question_answer

(i) With the help of a labelled diagram depict the organization of a typical embryo sac just after double fertilization.

(ii) How are seeds advantageous to angiosperms?

OR

What are out-breeding devices? Explain the different types.

Answer:

(i)

(ii) Advantages of seeds to angiosperms:

(a) Pollination and fertilization of seed plants are independent of water. Seed formation is therefore a dependable method.

(b) Hard seed coat protects the embryo from environment extremes.

(c) Seeds have abundant reserve food material for young seedlings.

(d) Seeds are dry and have protective coating and are therefore easy to store for their later use.

(e) Seeds are formed from sexual reproduction and thus have a number of variation for better adaptability to unfavourable conditions.

Since seeds are the product of sexual reproduction, it promotes diversity.

OR

Majority of flowering plants produce hermaphrodite flowers and pollen grains are likely to come in contact with the stigma of the same flower. Continued self-pollination results in inbreeding depression. Improved crops for depression by the devices are called outbreeding devices.

Flowering plants have developed many devices to discourage self-pollination and encourage cross-pollination.

(a) First device: In some species, pollen release and stigma receptivity are not synchronized. Either the pollen are released before the stigma becomes receptive or the stigma becomes receptive much before the release of pollen.

(b) Second device: In some other species, the anther and stigma are placed at different positions so that the pollen cannot come in contact with the stigma of the flower. Both first and second devices prevent autogamy

(c) Third device: Self-incompatibility prevents inbreeding. This is a genetic mechanism and prevents self-pollen (from the same flower or other flowers of the same plant) from fertilizing the ovules by inhibiting pollen germination.

(d) Another device to prevent self-pollination is the production of unisexual flowers.