes
  • Gymnosperms
  • Angiosperms
• The earliest systems of classification 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 (system given by Linnaeus). Such systems were artificial; they separated the closely related species since they were based on a few characteristics.
• Natural classification systems developed, which were based on natural affinities among the organisms and consider, not only the external features, but also internal features, like ultra-structure, anatomy, embryology and phytochemistry.
• At present 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 which is now 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. In this way each character is given equal importance and at the same time hundreds of characters can be considered.
• Cytotaxonomy that is based on cytological information like chromosome number, structure, behaviour and chemotaxonomy that uses the chemical constituents of the plant to resolve confusions, are also used by taxonomists these days.
• Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic (both fresh water and marine) organisms. 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).
• Algae


  • The form and size of algae is highly variable, ranging from colonial forms like Volvox and the filamentous forms like Ulothrix and Spirogyra. A few of the marine forms such as kelps, form massive plant bodies.
• Modes of reproduction in algae

  • Vegetative reproduction by fragmentation
  • Asexual reproduction by production of different types of spores, the most common being the zoospores
  • Sexual reproduction through fusion of two gametes, which can be flagellated and similar in size (isogamous), non-flagellated but similar in size (isogamous), or one large non-motile female gamete and a smaller motile male gamete (oogamous)
• Algae are useful to man in a variety of ways. At least a half of the total carbon dioxide fixation on earth is carried out by algae through photosynthesis. Being photosynthetic they increase the level of dissolved oxygen in their immediate environment. They are of paramount importance as primary producers of energy-rich compounds which form the basis of the food cycles of all aquatic animals.
• Algae used as food

  • Porphyra, Laminaria and Sargassum
• Algae that produce hydrocolloids

  • Brown algae (algin)
  • Red algae (carrageen)
• Algae used to grow microbes and in preparations of ice-creams and jellies

  • Gelidium and Gracilaria (agar)
• Chlorella, a unicellular alga rich in proteins, is used as food supplement even by space travellers.
• Main classes of algae

  • Chlorophyceae (green algae)
  • Phaeophyceae (brown algae)
  • Rhodophyceae (red algae)
• Chlorophyceae (green algae)


  • The plant body may be unicellular, colonial or filamentous. They are usually grass green due to the dominance of pigments chlorophyll a and b. The chloroplasts may be discoid, plate-like, reticulate, cup-shaped, spiral or ribbon-shaped in different species. Most of the members have one or more storage bodies called pyrenoids located in the chloroplasts. Vegetative reproduction usually takes place by fragmentation or by formation of different types of spores. Asexual reproduction is by flagellated zoospores produced in zoosporangia. The sexual reproduction shows considerable variation in the type and formation of sex cells and it may be isogamous, anisogamous or oogamous.
• Phaeophyceae (brown algae)


  • 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, which may be in the form of 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. 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.
• Rhodophyceae (red algae)


  • The 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. The red algae usually reproduce vegetatively by fragmentation. They reproduce asexually by non-motile spores and sexually by non-motile gametes. Sexual reproduction is oogamous and accompanied by complex post fertilisation developments.
• Bryophytes include the various mosses and liverworts that are found 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. They play an important role in plant succession on bare rocks/soil.
• Bryophytes


  • 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. 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. The sex organs in bryophytes are multicellular. The male sex organ is called antheridium. They produce biflagellate antherozoids. The female sex organ called archegonium is flask-shaped and produces a single egg. The zygotes produce a multicellular body called a sporophyte. The sporophyte is not free-living but attached to the photosynthetic gametophyte and derives nourishment from it.
• Bryophytes in general are of little economic importance but some mosses provide food for herbaceous mammals, birds and other animals. 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.
• Mosses along with lichens are the 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


  • Liverworts
  • Mosses
• Liverworts


  • The plant body of a liverwort is thalloid, e.g., Marchantia. The thallus is dorsiventral and closely appressed to the substrate. Asexual reproduction takes place by fragmentation of thalli, or by the formation of specialised structures called gemmae. During 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.
• Mosses


  • The predominant stage of the life cycle of a moss is the gametophyte which consists of two stages - the protonema stage, which develops directly from a spore and is a creeping, green, branched and frequently filamentous stage, and the leafy stage, which develops from the secondary protonema as a lateral bud and consists of upright, slender stems with leaves.
• Thalli


  Vegetative plant body of a bryophyte
• Gemmae


  Specialized asexual reproductive structures in bryophytes
• Vegetative reproduction in bryophytes

  1. Fragmentation of thalli

  2. Formation of gemmae
• Gametophyte in bryophytes


  • Dominant phase of the life cycle
  • Produces sex organs (antheridia and archegonia)
• Sexual reproduction in bryophytes

  1. Male and female sex organs produced on the same or different thalli

  2. Sporophyte develops from zygote

  3. Sporophyte differentiated into foot, seta and capsule

  4. Spores produced within capsule after meiosis

  5. Spores germinate to form free-living gametophytes
• Gametophyte in mosses


  • Consists of protonema and leafy stages
  • Protonema develops from spore
  • Leafy stage develops from secondary protonema
• Vegetative reproduction in mosses

  Fragmentation and budding in secondary protonema
• Sexual reproduction in mosses

  1. Sex organs (antheridia and archegonia) produced at apex of leafy shoots

  2. Zygote develops into sporophyte with foot, seta and capsule

  3. Spores formed after meiosis in capsule
• Mosses have elaborate mechanism of spore dispersal
• Common examples of mosses are Funaria, Polytrichum and Sphagnum
• Pteridophytes


  • First terrestrial plants to possess vascular tissues (xylem and phloem)
  • Found in cool, damp, shady places or sandy-soil conditions