Plant Life Cycle Innovations

Overview of Seed Plant Life Cycle

Introduction to Seed Plants

• Review of fundamental concepts from earlier studies.

• Introduction of pollen grain as a new structure in seed plants.

Pollen Grain

• Definition: The pollen grain represents the male gametophyte in seed plants.

• Characteristics:

  • Mobile structure that allows for sperm dispersion without the need for water.

  • Offers protection through a pollen coat.

• Composition:

  • Contains two cells (multicellular haploid structure).

  • Functions effectively in fertilization due to internalized processes.

Major Innovations in Seed Plants

1. Pollen Grain and Sperm Dispersal

• Functional transition from external sperm dispersal (needing water) to air-based dispersal (via pollen).

• Implication: Improves the chance for fertilization across greater distances.

2. Ovules and Internal Fertilization

• Structure: The ovule is surrounded by additional layers of tissue providing more protection.

• Definition of Ovule: A spore-containing structure known as a sporangium.

• Highlights the shift from external fertilization to internal fertilization, enhancing offspring survival.

3. Seeds as Offspring Dispersal Method

• Transition from dispersing spores to seeds.

• Seeds retain the embryo, providing resources and protective coatings, such as the endosperm.

• Importance: Seeds are multicellular and ensure greater survival potential by having protective resources.

Phylogenetic Transition in Life Cycle Stages

• Dominant life stage progression from:

  • Gametophyte-dominant (non-vascular plants)

  • To sporophyte-dominant structures.

• Sporophytes exhibit independent life stages (e.g., longer lifespan, photosynthesis capability).

Overview of Flower Structure

Flower as Strobilus

• Flowers are clusters of sporophylls at the end of the stem.

• Distinction of male and female reproductive organs within the same flower or separate flowers on the same or different plants.

Reproductive Processes in Flowers

Male Side of Reproductive Structure

• Stamen Structure: Comprises filament and anther (sporangium).

• Microsporangium Details: Houses microsporocytes which undergo meiosis to form microspores:

  • Each microsporocyte (diploid) develops into four haploid microspores (each pollen grain).

  • Each microspore is formed through mitotic division resulting in a pollen grain (2 cells: tube cell and generative cell).

Female Side of Reproductive Structure

• Structure: Contains pistil (style, stigma, ovary).

• Pistil vs Carpal:

  • Pistil refers to observable flower structures; carpal refers to evolutionary origin of the structure.

• Ovary Function: Houses ovules (megasporangium).

Ovule Formation and Development

• Definition: The integumented megasporangium transformed into ovule with single megasporocyte now made to endure meiosis.

• Development Process of Megasporocyte:

  • Results in one functional megaspore and three degenerate cells (programmed cell death).

  • Through three rounds of mitosis, the megaspore becomes a multicellular structure (embryo sac) containing seven cells and eight nuclei.

  • Key cells include:

  • Egg cell: Found near micropyle, conducive to fertilization.

  • Central cell: Contains two nuclei and plays a crucial role in fertilization.

Pollination and Fertilization Evolution

• Pollination defined as the movement of pollen grain from microsporangium to stigma.

• Transition from sperm as free-swimming cells to nuclei carried by pollen tubes into the ovules via fertilization; double fertilization occurs (fusion of one sperm with egg and another with central cell).

Final Developments in Seed Plants

• Endosperm Formation: The second fertilization leads to the creation of a triploid cell, forming the endosperm, vital for seed growth.

• Discussion of fruit development from the ovary as an adaptation for seed protection and dissemination, crucial for reproduction.

• Emphasizes the evolutionary advantages of these structures and processes in the plant kingdom.