NCERT Solution Part 2
Question 5: Write a note on economic importance of algae and gymnosperms.
Answer: Economic Importance of Algae:
- Almost 50% of carbon fixation on earth is carried out by algae. Thus, they are among the large producers on the earth; especially in aquatic habitat. About 70 species of marine algae are used as food, e.g. Porpyra, Laminaria and Sargassum.
- Some marine brown and red algae produce large amounts of hydrocolloids. Hydrocolloids are water holding substances and are used commercially.
- Agar, which is obtained from Gelidium and Gracilaria is used as culture medium in laboratories. It is also used in making ice-cream and jelly.
- Chlorella and Spirullina are used as food supplements; even during space travel.
Economic Importance of Gymnosperms:
- They are widely used as ornamental plants.
- Fibres from conifer are used as paper pulp.
- Conifers are widely used in reforestation projects.
- Turpentine and rosin are made from conifer resin.
- Commercially useful oils are produced from many conifers, e.g. juniper, pine, fir, etc.
Question 6: Both gymnosperms and angiosperms bear seeds, then why are they classified separately?
Answer: The seeds of gymnosperm are naked, while those of angiosperms are covered. Hence, they are kept in different groups.
Question 7: What is heterospory? Briefly comment on its significance. Give two examples.
Answer: When two types of spores are produced, the condition is called heterospory. One of the spores is bigger in size in this condition. Some pteriophytes are heterosporous, e.g. Selaginella and Salvinia. In heterospory, the megaspore germinates to produce female gametophyte, while the microspore germinates to produce the male gametophyte. This event is a precursor of the seed habit in higher plants.
Question 8: Explain briefly the following terms with suitable examples
Answer: A protonema forms the earliest stage of a bryophyte life cycle. It is a thread-like chain of cells. Protonema is formed just after the germination of spores. It subsequently develops into a leafy gametophyte.
Answer: An antheridium is the male sex organ of a plant. It produces the male gametes. It is present in the gametophyte phase of lower plants, but is present in the sporophyte phase of higher plants.
Answer: Archegonium is a structure which is present in the gametophyte phase of some plants. The archegonium produces the female gamete.
Answer: In this case, the diploid sporophyte is the dominant photosynthetic phase of the plant. The gametophyte is represented by the single to few-celled haploid gametophyte. Gymnosperms and angiosperms show this pattern.
Answer: A leaf or cluste of leaves which bears sporangia is called sporophyll. Sporophylls can produce microspores or megaspores as the case may be.
Answer: When gametes are similar in size, the condition is called isogamy, e.g. Spirogyra. The gametes can be flagellated or non-flagellated.