Plant Diversity II: Seed plants:Gymnosperms and Angiosperms

Question 1

What evolutionary innovations made seed plants so successful?

Answer 1

1. Seeds:
   ○ Provide protection and nourishment for the embryo.
   ○ Allow for dormancy until conditions are favorable for growth.
   2. Pollen:
   ○ Eliminates the need for water in fertilization.
   ○ Allows sperm to be transported over long distances by wind or animals.
   3. Reduced Gametophytes:
   ○ Protects the gametophyte within the sporophyte tissues (e.g., ovules and pollen grains).
   4. Vascular Tissue:
   ○ Efficiently transports water and nutrients, enabling larger plant sizes.
   5. Heterospory:
   ○ Production of two distinct types of spores: microspores (male gametophytes) and megaspores (female gametophytes).

Question 2

Describe the typical gymnosperm life cycle

Question 3

What are the major adaptations of angiosperms?

Answer 3

1. Flowers:
   ○ Specialized structures for reproduction, attracting pollinators to increase fertilization efficiency.
   2. Fruits:
   ○ Develop from mature ovaries and aid in seed dispersal.
   3. Double Fertilization:
   ○ One sperm fertilizes the egg, forming a zygote, while another sperm combines with two nuclei to form triploid endosperm (nutrient tissue).
   4. Efficient Vascular Tissue:
   ○ Xylem with vessel elements allows for more efficient water transport.
   5. Rapid Reproduction:
   Short life cycles allow for quick adaptation and diversification.

Question 4

Describe the life cycle of an angiosperm.

Answer 4

1. Mature Sporophyte (Diploid, 2n):
   The flower is the reproductive structure of the sporophyte generation.
   It contains:
   Anther (male structure) with microsporangia.
   Ovary (female structure) with ovules.
2. Formation of Gametophytes:
   a. Male Gametophyte (Pollen Grain):

Inside the anther, microspores (haploid, n) are produced via meiosis.
Each microspore undergoes mitosis to form a pollen grain (male gametophyte).
b. Female Gametophyte (Embryo Sac):

Inside the ovule in the ovary, a megasporocyte undergoes meiosis, producing four haploid megaspores.
Only one megaspore survives, which undergoes mitosis to form the embryo sac (female gametophyte). This sac contains the egg nucleus and other cells.
3. Pollination and Fertilization:
Pollination: Pollen grains are transferred to the stigma (part of the carpel) by wind, water, or animals.

A pollen tube grows from the pollen grain, carrying two sperm nuclei to the embryo sac.

Double Fertilization:

One sperm nucleus fertilizes the egg nucleus, forming a zygote (diploid, 2n).
The second sperm nucleus fuses with two nuclei in the embryo sac to form the endosperm (triploid, 3n), which serves as the seed’s food supply.
4. Seed and Fruit Development:
The zygote (2n) develops into an embryo.
The ovule becomes the seed, containing:
The embryo (2n).
The endosperm (3n) for nourishment.
A protective seed coat (2n).
The ovary develops into a fruit, which protects the seeds and aids in their dispersal.
5. Germination:
When conditions are favorable, the seed germinates, and the embryo grows into a new sporophyte (2n), completing the cycle.
Key Terms:
Sporophyte (2n): Dominant generation, represented by the mature flowering plant.
Gametophyte (n): Reduced, dependent on sporophyte:
Male: Pollen grain.
Female: Embryo sac.
Double Fertilization: Unique to angiosperms, forming both zygote and endosperm.
Seed: Contains the embryo, food supply, and seed coat.

Question 5

Why have angiosperms been so successful?

Answer 5

1. Pollinator Relationships:
   ○ Flowers attract specific pollinators, ensuring efficient pollen transfer.
   2. Seed Dispersal Mechanisms:
   ○ Fruits allow seeds to be dispersed by animals, wind, or water.
   3. Genetic Diversity:
   ○ Cross-pollination increases genetic variability, promoting adaptability.
   4. Rapid Growth and Reproduction:
   ○ Angiosperms can colonize and adapt to diverse habitats quickly.
   5. Specialized Vascular Tissue:
   ○ Efficient transport of water and nutrients supports larger and more productive plants.