lecture 6 - seedless plants

Lecture Overview

This lecture focuses on plant diversity, specifically the evolution of seed plants, including gymnosperms and angiosperms. The lecture aligns with Chapters 30, 35, and 38 of the related textbook and aims to enhance understanding of plant evolution and adaptation.

1. Longest Lived Tree

The longest-lived tree is the Bristle-Cone Pine, which resides in the White Mountains of California and is approximately 4,700 years old. This tree holds significant historical and ecological importance.

2. Dendrochronology

Dendrochronology, the study of tree rings, has helped calibrate carbon dating data and provides insights into climate changes over the last 11,500 years.

3. Timeline of Terrestrial Colonization by Plants

  • 1.2 billion years ago: Cyanobacteria colonized land.

  • 500 million years ago: Fungi and early non-vascular plants appeared.

  • Based on molecular data, plants share a common ancestor with Charophytes, which are predominantly freshwater algae.

4. Plant Origin and Characteristics

Plants share key features with primitive organisms:

  1. Multicellular and eukaryotic: Composed of multiple cells and characterized by complex cellular organization.

  2. Photosynthetic autotrophs: Includes brown, red, and green algae.

  3. Cell walls made of cellulose: Found in green algae and other organisms like Dinoflagellates and brown algae.

  4. Chloroplasts with chlorophyll: Contains chlorophyll a in Euglenids and Dinoflagellates; chlorophyll a and b found in green algae and charophytes.

5. Traits Shared with Charophytes

Evidence from morphologic and genetic studies supports the phylogenic relationship between land plants and Charophytes. Four ancestral traits include:

  1. Rings of cellulose-synthesizing proteins.

  2. Peroxisome enzymes.

  3. Flagellated sperm structure, absent in higher plants.

  4. Formation of a phragmoplast during cell division.

6. Defining the Plant Kingdom

  • Land plants and green algae form a clade called Viridiplantae.

  • 1.2 billion years ago: Divergence into two clades: Chlorophyta (algae) and Streptophyta (land plants and Charophyceans).

  • 475 million years ago: Evolution of non-vascular land plants from Charophyceans.

  • 425 million years ago: Emergence of vascular plants.

  • Traditional botanists refer to land plants as Embryophytes, as embryos develop within the archegonium/female gametophyte.

7. Derived Traits of Plants - Embryophytes

Five key derived traits present in nearly all plants but absent in Charophytes include:

  1. Alternation of generations:

  2. Multicellular, dependent embryos:

  3. Walled spores produced in sporangia:

  4. Multicellular gametangia:

  5. Apical meristems:

8. Vascular Plants Clades

Vascular plants consist of a single clade encompassing 93% of all species, categorized into:

  1. Lycophytes: Club mosses and relatives.

  2. Pterophytes (Monilophyta): Ferns and relatives.

  3. Seed vascular plants:

    • A. Gymnosperms: "Naked seed" plants.

    • B. Angiosperms: Flowering plants.

9. Heterospory in Seed Plants

  • All seed-bearing vascular plants exhibit heterospory: the production of two types of spores:

    • Macrospores (megaspore): Larger, female spores.

    • Microspores: Smaller, male spores.

  • Ferns are homosporous, yielding one kind of spore that develops into a bisexual gametophyte.

10. Reproductive Structures

  • Megasporangium: Located on modified leaves called megasporophylls. It produces megaspores that develop into female gametophytes.

  • Microsporangium: Located on microsporophylls, which generate microspores that evolve into male gametophytes. Both structures are part of specialized reproductive structures such as cones and flowers.

11. Monoecious and Dioecious Plants

  • Plants may be monoecious, with both male and female reproductive structures on the same individual, or dioecious, with distinct male and female plants (e.g., male and female palm trees).

12. Key Reproductive Adaptations in Seed Plants

Three essential reproductive adaptations in seed plants:

  1. Increased dominance of the sporophyte generation, leading to reduced size of the gametophyte.

  2. Development of seeds that include ovules and eggs.

  3. Evolution of pollen as an airborne dispersal agent.

13. Seeds and Their Dispersal

Seeds can be dispersed through various mechanisms, including wind, animals, and water. Each seed contains an embryo and a nutrient supply for early growth.

14. Pollen and Its Dispersal Mechanism

Pollen grains utilize wind and animals for dispersal, conveying male gametophytes to female reproductive structures.

15. Reduced Gametophytes

  • In vascular plants, gametophytes are reduced compared to those in mosses, which are the dominant stage in non-vascular plants. Vascular plant gametophytes are mostly microscopic, which allows for development within the sporangium of the parental sporophyte, providing protection against environmental stresses such as drought and UV radiation.

16. Ovaries and Seeds

  • The unique characteristic of seed plants is that the megasporangium is retained within the female sporophyte. This ovule structure consists of:

    • Megasporangium (2n): diploid.

    • Megaspore (n): haploid, forms through meiosis.

    • Surrounded by integuments: protective layers that aid in forming the seed coat.

17. Evolution of Pollen in Seed Plants

  • Pollen grains develop from microspores produced in microsporangia and contain the male gametophyte. The outer wall is fortified with sporopollenin for protection during dispersal.

  • Pollination occurs when pollen grains reach the ovule, and a pollen tube is formed to facilitate sperm delivery for fertilization.

18. Gymnosperms Overview

  • Gymnosperms, or “naked seeds,” carry exposed seeds on modified leaves called sporophylls, typically found within cones.

  • Historical context notes that gymnosperms underwent dominance around 250 million years ago, particularly benefiting from drier environments. Common existing gymnosperms include conifers such as spruce, pine, fir, and redwood.