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Systems of Biological classification
- It considers external features and internal features
(ultrastructure, anatomy, embryology & phytochemistry).
1. Artificial classification systems - E.g. Classification for flowering plants given by George
Bentham & Joseph Dalton Hooker.
- Earliest systems of classification.
- They were based on vegetative characters or superficial
morphological characters such as habit, colour, number
and shape of leaves, etc.
3. Phylogenetic classification systems
- It is based on evolutionary relationships among organisms.
- This assumes that organisms in the same taxa have a
common ancestor.
- Linnaeus’s artificial system of classification was based
on the androecium structure.
Other sources to resolve the problems in classification:
Drawbacks:
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.
- 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.
o Cytotaxonomy: It is based on cytological information like
chromosome number, structure, behaviour etc.
2. Natural classification systems
- These are based on natural affinities among organisms. o^ Chemotaxonomy:^ It uses chemical constituents of^ plants.
- Algae are simple, thalloid, autotrophic, chlorophyll-
bearing and aquatic (fresh water & marine) organisms.
☺ Some marine brown & red algae produce hydrocolloids
(water holding substances). E.g. algin (brown algae) and
- They also occur in moist stones, soils and wood. carrageen (red algae). These are used commercially.
- Some occur in association with fungi (lichen) and animals
(e.g., on sloth bear).
☺ Protein-rich unicellular algae like Chlorella & Spirullina
are used as food supplements by space travellers.
- The form and size of algae is highly variable. Algae include 3 classes: Chlorophyceae, Phaeophyceae
o Microscopic unicellular forms: E.g. Chlamydomonas. and Rhodophyceae.
o Colonial forms: E.g. Volvox.
1. Chlorophyceae (green algae)
o Filamentous forms: E.g. Ulothrix and Spirogyra.
- Unicellular, colonial or filamentous. Reproduction:
- They are usually grass green due to the pigments
- Vegetative reproduction: By fragmentation. Each
fragment develops into a thallus.
chlorophyll a and b in chloroplasts.
- The chloroplasts may be discoid, plate-like, reticulate, cup-
shaped, spiral or ribbon-shaped in different species.
- Asexual reproduction: By the production of spores. E.g.
zoospores (most common). They are flagellated (motile)
and on germination gives rise to new plants.
- Most of them have one or more pyrenoids (storage bodies)
located in the chloroplasts. Pyrenoids contain protein
besides starch.
- Sexual reproduction: Through fusion of two gametes. It
is many types:
- Some algae store food as oil droplets. o Isogamous: Fusion of gametes similar in size. They
may be flagellated (e.g. Ulothrix ) or non-flagellated
(non-motile, e.g. Spirogyra ).
- They have a rigid cell wall made of an inner layer of
cellulose and an outer layer of pectose.
o Anisogamous: Fusion of two gametes dissimilar in size.^ -^ E.g.^ Chlamydomonas, Volvox, Ulothrix, Spirogyra^ &^ Chara.
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 CO 2 on
earth and increase the level of dissolved oxygen.
☺ They are primary producers and the basis of the food
cycles of all aquatic animals.
Reproduction:
o Vegetative reproduction: By fragmentation or by
formation of different types of spores.
☺ Many marine algae (70 species) are used as food. E.g.
Porphyra, Laminaria and Sargassum. o Asexual reproduction: By flagellated zoospores
produced in zoosporangia.
☺ Agar (from Gelidium & Gracilaria ) is used to grow
microbes and in ice-creams and jellies. o Sexual reproduction: Isogamous, anisogamous or
oogamous.
ALGAE
BRYOPHYTES
2. Phaeophyceae (brown algae)
o Sexual reproduction: Isogamous, anisogamous or
oogamous. Union of gametes occurs in water or within the
oogonium (oogamous species). Gametes are pear-shaped
(pyriform) bearing 2 laterally attached flagella.
- They are mostly marine forms.
- They show great variation in size & form. They range from
simple branched, filamentous forms (E.g. Ectocarpus ) to
profusely branched forms (e.g. kelps- 100 m in height). 3. Rhodophyceae (red^ algae)
- They have a red pigment, r-phycoerythrin.
- 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.
- Majority are marine especially in the warmer areas.
- They occur in both well-lighted regions close to the surface
of water and at great depths in oceans where relatively little
light penetrates.
- Food is stored as complex carbohydrates (laminarin or
mannitol).
- The vegetative cells have a cellulosic wall covered by a
gelatinous coating of algin.
- The red thalli of most of the red algae are multicellular.
- Some of them have complex body organisation.
- Protoplast contains plastids, central vacuole and nucleus.
- The food is stored as floridean starch which is very
similar to amylopectin and glycogen in structure.
- Plant body is attached to substratum by a holdfast , and has
a stalk (stipe) and leaf like photosynthetic organ (frond).
- E.g. Polysiphonia, Porphyra , Gracilaria and Gelidium.
- E.g. Ectocarpus, Dictyota, Laminaria, Sargassum & Fucus.
Reproduction:
Reproduction: o Vegetative reproduction: By fragmentation.
o Vegetative reproduction: By fragmentation. (^) o Asexual reproduction: By non-motile spores.
o Asexual reproduction: By pear-shaped biflagellate
zoospores (have 2 unequal laterally attached flagella).
o Sexual reproduction: Oogamous. By non-motile
gametes. It has complex post fertilisation developments.
Classes Chlorophyceae (Green algae) Phaeophyceae (brown algae) Rhodophyceae (Red algae)
Major pigments Chlorophyll a, b Chlorophyll a, c, Fucoxanthin Chlorophyll a, d, Phycoerythrin
Stored food Starch Mannitol, laminarin Floridean Starch
Cell wall Cellulose Cellulose and algin Cellulose
Flagellar number &
position of insertion
2 - 8, equal, apical 2, unequal, lateral Absent
Habitat
Fresh water, salt water &
brackish water
Fresh water (rare), salt water &
brackish water
Fresh water (some), salt water
(most) & brackish water
- They are called amphibians of the plant kingdom because
they can live in soil but need water for sexual reproduction.
- Zygotes do not undergo meiosis immediately. They
produce a multicellular body called a sporophyte.
- They occur in damp, humid and shaded localities. - 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.
- Their body 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.
Importance of Bryophytes:
- They lack true roots, stem or leaves. They may possess
root-like, leaf-like or stem-like structures.
☺ Some mosses provide food for herbaceous mammals,
birds and other animals.
- The main plant body is haploid. It produces gametes, hence
is called a gametophyte.
☺ 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.
- The sex organs in bryophytes are multicellular. - The male sex organ (antheridium) produces biflagellate
antherozoids. The female sex organ (archegonium) is
flask-shaped and produces a single egg.
☺ 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.
- Antherozoids are released to water and meet archegonium.
An antherozoid fuses with the egg to form zygote.
GYMNOSPERMS
female gametophytes are retained on the parent
sporophytes for variable periods.
- Within female gametophytes, zygotes develop into young
embryos. This event is a precursor to the seed habit. It is
considered as an important step in evolution.
- The pteridophytes have 4 classes: 1. Psilopsida: E.g. Psilotum 2. Lycopsida: E.g. Selaginella, Lycopodium 3. Sphenopsida: E.g. Equisetum 4. Pteropsida: E.g. Dryopteris, Pteris, Adiantum
microspores develop into male gametophytes. It is
highly reduced and confined to only a limited number of
cells. This gametophyte is called a pollen grain. The
pollen grains are developed within the microsporangia.
- 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 that
develop post-fertilization are not covered (naked).
- They include medium-sized trees or tall trees and shrubs. o Megasporophylls: They are arranged to female strobili
(macrosporangiate). They bear megasporangia (ovules).
Megasporangium mainly consists of a body called
nucellus. It is protected by envelopes. 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.
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 N 2 - fixing cyanobacteria.
- Stems are unbranched ( Cycas ) or branched ( Pinus, Cedrus ). - Leaves are simple or compound. They are well-adapted to
withstand extreme temperature, humidity and wind.
- 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.
- The male or female cones may be borne on the same tree
( Pinus ) or on different trees ( Cycas ).
- Unlike bryophytes and pteridophytes, 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.
REPRODUCTION:
- Gymnosperms are heterosporous. They produce haploid
microspores and megaspores.
- 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.
- Some leaves are modified into sporophylls. They are
compactly and spirally arranged along an axis to form lax
or strobili or cones.
- Sporophylls bear sporangia in which spores are produced. - Sporophylls are 2 types: - After fertilization, zygote develops into an embryo and the
ovules into seeds.
o Microsporophylls: They are arranged to male strobili
(microsporangiate). They bear microsporangia. The
- They are an exceptionally large group of plants. - Male sex organ in a flower is the stamen. Each stamen
consists of a filament with an anther at the tip. Within the
anthers, the pollen mother cell divides by meiosis to
produce microspores which matures into pollen grains.
- They range in size from tiny, almost microscopic Wolffia
to tall trees of Eucalyptus (over 100 metres).
- They include 2 classes: Dicotyledons & Monocotyledons. - Female sex organ in a flower 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.
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). -^ Each embryo-sac has a 3-celled^ egg apparatus^ (one^ egg
cell & two synergids) , 3 antipodal cells & 2 polar nuclei.
The polar nuclei eventually fuse to produce a diploid
secondary nucleus.
REPRODUCTION:
- Flower is the reproductive structure.
ANGIOSPERMS (FLOWERING PLANTS)
- Pollen grains dispersed from anthers are carried by wind or
other agencies to the stigma of pistil. It is called pollination.
- The seeds are enclosed by fruits. - 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 2 male gametes
are discharged. One male gamete fuses with egg cell to
form zygote (syngamy). The other male gamete fuses with
diploid secondary nucleus to produce triploid primary
endosperm nucleus (PEN). Because of the involvement
of two fusions, this event is called double fertilisation. It
is an event unique to angiosperms.
- The zygote develops into an embryo (with one or two
cotyledons). The PEN develops into endosperm which
provides nourishment to the developing embryo.
- Synergids & antipodals degenerate after fertilization. - During these events, the ovules develop into seeds and the
ovaries develop into fruit.
- In plants, both haploid and diploid cells can divide by
mitosis. This forms haploid and diploid plant bodies.
Zygote undergoes meiosis to form haploid spores. They
divide mitotically to form gametophyte. The dominant,
photosynthetic phase is the free-living gametophyte. E.g.
Algae such as Volvox, Spirogyra and some species of
Chlamydomonas.
- 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.
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. An alga, Fucus sp., all seed-bearing
plants (gymnosperms & angiosperms - the gametophytic
phase is few to multi-celled).
- 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).
3. Haplo-diplontic: It is the intermediate condition between
haplontic & diplontic. Both gametophyte & sporophyte are
multicellular and often free-living. But they have different
dominant phases. E.g. Bryophytes & Pteridophytes.
Patterns of Plant life cycles
1. Haplontic: In this, sporophytic generation is represented
only by the zygote. There are no free-living sporophytes.
PLANT LIFE CYCLES AND ALTERNATION OF GENERATIONS
