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. We now use information from many other sources too to help
resolve difficulties in classification. These become more important when there
is no supporting fossil evidence.
Numerical Taxonomy 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
Habit & Habitat: 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).
Size: The size ranges from the
microscopic unicellular forms like Chlamydomonas, to colonial forms like Volvox
and to the filamentous forms like Ulothrix and Spirogyra. A few of the marine
forms, such as kelps, form massive plant bodies.
Reproduction: The algae reproduce by
vegetative, asexual and sexual methods.
Vegetative Reproduction: Vegetative reproduction is by fragmentation. Each fragment
develops into a thallus.
Asexual Reproduction: Asexual reproduction is 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: Sexual reproduction takes
place through fusion of two gametes. The fusion of gametes can be of following
types in algae:
Isogamous Fusion: These gametes can be
flagellated and similar in size (as in Chlamydomonas) or non-flagellated
(non-motile) but similar in size (as in Spirogyra). Such reproduction is called
isogamous.
Anisogamous Fusion: Fusion of two gametes
dissimilar in size, as in some species of Chlamydomonas is termed as
anisogamous.
Oogamous Fusion: Fusion between one large,
non-motile (static) female gamete and a smaller, motile male gamete is termed
oogamous, e.g., Volvox, Fucus.
Economic Importance of Algae: 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. 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), e.g., algin (brown algae) and carrageen (red algae) are used
commercially. 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 and Spirullina are unicellular algae, rich in proteins and
are used as food supplements even by space travellers.
The algae are divided into three main classes:
Chlorophyceae,
Phaeophyceae and
Rhodophyceae.
Chlorophyceae
Characteristics: The members of
chlorophyceae are commonly called 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 pigments are localised in
definite chloroplasts. 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.
Pyrenoids contain protein besides starch. Some algae may store food in the form
of oil droplets. Green algae usually have a rigid cell wall made of an inner
layer of cellulose and an outer layer of pectose.
Reproduction: 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.
Common Examples: Chlamydomonas, Volvox,
Ulothrix, Spirogyra and Chara
Phaeophyceae
Characteristics: The members of
phaeophyceae or brown algae are found primarily in marine habitats. They show
great variation in size and form. They range from simple branched, filamentous
forms (Ectocarpus) to profusely branched forms as represented by kelps, which
may reach a 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, 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 protoplast
contains, in addition to plastids, a centrally located vacuole and nucleus. 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. Vegetative
reproduction takes place by fragmentation.
Reproduction: 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.
Common Examples: Ectocarpus, Dictyota,
Laminaria, Sargassum and Fucus
Rhodophyceae
Characteristics: Rhodophyta are commonly
called red algae because of the predominance of the red pigment,
r-phycoerythrin in their body. Majority of the red algae are marine with
greater concentrations found in the warmer areas. They occur in both
well-lighted regions close to the surface of water and also at great depths in
oceans where relatively little light penetrates. 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.
Reprodcution: 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.
Common Examples: Polysiphonia, Porphyra,
Gracilaria and Gelidium.
BRYOPHYTES
Habits & Habitats: 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.
Characteristics: They play an important
role in plant succession on bare rocks/soil. 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.
Reproduction: 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 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.
Economic Importance of Bryophytes: 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 because of
their capacity to hold water as packing material for trans-shipment of living
material. 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.
The bryophytes are divided into following classes:
1. liverworts and
2. mosses.
Liverworts
Characteristics: The 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. The thallus is dorsiventral and closely appressed
to the substrate. The leafy members have tiny leaf-like appendages in two rows
on the stem-like structures.
Reproduction: Asexual reproduction in
liverworts takes place by fragmentation of thalli, or by the formation of
specialised structures called gemmae (sing. gemma). 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. 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. After meiosis,
spores are produced within the capsule. These spores germinate to form
free-living gametophytes.
Mosses
Characteristics: The predominant stage of
the life cycle of a moss is the gametophyte which consists of two stages. The
first stage is the protonema stage, which develops directly from a spore. It is
a creeping, green, branched and frequently filamentous stage. The second stage
is the 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: Vegetative reproduction in
mosses is by fragmentation and budding in the secondary protonema. In sexual
reproduction, the 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. The sporophyte in mosses is
more elaborate than that in liverworts. The capsule contains spores. Spores are
formed after meiosis. The mosses have an elaborate mechanism of spore
dispersal.
Common Examples: Funaria, Polytrichum and
Sphagnum
PTERIDOPHYTES
Characteristics: The pteridophytes are
found in cool, damp, shady places though some may flourish well in sandy-soil
conditions. 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. The leaves in pteridophyta are small
(microphylls) as in Selaginella or large (macrophylls) as in ferns. 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).
Reproduction: 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. 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. The gametophytes bear male and
female sex organs called antheridia and archegonia, respectively. Water is
required for transfer of antherozoids – the male gametes released from the
antheridia, to the mouth of archegonium. Fusion of male gamete with the egg
present in the archegonium result in the formation of zygote. Zygote thereafter
produces a multicellular well-differentiated sporophyte which is the dominant
phase of the pteridophytes. 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. 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. 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.
The pteridophytes are divided into four classes:
1. Psilopsida(Psilotum);
2. Lycopsida (Selaginella, Lycopodium),
3. Sphenopsida (Equisetum) and
4. Pteropsida (Dryopteris, Pteris, Adiantum).
GYMNOSPERMS
Characteristics: 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. The seeds
that develop post-fertilisation, are not covered, i.e., are naked. Gymnosperms
include medium-sized trees or tall trees and shrubs. One of the gymnosperms,
the giant redwood tree Sequoia is one of the tallest tree species. The roots
are generally tap roots. Roots in some genera have fungal association in the
form of mycorrhiza (Pinus), while in some others (Cycas) small specialized
roots called coralloid roots are associated with N2- fixing cyanobacteria. The
stems are unbranched (Cycas) or branched (Pinus, Cedrus). The leaves may be
simple or compound. In Cycas the pinnate leaves persist for a few years. The
leaves in gymnosperms are well-adapted to withstand extremes of temperature,
humidity and wind. In conifers, the needle-like leaves reduce the surface area.
Their thick cuticle and sunken stomata also help to reduce water loss.
Reproduction: 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. The
strobili bearing microsporophylls and microsporangia are called
microsporangiate or male strobili. 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 takes place within the microsporangia. The cones
bearing megasporophylls with ovules or megasporangia are called
macrosporangiate or female strobili. The male or female cones or strobili may
be borne on the same tree (Pinus) or on different trees (Cycas). 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. One of the megaspores enclosed within the megasporangium (nucellus)
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.
Fertilization: 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. 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.
ANGIOSPERMS
Characters: In the angiosperms or
flowering plants, the pollen grains and ovules are developed in specialized
structures called flowers. In angiosperms, the seeds are enclosed by fruits.
The angiosperms are an exceptionally large group of plants occurring in wide
range of habitats. They range in size from tiny, almost microscopic Wolfia to
tall trees of Eucalyptus (over 100 metres). They provide us with food, fodder,
fuel, medicines and several other commercially important products.
Angiosperms are divided into two classes:
1. Dicotyledons and
2. Monocotyledons
The dicotyledons are characterised by having two cotyledons in
their seeds while the monocolyledons have only one.
Reproduction:
Male Sex Organ: The male sex organ in a
flower is the stamen. Each stamen consists of a slender filament with an anther
at the tip. The anthers, following meiosis, produce pollen grains.
Female Sex Organs: The female sex organs in a
flower is the pistil or the carpel. Pistil consists of an ovary enclosing one
to many ovules. Within ovules are present highly reduced female gametophytes
termed embryosacs. The embryo-sac formation is preceded by meiosis. Hence, each
of the cells of an embryo-sac is haploid. Each embryo-sac has a three-celled
egg apparatus – one egg cell and two synergids, three antipodal cells and two
polar nuclei. The polar nuclei eventually fuse to produce a diploid secondary
nucleus.
Pollination: Pollen grains, after
dispersal from the anthers, are carried by wind or various other agencies to
the stigma of a pistil. This is termed as pollination.
Fertilization: The pollen grains
germinate on the stigma and the resulting pollen tubes grow through the tissues
of stigma and style and reach the ovule. The pollen tubes enter the embryo-sac
where two male gametes are discharged. One of the male gametes fuses with the
egg cell to form a zygote (syngamy). The other male gamete fuses with the
diploid secondary nucleus to produce the triploid primary endosperm nucleus
(PEN). Because of the involvement of two fusions, this event is termed as
double fertilisation, an event unique to angiosperms.
Embryo: The zygote develops into
an embryo (with one or two cotyledons) and the PEN develops into endosperm
which provides nourishment to the developing embryo. The synergids and
antipodals degenerate after fertilisation. During these events the ovules
develop into seeds and the ovaries develop into fruit.
PLANT LIFE CYCLES AND ALTERNATION OF GENERATIONS
However, different plant groups, as well as individuals representing them, differ in the following patterns:
1. Sporophytic generation is represented only by the one-celled zygote. There are no free-living sporophytes. Meiosis in the zygote results in the formation of haploid spores. The haploid spores divide mitotically and form the gametophyte. The dominant, photosynthetic phase in such plants is the free-living gametophyte. This kind of life cycle is termed as haplontic. Many algae such as Volvox, Spirogyra and some species of Chlamydomomas represent this pattern.
2. On the other extreme, is the type wherein the diploid sporophyte is the dominant, photosynthetic, independent phase of the plant. The gametophytic phase is represented by the single to few-celled haploid gametophyte. This kind of lifecycle is termed as diplontic. All seed-bearing plants i.e. gymnosperms and angiosperms, follow this pattern.
3. Bryophytes and pteridophytes, interestingly, exhibit an intermediate condition (Haplo-diplontic); both phases are multicellular and often free-living. However, they differ in their dominant phases.
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