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CLASS XI BIOLOGY
Plant Kingdom
1. What is the basis of classification of algae?
Answer: The main basis of classification of algae has been done on the basis of
presence or absence of pigments, which impart an algae its colour. Chlorophyceae
contains chlorophyll a and b, giving it the green colour and the name ‘blue-green
algae’. Phaeophyceae contains chlorophyll a and c and fucoxanthin. The fucoxanthin
gives it the brown colour and hence the name ‘brown algae’. Rhodophyceae contains
chlorophyll a and d and phycoerythrin. The phycoerythrin gives the disitinct red
colour and hence the name ‘red algae’.
2. When and where does reduction division take place in the life cycle of a
liverwort, a moss, a fern, a gymnosperm and an angiosperm?
Answer: Reduction division or meiosis takes place in gametic cells. The role of
meiosis is to halve the number of chromosomes so that once fertilization takes place
the embryo thus formed will be having similar number of chromosomes as in parent
somatic cells. In plants sometimes the dominant phase will contain all of diploid
stage or all of haploid stage. There can be a brief interspersal of other stage. This is
known as the alteration of generation.
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. e.g., algae
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. Meiosis results in formation of male and female gametes.
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. The meiosis takes place in the sporophytes to
produce haploid spores.
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3. Name three groups of plants that bear archegonia. Briefly describe the
life cycle of any one of them.
Answer: Bryophytes, Pteridophytes and Gymnosperms bear distinct archegonia. In
angiosperms the archegonia is reduced to just a few cells so the term is not used in
case of angiosperms.
Life Cycle of Gymnosperms:
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.
Male Gamete: 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.
Female Gamete: 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
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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: 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.
4. Mention the ploidy of the following: protonemal cell of a moss; primary
endosperm nucleus in dicot, leaf cell of a moss; prothallus cell of a ferm;
gemma cell in Marchantia; meristem cell of monocot, ovum of a liverwort,
and zygote of a fern.
Answer:
(a) Protonemal cell of a moss develop from spores and are haploid.
(b) Primary endosperm nucleus is formed after fertilization and is diploid.
(c) Leaf cell of a moss is haploid as it develops from protonemal cells.
(d) Prothallus of a fern is haploid and bears male and female sex organs and is called
gametophyte. This upon fertilization starts the diploid stage.
(e) Gemma cells in Marchantia are meant for asexual reproduction and are diploid.
(f) Meristem cell of a monocot is somatic cell and is diploid.
(g) Ovum and zygote will always be diploid.
5. Write a note on economic importance of algae and gymnosperms.
Answer:
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.
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Economic Importance of Gymnosperms:
Gymnospermous plants are widely used as ornamentals. Conifers are often featured
in formal gardens and are used for bonsai. Yews and junipers are often low-growing
plants cultivated for ground cover. Conifers are effective windbreaks, especially those
that are evergreen.
Most plywood is gymnospermous. Fibres of conifers make up paper pulp and may
occasionally be used for creating artificial silk or other textiles. Conifers are
frequently planted in reforestation projects. Conifer bark is often the source of
compounds involved in the leather tanning industry. Bark is also used extensively as
garden mulch.
From conifer resins are derived turpentine and rosin. A hardened form of resin from
a kauri (Agathis australis), called copal, is used in the manufacture of paints and
varnishes. Some resins, such as balsam (from hemlock) and dammar (from Agathis)
are used in the preparation of mounting media for microscope slides. Resins may
also have medicinal uses. Many types of amber are derived from fossilized resin of
conifers. Commercially useful oils are derived from such conifers as junipers, pines,
hemlock, fir, spruces, and aborvitae. These oils serve as air fresheners, disinfectants,
and scents in soaps and cosmetics. Seeds are often food sources. Pine seeds are a
delicacy eaten plain or used as a garnish on bakery products. Seeds of Ginkgo and
cycads may be poisonous unless detoxified. "Berries" (in reality the fleshy cones) of
juniper are used to flavour gin.
6. Both gymnosperms and angiosperms bear seeds, then why are they
classified separately?
Answer: The seeds of gymnosperms are naked, while that of angiosperms are
covered by a membrane. That is why they are classified separately.
7. What is heterospory? Briefly comment on its significance. Give two
examples.
Answer: Certain pteridophytes produce two kinds of spores. This phenomenon is
called heterospory. The smaller one are called microspore and the bigger ones are
called megaspore. Microspore produce male gametophytes and megaspores produces
female gametophyte. In fact heterospory is the crucial step in evolution. This
ultimately led to seed development in gymnosperms and angiosperms.
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8. Explain briefly the following terms with suitable examples:(i) protonema
(ii) antheridium
(iii) archegonium
(iv) diplontic
(v) sporophyll
(vi) isogamy
Answer: (i) Protonema: A protonema is a thread-like chain of cells that forms the
earliest stage (the haploid phase) of a bryophyte life cycle. When a moss or liverwort
first grows from the spore, it grows as a protonema which develops into a leafy
gametophore.
Moss spores germinate to form an alga-like filamentous structure called the
protonema. It represents the juvenile gametophyte. While the protonema is growing
by apical cell division, at some stage, under the influence of the phytohormone
cytokinin, buds are induced which grow by three-faced apical cells. These give raise
to gametophores, stems and leaf like structures (bryophytes do not have true leaves
(megaphyll)). These gametophores are the adult form of the gametophyte.
Protonema are characteristic of all mosses and some liverworts but are absent from
hornworts. The protonema is also the photosynthetic part of a germinating fern
spore.
(ii) Antheridium: An antheridium (plural: antheridia) is a haploid structure or organ
producing and containing male gametes (called antherozoids or sperm). It is present
in the gametophyte phase of lower plants like mosses and ferns, and also in the
primitive vascular psilotophytes. Many algae and some fungi, for example
ascomycetes and water moulds, also have antheridia during their reproductive
stages.
An antheridium typically consists of sterile cells and spermatogenous tissue. The
sterile cells may form a central support structure or surround the spermatogenous
tissue as a protective jacket. The spermatogenous cells give rise to spermatids via
mitotic cell division. In bryophytes, the antheridium is borne on an antheridiophore,
a stalk-like structure that carries the antheridium at its top.
(iii) Archegonium: An archegonium is a multicellular structure or organ of the
gametophyte phase of certain plants producing and containing the ovum or female
gamete. The archegonium has a long neck and a swollen base. Archegonia are
typically located on the surface of the plant thallus, although in the horned liverworts
they are embedded.
(iv) Diplontic: The major part of the life cycle is composed of gametophytic satge
and gametophytes produce haploid male and female gametes. This happens in all
the higher plants and animals. The life cycle is called diplontic life cycle.
(v) Sporophylls: Sporophyll is a leaf that bears sporangia. Both microphylls and
megaphylls can be sporophylls. In heterosporous plants, sporophylls (whether they
are microphylls or megaphylls) bear either megasporangia (and thus are called
megasporophylls), or microsporangia (microsporophylls). The overlap of the prefixes
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and roots makes
nomenclature.
these
terms
a
particularly
confusing
subset
of botanical
Sporophylls vary greatly in appearance and structure, and may or may not look
similar to sterile leaves.
(vi) Isogamy: Isogamy refers to a form of sexual reproduction involving gametes of
similar morphology, differing only in allele expression in one or more mating-type
regions. Since both gametes look alike, they cannot be classified as "male" or
"female." Instead, organisms undergoing isogamy are said to have different mating
types, most commonly noted as "+" and "-" strains. Fertilization occurs when "+"
and "-" gametes fuse to form a zygote.
In many cases, isogamous fertilization is used by organisms that can also reproduce
asexually through binary fission, budding, or asexual spore formation. The switch to
sexual reproduction mode is often triggered by a change from favorable to
unfavorable growing conditions. Fertilization frequently leads to the formation of a
thick-walled zygotic resting spore that can withstand harsh environments and will
germinate once growing conditions turn favorable again.
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