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OBJECTIVE QUESTIONS
A. Gridling leaf traces are the characteristic feature of the stem of –
(i) Cycas
B. Pollination of Pinus is –
(i) Anemophilous
C. In gymnosperms, the archegonia lack –
(i) Neck Canal Cells
D. Which of the following is the living fossil?
(ii) Cycas
E. In Cycas, pollen grains are shed at the –
(iii) 2-celled stage
SUBJECTIVE QUESTIONS
Answer any two questions of following:-
2. Give an account of the gametophyte of Anthoceros
The gametophyte of Anthoceros plays a crucial role in its reproductive cycle and represents the dominant, photosynthetic phase.
1. Structure of the Gametophyte
- Thallus:
- The gametophyte is thalloid (flat, lobed, and irregularly branched).
- It grows close to the ground on moist soil, rocks, or tree barks, forming a green mat-like structure.
2. Rhizoids and Absorption
- Rhizoids:
- Unicellular structures on the lower surface anchor the thallus to the substrate and help absorb water.
- Mucilage Pores:
- Openings present in the thallus release mucilage, which keeps the thallus moist and aids in the entry of nitrogen-fixing cyanobacteria.
3. Symbiosis with Nostoc
- Nitrogen fixation:
Mucilage-filled cavities house colonies of cyanobacteria (Nostoc), which fix atmospheric nitrogen. This provides essential nutrients to the plant.
4. Photosynthetic Cells
- Each cell contains a large chloroplast with a pyrenoid. This makes Anthoceros unique among bryophytes, as pyrenoids are more common in algae.
- These cells carry out photosynthesis, providing the energy needed for growth and reproduction.
5. Reproduction in the Gametophyte
- Antheridia (male gametangia):
- Embedded in small chambers on the upper surface of the thallus.
- Each antheridium produces biflagellate sperm cells, which require water for movement.
- Archegonia (female gametangia):
- These also develop inside cavities. Each archegonium contains a single egg.
6. Fertilization and Sporophyte Development
- Fertilization:
Sperm swims to the egg in the archegonium through water, forming a zygote. - The sporophyte remains attached to the gametophyte throughout its life and depends on it for nutrients.
3. T.S. of Anthoceros Thallus (Cellular)
To draw the T.S. (transverse section) of Anthoceros thallus, include the following components:
1. Epidermis:
- The outermost single-layered structure covering the thallus.
- It is protected by a thin cuticle to prevent water loss.
2. Photosynthetic Zone:
- Just below the epidermis is a layer of chloroplast-rich cells that perform photosynthesis.
- The chloroplasts contain pyrenoids to enhance CO₂ fixation.
3. Mucilage Cavities:
- These irregular cavities are distributed inside the thallus and contain mucilage and Nostoc colonies.
- They play a role in moisture retention and nitrogen fixation.
4. Storage Cells:
- The inner part of the thallus contains parenchyma cells for storage of nutrients and water.
5. Rhizoids:
- Unicellular rhizoids are located on the lower surface and assist with anchorage and absorption.
4. What is Heterospory?
Definition
Heterospory is the production of two distinct types of spores—microspores and megaspores—that develop into male and female gametophytes, respectively.
Types of Spores:
- Microspores:
- Small in size.
- Develop into male gametophytes that produce sperm cells.
- Megaspores:
- Larger in size.
- Develop into female gametophytes that produce egg cells.
Examples of Heterosporous Plants:
- Some pteridophytes:
- Selaginella
- Marsilea
- All gymnosperms and angiosperms are heterosporous.
Evolutionary Significance of Heterospory:
- Seed Formation:
- Heterospory is considered a major evolutionary step toward the formation of seeds.
- The female gametophyte is retained within the megaspore, which later develops into a seed after fertilization.
- Protection and Nutrition:
- The female gametophyte develops inside the ovule, which protects it from environmental stresses and provides nutrition.
- Efficient Reproduction:
- The separation of sexes in spores ensures better genetic diversity and specialization.
5. Describe the Pinus Cone
Pinus produces two types of cones: male cones and female cones, which serve as the reproductive structures.
1. Male Cone (Microsporangiate Cone):
- Location: Found in clusters at the base of young shoots.
- Size: Small, soft, and cylindrical.
- Structure:
- Each male cone is made up of numerous microsporophylls arranged in a spiral.
- Each microsporophyll bears two microsporangia on its lower surface, where microspores (pollen grains) are produced.
- Pollen Grains:
- The pollen grains have two air sacs (wings) that help them disperse through wind (anemophily).
2. Female Cone (Megasporangiate Cone):
- Location: Found on the upper branches, singly or in pairs.
- Size: Larger, woody, and persistent.
- Structure:
- The female cone consists of ovuliferous scales, each with two ovules on its upper surface.
- The ovule contains a megaspore mother cell, which undergoes meiosis to produce a megaspore.
- Development:
- The megaspore develops into a female gametophyte, which contains an archegonium with an egg cell.
3. Pollination and Fertilization:
- Pollination:
- Pollen grains are carried by wind and land on the micropyle (an opening in the ovule).
- A pollen tube grows from the pollen grain to deliver sperm cells to the egg cell.
- Fertilization:
- The fusion of sperm and egg forms a zygote, which develops into an embryo.
4. Seed Development:
- Seed Formation:
- After fertilization, the ovule develops into a seed enclosed within the woody cone.
- Dispersal:
- The mature cone becomes dry and opens up, releasing winged seeds, which are dispersed by wind.
Significance of Pinus Cones:
- Adaptation to Wind Pollination:
- Both male and female cones exhibit features that facilitate anemophily (wind pollination).
- Seed Protection:
- The woody structure of the female cone protects the developing seeds until they are mature and ready for dispersal.