Notes " Plant Kingdom"
Notes " Plant Kingdom"
By:- Nagraj Sir
Systems of Biological classification
1. Artificial classification systems
- Earliest systems of classification.
-
They were based
on vegetative characters or superficial morphological characters such as habit,
colour, number and shape of leaves, etc.
- Linnaneus Artificial system of classification was based on the androecium structure.
Drawbacks:
-
They separated
the closely related
species since they were
based on a few characteristics.
- Equal weightage to vegetative and sexual characteristics. This is not acceptable since the vegetative characters are more easily affected by environment.
2. Natural classification systems
- These are based on natural
affinities among organisms.
-
It considers
external features and internal features (ultrastructure,
anatomy, embryology & phytochemistry).
- E.g. Classification for flowering plants given by George Bentham & Joseph Dalton Hooker.
3. Phylogenetic classification systems
- It is based on evolutionary relationships among organisms.
- This assumes that organisms in the same taxa have a
common ancestor.
Other sources to classification:
o Numerical
Taxonomy: It is based on all observable
characteristics. It is easily carried out using computers. Number & codes are assigned
to all the characters and the
data are processed. Thus, hundreds of characters can be equally considered.
o Cytotaxonomy: It is based on cytological information like
chromosome number, structure, behaviour etc.
o
Chemotaxonomy: It uses chemical constituents of plants.
ALGAE
- They are simple, thalloid, autotrophic, chlorophyll- bearing and aquatic (fresh water & marine) organisms.
- They also occur in moist stones, soils and wood.
- Some occur in association with fungi (lichen) and animals (e.g., on sloth bear).
- The form and size of algae is highly variable.
o Microscopic unicellular forms: E.g. Chlamydomonas.
o Colonial forms: E.g. Volvox.
o Filamentous forms: E.g. Ulothrix
and Spirogyra.
Reproduction:
-
Vegetative reproduction: By fragmentation. Each fragment develops into a thallus.
-
Asexual reproduction:
By the
production of spores. E.g. zoospores (most common). They are
flagellated (motile) and on germination gives rise to new plants.
-
Sexual reproduction: Through fusion of two gametes.
It is many types:
o Isogamous: Fusion of gametes similar in size. They may be flagellated
(e.g. Ulothrix) or non-flagellated
(non-motile, e.g. Spirogyra).
o
Anisogamous: Fusion of two gametes dissimilar in size.
E.g. Some species of Eudorina.
o Oogamous: Fusion between one large, non-motile (static) female gamete and a smaller, motile male gamete. E.g. Volvox, Fucus.
Benefits of algae:
Through photosynthesis, they fix half of the total
CO2
on earth and increase the level of dissolved oxygen.
They are primary producers and the basis of the food cycles of all aquatic animals.Many marine algae (70 species) are used as food. E.g.Porphyra, Laminaria and Sargassum.
Agar (from Gelidium & Gracilaria) is used to grow
microbes and in ice-creams and jellies.
Some marine brown & red algae produce hydrocolloids (water holding substances). E.g. algin (brown algae) and carrageen (red algae). These are used commercially.
Protein-rich unicellular algae like Chlorella & Spirullina are used as food supplements by space travellers.
Algae include 3 classes: Chlorophyceae, Phaeophyceae and Rhodophyceae.
1. Chlorophyceae (green algae)
- Unicellular, colonial or filamentous.
- They are usually
grass green due to the pigments
chlorophyll a and b in chloroplasts.
-
The chloroplasts may be discoid,
plate-like, reticulate, cup- shaped, spiral
or ribbon-shaped in different species.
-
Most of them have one or more pyrenoids (storage bodies) located in the chloroplasts. Pyrenoids contain
protein besides starch.
- Some algae store food as oil droplets.
-
They have a
rigid cell wall made of an inner layer of cellulose and an outer layer of pectose.
- E.g. Chlamydomonas, Volvox,
Ulothrix, Spirogyra &
Chara.
Reproduction:
o Vegetative reproduction: By fragmentation or by
formation of different types of spores.
o Asexual reproduction: By flagellated zoospores produced in zoosporangia.
o Sexual reproduction: Isogamous, anisogamous or oogamous.
2. Phaeophyceae (brown algae)
- They are mostly marine forms.
- They range from simple branched, filamentous forms (E.g. Ectocarpus) to profusely branched forms (e.g. kelps- 100 m in height).
- They have chlorophyll a, c, carotenoids & xanthophylls.
- They vary in colour from olive green to brown depending upon the amount of a xanthophyll pigment, fucoxanthin.
- Food is stored as complex carbohydrates (laminarin or mannitol).
- The vegetative cells have a cellulosic wall covered by a gelatinous coating of algin.
- Plant body is attached to substratum by a holdfast, and has a stalk (stipe) and leaf like photosynthetic organ (frond).
E.g. Ectocarpus, Dictyota, Laminaria, Sargassum & Fucus.
Reproduction:
*Vegetative reproduction: By fragmentation.
*Asexual reproduction: By pear-shaped biflagellate zoospores (have 2 unequal laterally attached flagella).
* Sexual
reproduction: Isogamous, anisogamous or
oogamous. Union of gametes occurs
in water or with in the
oogonium (oogamous species). Gametes are pear-shaped (pyriform) bearing 2
laterally attached flagella.
3. Rhodophyceae (red algae)
- They have a red pigment,
r-phycoerythrin.
- Majority are marine
- The red thalli of most of the red algae are multicellular.
- The food is stored as floridean starch which is very similar to amylopectin and glycogen in structure.
- E.g. Polysiphonia, Porphyra, Gracilaria and
Gelidium.
Reproduction:
o Vegetative reproduction: By fragmentation.
o Asexual reproduction: By non-motile spores.
o Sexual reproduction: Oogamous. By non-motile gametes.
BRYOPHYTES
- They are called
amphibians of the plant kingdom
because they can live in soil but need water for sexual
reproduction.
- They occur in damp, humid and shaded localities.
- It is thallus-like and prostrate or erect, and attached to the substratum by rhizoids.
- They lack true roots, stem or leaves. They may possess root-like, leaf-like or stem-like structures.
- The main plant body is haploid. It produces gametes, hence is called a gametophyte.
- The male sex organ (antheridium) produces biflagellate antherozoids. The female sex organ (archegonium) is flask-shaped and produces a single egg.
- Antherozoids are released to water and meet archegonium. An antherozoid fuses with the egg to form zygote.
-
Zygotes do not
undergo meiosis immediately. They produce a multicellular body called a sporophyte.
-
Sporophyte is
not free-living but attached to the photosynthetic gametophyte
and derives nourishment from it. Some cells
of the sporophyte undergo meiosis to form haploid spores. They germinate to
form gametophyte.
Importance of Bryophytes:
Species of Sphagnum (a moss) provide peat. It is used as fuel. It has water holding capacity so that used as packing material for trans-shipment of living material.
They are ecologically important because of their role in plant succession on bare rocks/soil. Mosses along with lichens decompose rocks making the substrate suitable for the growth of higher plants.
mosses form dense mats on the soil, they can
prevent soil erosion.
The bryophytes are divided into liverworts
and mosses.
Liverworts
- They grow usually in moist, shady , marshy ground, damp soil.
- Their plant body is thalloid. E.g. Marchantia. Thallus is dorsi-ventral and attached to the substrate.
-
Asexual reproduction: By fragmentation of thallus body or by the formation of gemmae green multicellular, asexual buds that develop in small receptacles (gemma cups) on the thallus. Gemmae are
detached from the parent body and germinate
to form new individuals.
-Sexual reproduction: Male and female sex organs are produced on the same or different thalli. 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.
Mosses
- The predominant stage of the life cycle of a moss is the gametophyte. It consists of two stages.
o Protonema stage: The first stage which develops directly from a spore. It
is a creeping, green, branched and frequently filamentous stage.
o Leafy stage:
The second 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 soil through multicellular and branched rhizoids. This stage
bears the sex organs.
- Vegetative reproduction: By fragmentation and budding in the secondary protonema.
-
Sexual reproduction: The antheridia & archegonia are produced at the apex of leafy
shoots. After fertilisation, zygote develops into a sporophyte, consisting of a
foot, seta and capsule. The sporophyte in mosses is more elaborate than that in
liverworts. The capsule contains spores. Spores are formed after meiosis.
Mosses have an elaborate mechanism of spore dispersal.
- E.g. Funaria, Polytrichum and
Sphagnum.
PTERIDOPHYTES
- They include horsetails and ferns.
- They are found in cool, damp, shady places
- they are the first terrestrial plants to possess vascular tissues (xylem & phloem).
- In pteridophytes, the dominant phase (main plant body) is a sporophyte. It is differentiated to true root, stem & leaves. These organs have well-differentiated vascular tissues.
- The leaves in pteridophyta are small (microphylls) as in Selaginella or large (macrophylls) as in ferns.
- Economic importance: They are used for medicinal purposes and as soil-binders and ornamentals.
REPRODUCTION:
-
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 (E.g. Selaginella, Equisetum).
- Sporangia produce
spores by meiosis
in spore mother cells.
-
The spores
germinate to give small, multicellular, free-living, mostly
photosynthetic thalloid gametophytes called prothallus.
- Prothallus requires water for fertilization.
-
The
gametophytes (prothallus) bear male and female sex organs called antheridia and archegonia, respectively.
-
Water is needed
for transfer of antherozoids (male
gametes from antheridia) to the mouth of archegonium.
-
Antherozoid fuses with the egg in the archegonium to form zygote.
Zygote develops to a multicellular well- differentiated sporophyte.
-
Most of the pteridophytes produce similar kinds of spores (homosporous plants). Others produce two kinds of spores, macro (mega) & micro spores. They are heterosporous.
E.g. Selaginella & Salvinia.
- The megaspores & microspores germinate and give rise to female and male gametophytes, respectively.
- Within female gametophytes, zygotes develop into young embryos.
- The pteridophytes have 4 classes:
1.
Psilopsida: E.g. Psilotum
2.
Lycopsida: E.g. Lycopodium
3.
Sphenopsida: E.g. Equisetum
4. Pteropsida: E.g. Pteris,
GYMNOSPERMS
- Gymnosperms (gymnos: naked, sperma: seeds) are plants in which the ovules are not enclosed by ovary wall and remain exposed before and after fertilization. Seeds are (naked).
- They include medium-sized trees or tall trees and shrubs. Sequoia (giant redwood) is the tallest tree species.
- The roots are generally tap roots.
- Roots in some genera have fungal association in the form of mycorrhiza (E.g. Pinus).
- In plants like Cycas, small specialized roots (coralloid roots) are associated with N2- fixing cyanobacteria.
- Stems are unbranched or branched
- Leaves are simple or compound.
- In Cycas, the pinnate leaves persist for a few years.
- In conifers (Pinus, Cedrus etc.), the needle-like leaves reduce the surface area. Their thick cuticle and sunken stomata also help to reduce water loss.
REPRODUCTION:
- Gymnosperms are heterosporous. They produce haploid microspores and megaspores.
- Some leaves are modified into sporophylls. They are compactly and spirally arranged along an axis to form strobili or cones.
- Sporophylls bear sporangia in which spores are produced.
- Sporophylls are 2 types:
o Microsporophylls: They are arranged to male strobili (microsporangia). They
bear microsporangia. The
o Megasporophylls: They are arranged
to female strobili (macrosporangia). They bear megasporangia (ovules). Megasporangium
mainly consists of a body called nucellus.
The megaspore mother cell is differentiated from a
cell of the nucellus.
Megaspore mother cell undergoes meiosis
to form four megaspores. One of the megaspores enclosed
within the Megasporangium (nucellus) develops into
a multicellular female gametophyte that bears two or more archegonia. The multicellular female gametophyte is also retained
within megasporangium.
- The male or female cones may be borne on the same tree (Pinus) or on different trees (Cycas).
-
The pollen
grain released from the microsporangium are carried in air currents
and meet the opening
of the ovules. The pollen
tube carrying the male gametes
grows towards archegonia in the ovules
and discharges their contents near the mouth of the archegonia.
-
After fertilization, zygote develops into an embryo
and the ovules into seeds.
ANGIOSPERMS (FLOWERING PLANTS)
-They are large group of plants.
- They range in size from tiny, almost microscopic Wolffia to tall trees of Eucalyptus (over 100 metres).
- They include 2 classes: Dicotyledons & Monocotyledons.
o Dicotyledons: Have 2 cotyledons in seeds, reticulate venations in leaves and tetramerous or pentamerous flowers (4 or 5 members in each floral whorl).
o Monocotyledons: Have only one cotyledon, parallel venation in leaves and trimerous flowers (3 members in each floral whorl).
REPRODUCTION:
- Flower is the reproductive structure.
-
Female sex
organ is the pistil. It
consists of a swollen ovary at its
base, a long slender style & stigma. Ovary
contains ovules. An ovule has a megaspore mother cell that undergoes meiosis
to form 4 haploid megaspores.
3 of them degenerate and one divides
to form embryo sac.
- Pollen grains dispersed from anthers are carried by wind or other agencies to the stigma of pistil. It is called pollination.
-
Pollen grains
germinate on the stigma and the resulting pollen tubes
grow through the tissues
of stigma and style
and reach the ovule.
- Pollen tubes enter the embryo-sac where male gametes are discharged. male gamete fuses with egg cell to form zygote (syngamy).
- During these events, the ovules develop into seeds and the ovaries develop into fruit. The seeds are enclosed by fruits.
PLANT LIFE CYCLES AND ALTERNATION OF GENERATIONS
- In plants, both haploid and diploid cells can divide by mitosis. This forms haploid and diploid plant bodies.
- Haploid plant body (gametophyte) produces gametes by mitosis. After fertilization, the zygote also divides by mitosis to produce a diploid plant body (sporophyte). This produces haploid spores by meiosis.
- Spores divide by mitosis to form a haploid plant body. Thus, during the life cycle of any sexually reproducing plant, there is an alternation of generations between gametophyte (n) and sporophyte (2n).
Patterns of Plant life cycles
1. Haplontic: In this, sporophytic generation is represented only by the zygote.
There are no free-living
sporophytes.
Zygote undergoes meiosis to form haploid spores. They divide mitotically to form gametophyte. E.g. Algae
2.Diplontic: In this, diploid sporophyte is the dominant, photosynthetic, independent phase. Gametophytic phase is represented by the single to few-celled haploid gametophyte. E.g. gymnosperms & angiosperms
3.Haplo-diplontic: It is the intermediate condition between haplontic & diplontic. Both gametophyte & sporophyte are multicellular and often free-living. E.g. Bryophytes & Pteridophytes.
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