Biology - Class Twelve


NCERT Solution - Part-3

Question-(11) Identify each part in a flowering plant and write whether it is haploid (n) or diploid (2n).

(a) Ovary - - - - - - - - - -

(b) Anther - - - - - - - - - -

(c) Egg - - - - - - - - - -

(d) Pollen - - - - - - - - - -

(e) Male gamete - - - - - - - - - -

(f) Zygote - - - - - - - - - -

Answer: Haploid (n): Pollen, egg and male gamete

Diploid (2n): Ovary, anther and zygote

Question-(12) Define external fertilisation. Mention its disadvantages.

Answer: When fertilization takes place outside the body of an organism, it is called external fertilization. Some of the disadvantages of external fertilization are as follows:

(a) Eggs and sperms are at risk of getting destroyed by flow of water and by predators.

(b) Developing embryos are at risk of getting destroyed by flow of water and by predators.

(c) Percentage of survival is very low among progeny.

Question-(13) Differentiate between a zoospore and a zygote.

Answer: Zoospore is formed by asexual means, while zygote is formed by sexual means. Zoospores are seen in lower animals and plants, while zygote is seen in higher animals and plants.

Question-(14) Differentiate between gametogenesis from embryogenesis.

Answer: Formation of gametes is called gametogenesis. Gametogenesis is part of pre-fertilisation stage of sexual reproduction. Formation of embryo is called embryogenesis. Embryogenesis is part of post-fertilisation stage of sexual reproduction.

Question-(15) Describe the post-fertilisation changes in a flower.

Answer: After fertilization; sepals, petals and stamens wither and fall off. The pistil remains attached to the plant. The zygote develops into the embryo and ovules develop into the seed. The ovary develops into a fruit with a thick wall called pericarp. Pericarp is protective in nature. Seeds get dispersed once they become mature. Subsequently, seeds germinate and give rise to new plants.

Question-(16) What is a bisexual flower? Collect five bisexual flowers from your neighbourhood and with the help of your teacher find out their common and scientific names.

Answer: A flower which contains both the male and the female parts is called bisexual flower. In other words, a bisexual flower produces both the gametes, i.e. male and female gametes. Some bisexual flowers are as follows: Pea (Pisum sativum), Soybean (Glycine max), Tomato (Solanum lycopersicum), Potato (Solanum tuberosum), etc.

Question-(17) Examine a few flowers of any cucurbit plant and try to identify the staminate and pistillate flowers. Do you know any other plant that bears unisexual flowers?

Answer: Flowers of pumpkin, cucumber, etc. are unisexual. Similarly, flowers of maize are unisexual. The male flowers are at the top of the plant in maize; while female flowers appear from the nodes on the stem.

Question-(18) Why are offspring of oviparous animals at a greater risk as compared to offspring of viviparous animals?

Answer: An oviparous animal lays eggs. The eggs are either left alone or incubated by the parents. In both the cases, eggs are at the risk of being eaten by predators. An egg can also get broken because of various natural causes. Thus, survival rate of progenies from eggs is very low. On the other hand, offspring of viviparous animals are borne only after they are fully developed. In most of the cases, the parents also take care of the offspring for a considerable duration. Thus, the offspring borne of viviparous animals are better equipped to withstand the vagaries of outer world.


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