NY Botanical Gardens: High School

Plant Reproduction and Development

Grade level: 10-12

OBJECTIVES: Students will be able to

- differentiate sexual and asexual reproductions

- identify the female and male parts of the flower

- understand the gametophyte development in flowering plants

- understand the development of seeds and fruits

- comprehend the importance of environmental factors for germination

- comprehend the adaptations of the plants for the survival

MATERIALS NEEDED: general figures of sexual and asexual reproduction and pollen and embryo sac (these can be easily found in any college biology textbooks)

DISCRIPTION OF ACTIVITIES

Students will be lectured on reproduction and development of the plants including sexual and asexual reproduction, plant life cycle, gametophyte development, pollination, development of seeds and fruits, germination of the seeds, and special adaptation of the plants (evolution).

PROCEDURE

1. Lecture on reproduction

A. Sexual vs. Asexual Reproduction

· Many plants can reproduce either sexually or asexually. Asexual reproduction in plants usually involves part of a single plant, giving rise to new plants. These offspring are genetically identical to the parent.

B. Plant life cycles

· Plants have two distinct, multicellular adult forms, one diploid and one haploid, which give rise to each other. For this reason, the plant life cycle is named ALTERNATION OF GENERATIONS, as diploid adult alternate with haploid adults.

C. Gametophyte development in flowering plants

· Pollen - An anther consists of 4 chambers called pollen sacs. Within each sac, hundreds to thousands of diploid microspore mother cells develop. Each microspore mother cell undergoes meiosis to produce four haploid microspores. Each microspore divides once, by mitosis, to produce a male gametophyte, or pollen grain. When ripe, the pollen sac splits open. In wind pollinated flowers, the pollen spills out.

· Embryo sac - Within an ovary, one or more dome-shaped masses of cells differentiate into ovules. Each ovule consists of outer layer of cells called integument that surround a single, diploid megaspore mother cell. The megaspore mother cell divides by meiosis to produce 4 large haploid megaspores. Three megaspores degenerate and only one survives. This remaining megaspore undergoes an unusual set of mitotic divisions. Three nuclear divisions produce a total of eight haploid nuclei. Plasma membranes then divide up the cytoplasm into seven, not 8, cells: three small cells at each end, with one nucleus apiece and one remaining large cell in the middle with 2 nuclei. This 7-celled organism, called embryo sac, is the haploid female gametophyte. The central binucleated cell is the primary endosperm cell. The egg is one of the cells at the bottom of the embryo sac, near a pore in the integument.

D. Pollination

Beetle-pollinated flowers

Many beetles preferably feed on animal materials because animal tissue usually has more protein. Beetle pollinated flowers often smell like rotting carrion or dung, which attracts scavenging beetles.

· Bee-pollinated flowers

- To attract a bee from a far, bee-pollinated flowers must look brightly-colored to a bee. Typically, these flowers are white, yellow, or blue. Many bee-pollinated flowers produce nectar at the bottom of a tube.

· Moth- and Butterfly- pollinated flowers

- Many moths and butterflies also feed on nectar. Flowers adapted for these pollinators are often superficially similar to bee-pollinated flowers, except that the nectar tubes are usually deep and narrow.

· Hummingbird-pollinated flowers

- Flowers produce large amounts of nectar. To keep insects away, these flowers are always tubular. The tube is much deep for bees to reach nectar at its base. Most are red or orange colored, which is brightly attractive to a bird, but drab to a bee.

E. The Development of Seeds and Fruits

· The embryo develops a root, shoot, and cotyledons. Cotyledons digest and absorb food from the endosperm, and transfer it to the growing embryo. Monocot embryos have one cotyledon, while dicot embryos have two. The seed is enclosed within a fruit that develops from the ovary wall. The function of the fruit is to disperse the seeds away from the parent plant. Seeds often remain dormant for some time after fruit ripening. Environmental conditions involved in breaking dormancy may include an initial desiccation, exposure to cold, or disruption of the seed coat.

F. Germination and Growth of Seedlings

· Seed germination requires warmth and moisture. Energy for germination comes from food stored in the endosperm, transferred to the embryo by the cotyledons.

G. Special Adaptation of roots, stems, and leaves

-The evolution

1) Root adaptation - Some roots have extreme specialization for storage

(e.g., beet, carrot, or sweet potato)

2) Stem adaptation

· runners (e.g., strawberries)

· Many plants store carbohydrates in underground stems (e.g., white potato)

· thorns

3) Leaf adaptation

· Cacti - reducing leaves into thin spines that protect that plant from

herbivores and reduce water loss

Note:

1. Through evolution, the roots, stems, and leaves of many plants have been modified into diverse structures such as spines, tendrils, thorns and bulbs. These unusual structures are often involved in water or energy storage,

support or protection.

2. This lesson plan was the last section of the unit plan (on plants).

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