Plant KingdomClass 11 Biology NCERT Solutions
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Q1Exercises
What is the basis of classification of algae?
Solution
The primary basis for the classification of algae into its three main classes—Chlorophyceae, Phaeophyceae, and Rhodophyceae—is the type of pigments they possess and the type of stored food material.
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Pigments: The presence of specific photosynthetic pigments gives each class its characteristic colour.
- Chlorophyceae (Green algae): Dominance of Chlorophyll a and b.
- Phaeophyceae (Brown algae): Presence of Chlorophyll a, c, and fucoxanthin.
- Rhodophyceae (Red algae): Presence of Chlorophyll a, d, and phycoerythrin.
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Stored Food: The form in which they store their food is also a key distinguishing feature.
- Chlorophyceae: Store food as starch.
- Phaeophyceae: Store food as complex carbohydrates like mannitol or laminarin.
- Rhodophyceae: Store food as floridean starch.
Other characteristics like cell wall composition and the number and position of flagella are also used in their classification (as shown in Table 3.1 of the chapter).
Q2Exercises
When and where does reduction division take place in the life cycle of a liverwort, a moss, a fern, a gymnosperm and an angiosperm?
Solution
Reduction division (meiosis) is the process that produces haploid cells from diploid cells. In the life cycle of plants, it typically occurs to produce spores from a diploid sporophyte.
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Liverwort: Meiosis takes place inside the capsule of the sporophyte. The diploid spore mother cells undergo reduction division to form haploid spores.
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Moss: Similar to liverworts, meiosis occurs inside the capsule of the sporophyte. Diploid spore mother cells divide by meiosis to produce haploid spores.
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Fern (Pteridophyte): Meiosis occurs in the sporangia, which are borne on the sporophylls of the diploid sporophyte. Spore mother cells within the sporangia undergo reduction division to form haploid spores.
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Gymnosperm: Meiosis occurs at two locations within the diploid sporophyte:
- In the microsporangia (within male cones), where diploid microspore mother cells divide to form haploid microspores (pollen grains).
- In the megasporangia (ovules within female cones), where the diploid megaspore mother cell divides to form four haploid megaspores.
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Angiosperm: Meiosis also occurs at two locations within the flower of the diploid sporophyte:
- In the microsporangia (pollen sacs of the anther), where diploid microspore mother cells divide to form haploid microspores.
- In the megasporangium (ovule), where the diploid megaspore mother cell divides to form the haploid megaspore.
Q3Exercises
Name three groups of plants that bear archegonia. Briefly describe the life cycle of any one of them.
Solution
The three groups of plants that bear archegonia (the female sex organ) are:
- Bryophytes
- Pteridophytes
- Gymnosperms
Life Cycle of a Moss (a Bryophyte):
The life cycle of a moss shows alternation of generations between a dominant haploid gametophyte and a dependent diploid sporophyte.
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Gametophyte Generation (Haploid, n):
- The life cycle begins with the germination of a haploid spore.
- The spore develops into a juvenile filamentous stage called the protonema.
- A lateral bud from the protonema develops into the mature, upright, leafy gametophyte, which is the main plant body.
- This leafy stage bears the multicellular sex organs: the male antheridia and the female archegonia at its apex.
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Fertilisation:
- The antheridium produces biflagellate antherozoids (male gametes), and the flask-shaped archegonium produces a single egg.
- Antherozoids are released into water and swim to the archegonium, where one fuses with the egg.
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Sporophyte Generation (Diploid, 2n):
- Fertilisation results in the formation of a diploid zygote.
- The zygote develops into a multicellular sporophyte while still attached to the gametophyte, from which it derives nourishment.
- The sporophyte is differentiated into a foot, seta, and capsule.
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Spore Formation:
- Within the capsule, diploid spore mother cells undergo meiosis (reduction division) to produce numerous haploid spores.
- When mature, the capsule releases the spores, which are then dispersed by wind. Each spore can germinate into a new protonema, starting the cycle again.
Q4Exercises
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.
Solution
The ploidy of the given structures is as follows:
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Protonemal cell of a moss: Haploid (n), as it develops directly from the germination of a haploid spore.
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Primary endosperm nucleus in dicot: Triploid (3n), as it is formed by the fusion of one male gamete (n) with the central cell containing two polar nuclei (n + n).
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Leaf cell of a moss: Haploid (n), as the leafy stage is part of the haploid gametophyte, which is the main plant body.
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Prothallus cell of a fern: Haploid (n), because the prothallus is the gametophyte that develops from a haploid spore.
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Gemma cell in Marchantia: Haploid (n), as gemmae are asexual buds produced on the haploid gametophytic thallus.
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Meristem cell of monocot: Diploid (2n), as the main plant body of a monocot (an angiosperm) is the diploid sporophyte.
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Ovum of a liverwort: Haploid (n), as it is the female gamete produced within the archegonium on the haploid gametophyte.
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Zygote of a fern: Diploid (2n), as it is formed by the fusion of a haploid male gamete (antherozoid) and a haploid female gamete (egg).
Q5Exercises
Write a note on economic importance of algae and gymnosperms.
Solution
Economic Importance of Algae
Algae are of great economic importance to humans in various ways:
- Carbon Dioxide Fixation: Algae perform about half of the total carbon dioxide fixation on Earth through photosynthesis, playing a crucial role in the carbon cycle and producing oxygen.
- Primary Producers: They are the primary producers of energy-rich compounds in aquatic ecosystems, forming the base of the food cycles for all aquatic animals.
- Food Source: Many species of marine algae are consumed as food, such as Porphyra, Laminaria, and Sargassum.
- Hydrocolloids: Certain marine brown and red algae produce hydrocolloids (water-holding substances). Algin, from brown algae, and carrageen, from red algae, are used commercially as thickeners and emulsifiers.
- Agar: This commercial product is obtained from red algae like Gelidium and Gracilaria. It is used as a culture medium for microbes in laboratories and in the preparation of jellies and ice creams.
- Food Supplements: Unicellular algae like Chlorella are rich in proteins and are used as food supplements, even by space travellers.
Economic Importance of Gymnosperms
Gymnosperms are also economically important, primarily as a source of wood and other products:
- Timber: Many gymnosperms like Pinus (pine), Cedrus (cedar), and Sequoia are sources of softwood, which is used extensively in construction, making furniture, plywood, and packing cases.
- Paper Production: The wood of many conifers is used to manufacture paper.
- Resins: Conifers like pines yield resins, which are commercially used to manufacture turpentine and other products.
- Ornamental Plants: Many species of gymnosperms, such as Cycas, Pinus, and Ginkgo, are grown as ornamental plants in gardens and parks due to their evergreen nature and aesthetic appeal.
- Food Source: The seeds of some gymnosperms, like those of Pinus gerardiana (chilgoza), are edible.
Q6Exercises
Both gymnosperms and angiosperms bear seeds, then why are they classified separately?
Solution
Although both gymnosperms and angiosperms are seed-bearing plants (spermatophytes), they are classified separately due to fundamental differences in how their ovules and seeds are borne. The primary reasons for their separate classification are:
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Ovary and Fruit Formation:
- Gymnosperms: The term 'gymnosperm' means 'naked seed' (gymnos - naked, sperma - seed). In these plants, the ovules are not enclosed within an ovary wall. They are exposed on the surface of megasporophylls. After fertilisation, the resulting seeds remain naked and are not enclosed within a fruit.
- Angiosperms: The term 'angiosperm' means 'enclosed seed' (angios - vessel, sperma - seed). In these plants, the ovules are enclosed within a hollow ovary, which is part of a specialised structure called a flower. After fertilisation, the ovary develops into a fruit, and the ovules develop into seeds, which remain enclosed within the fruit.
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Presence of Flowers:
- Gymnosperms: They do not produce true flowers. Their reproductive structures are arranged in the form of cones or strobili.
- Angiosperms: They are also known as 'flowering plants' because they possess true flowers, which are highly specialised reproductive structures containing the ovary, stamens, petals, etc.
Q7Exercises
What is heterospory? Briefly comment on its significance. Give two examples.
Solution
Heterospory is the phenomenon of producing two different kinds of spores by a plant: smaller microspores and larger megaspores.
- Microspores germinate to give rise to the male gametophyte.
- Megaspores germinate to give rise to the female gametophyte.
Significance of Heterospory:
Heterospory is considered a crucial step in evolution as it is a precursor to the seed habit. The development of two types of spores leads to the following significant events:
- Differentiation of Gametophytes: It leads to the development of distinct male and female gametophytes.
- Retention of Female Gametophyte: In heterosporous plants, the female gametophyte is retained on the parent sporophyte for a considerable period. This provides better protection and nourishment to the developing female gametophyte and the subsequent embryo.
- In-situ Embryo Development: The development of the zygote into a young embryo takes place within the female gametophyte, which is still attached to the parent sporophyte. This ensures a more reliable and protected development for the next generation.
These events paved the way for the evolution of the seed, a protected structure containing the embryo, which is a hallmark of higher plants like gymnosperms and angiosperms.
Examples:
Two examples of heterosporous pteridophytes mentioned in the chapter are:
- Selaginella
- Salvinia
Q8Exercises
Explain briefly the following terms with suitable examples:-
(i)
protonema
(ii)
antheridium
(iii)
archegonium
(iv)
diplontic
(v)
sporophyll
(vi)
isogamy
Solution
(i)
Protonema: It is the first, juvenile stage in the development of the gametophyte of mosses. It develops directly from a germinating spore and is a creeping, green, branched, and often filamentous structure. The mature leafy gametophyte arises from a lateral bud on the protonema.
* Example: Found in mosses like Funaria and Sphagnum.
(ii)
Antheridium: It is the multicellular male sex organ found in bryophytes and pteridophytes. It is typically a globular or club-shaped structure that produces the male gametes, called antherozoids.
* Example: Found in bryophytes like Marchantia and mosses like Funaria, and in pteridophytes like ferns.
(iii)
Archegonium: It is the multicellular, flask-shaped female sex organ found in bryophytes, pteridophytes, and gymnosperms. It consists of a swollen base called a venter, which contains the single egg cell, and an elongated neck.
* Example: Found in bryophytes like Marchantia, pteridophytes like Selaginella, and gymnosperms like Cycas.
(iv)
Diplontic: This term describes a life cycle in which the diploid (2n) sporophyte is the dominant, photosynthetic, and independent phase of the plant. The gametophytic phase is highly reduced and is represented only by the haploid gametes.
* Example: All seed-bearing plants, i.e., gymnosperms and angiosperms, exhibit a diplontic life cycle. The alga Fucus is also an example.
(v)
Sporophyll: It is a modified, leaf-like appendage that bears sporangia. In some plants, sporophylls may be aggregated to form compact structures called strobili or cones.
* Example: In pteridophytes like Selaginella and Equisetum, sporophylls form cones. In gymnosperms like Pinus, microsporophylls and megasporophylls form the male and female cones, respectively.
(vi)
Isogamy: It is a type of sexual reproduction where the fusing gametes are morphologically similar; they cannot be distinguished as male and female. These gametes can be flagellated (motile) or non-flagellated (non-motile) but are of the same size.
* Example: Ulothrix produces similar-sized flagellated gametes, while Spirogyra produces similar-sized non-flagellated gametes.
Q9Exercises
Differentiate between the following:-
(i)
red algae and brown algae
(ii)
liverworts and moss
(iii)
homosporous and heterosporous pteridophyte
Solution
(i)
Red algae vs. Brown algae
| Feature | Red Algae (Rhodophyceae) | Brown Algae (Phaeophyceae) |
|---|---|---|
| Major Pigments | Chlorophyll a, d, and r-phycoerythrin (red pigment). | Chlorophyll a, c, and fucoxanthin (brown pigment). |
| Stored Food | Floridean starch. | Mannitol and laminarin. |
| Cell Wall | Cellulose, pectin, and polysulphate esters. | Cellulose and an outer gelatinous coating of algin. |
| Flagella | Absent throughout the life cycle. | Two unequal, laterally attached flagella in motile cells (zoospores and gametes). |
| Habitat | Mostly marine, abundant in warmer areas, found at great depths. | Primarily marine, found in cooler coastal waters. |
(ii)
Liverworts vs. Moss
| Feature | Liverworts | Mosses |
|---|---|---|
| Plant Body | The plant body is a thalloid (e.g., Marchantia), which is dorsiventrally flattened. | The plant body is differentiated into an upright, slender axis bearing spirally arranged leaves. |
| Gametophyte Stages | Spore germinates directly into the thallus. No protonema stage. | Spore germinates to form a filamentous protonema, from which the leafy stage develops. |
| Rhizoids | Unicellular and unbranched. | Multicellular and branched. |
| Sporophyte | Simpler, differentiated into foot, seta, and capsule. | More elaborate and differentiated into foot, seta, and capsule, with a complex mechanism for spore dispersal. |
(iii)
Homosporous vs. Heterosporous pteridophyte
| Feature | Homosporous Pteridophyte | Heterosporous Pteridophyte |
|---|---|---|
| Type of Spores | Produces only one type of spore, which are all of similar size. | Produces two distinct types of spores: small microspores and large megaspores. |
| Gametophyte | The spores germinate to form a single type of gametophyte, which is typically bisexual (monoecious). | Microspores develop into male gametophytes, and megaspores develop into female gametophytes (dioecious). |
| Significance | Represents a more primitive condition. | Considered an advanced feature and a precursor to the seed habit. |
| Examples | Majority of ferns like Dryopteris, Pteris. | Selaginella, Salvinia. |
Q10Exercises
Match the following (column I with column II) Column I (a) Chlamydomonas (b) Cycas (c) Selaginella (d) Sphagnum Column II
(i)
Moss
(ii)
Pteridophyte
(iii)
Algae
(iv)
Gymnosperm
Solution
The correct matches are as follows:
- (a) Chlamydomonas matches with (iii) Algae. (Chlamydomonas is a unicellular green alga).
- (b) Cycas matches with (iv) Gymnosperm. (Cycas is a genus of gymnosperms).
- (c) Selaginella matches with (ii) Pteridophyte. (Selaginella is a genus of pteridophytes).
- (d) Sphagnum matches with (i) Moss. (Sphagnum is a genus of moss, also known as peat moss).
Final Answer:
(a) - (iii)
(b) - (iv)
(c) - (ii)
(d) - (i)
Q11Exercises
Describe the important characteristics of gymnosperms.
Solution
The important characteristics of gymnosperms are as follows:
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Naked Seeds: The most defining feature is that their ovules are not enclosed by any ovary wall and remain exposed, both before and after fertilisation. Consequently, the seeds that develop are not enclosed within a fruit and are thus 'naked'.
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Plant Body: The main plant body is the diploid sporophyte, which is differentiated into true roots, stem, and leaves. They are typically medium-sized to tall trees or shrubs. The giant redwood tree Sequoia is one of the tallest tree species.
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Root System: They generally have a tap root system. Some genera show symbiotic associations, such as mycorrhiza in Pinus and nitrogen-fixing coralloid roots in Cycas.
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Leaves: The leaves are well-adapted to withstand extreme environmental conditions. They may be simple or compound. In conifers, needle-like leaves with a thick cuticle and sunken stomata help reduce water loss.
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Heterospory: All gymnosperms are heterosporous, producing haploid microspores and megaspores.
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Reproductive Structures: The reproductive structures are organised into compact strobili or cones. There are male cones (microsporangiate) which bear microsporophylls, and female cones (macrosporangiate) which bear megasporophylls.
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Gametophytes: The male and female gametophytes are highly reduced and do not have an independent, free-living existence. They are dependent on and retained within the sporangia on the parent sporophyte.
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Pollination and Fertilisation: Pollination occurs through wind (anemophily). The pollen grain germinates to form a pollen tube that carries the male gametes to the archegonium in the ovule. Water is not required for fertilisation.