Plant Diversity II: The Evolution of Seed Plants (Chapter 30) – Study Notes

Plant Diversity II: The Evolution of Seed Plants

Introduction

This chapter explores the evolutionary innovations and diversity of seed plants, focusing on their derived characteristics, major clades, and reproductive adaptations. Seed plants are a dominant group in terrestrial ecosystems and include both gymnosperms and angiosperms.

Major Groups of Extant Plants

Classification Overview

• Nonvascular Plants (Bryophytes): Lack vascular tissue; dominant gametophyte generation.

• Vascular Plants:

  • Seedless (Spore-bearing): Includes ferns and their relatives; dominant sporophyte generation.

  • Seed-producing: Includes gymnosperms and angiosperms; dominant sporophyte generation.

Seed plants are divided into two sister clades:

1. Gymnosperms

2. Angiosperms

Life Cycles and Dominant Generations

Sporophyte and Gametophyte Generations

• Sporophyte (2n): The diploid, spore-producing generation; dominant in vascular plants.

• Gametophyte (n): The haploid, gamete-producing generation; dominant in nonvascular plants.

Example: When you see a fern frond in the forest, you are looking at a sporophyte.

Derived Characteristics of Vascular Seed-Bearing Plants

Key Innovations

• Reduced Gametophytes: Gametophytes are microscopic and develop within the tissues of the sporophyte, providing protection and nutrients.

• Heterospory: Production of two types of spores: megaspores (female) and microspores (male).

• Ovules: Structures that develop into seeds after fertilization; consist of a megasporangium, megaspore, and protective integuments.

• Pollen: Microspores develop into pollen grains, which contain the male gametophyte and facilitate fertilization without water.

• Seeds: Structures containing the embryo, food supply, and protective coat; allow for dormancy and dispersal.

Comparison of Plant Groups

Life Cycle and Gametophyte/Sporophyte Relationships

	Mosses and other nonvascular plants	Ferns and other seedless vascular plants	Seed plants (gymnosperms and angiosperms)
Sporophyte	Reduced, dependent on gametophyte for nutrition	Dominant	Dominant
Gametophyte	Dominant	Reduced, independent (photosynthetic and free-living)	Reduced (usually microscopic), dependent on surrounding sporophyte tissue for nutrition

Homosporous vs. Heterosporous Spore Production

Definitions and Processes

• Homosporous: One type of spore develops into a bisexual gametophyte (common in seedless vascular plants).

• Heterosporous: Two types of spores:

  • Megaspore: Develops into a female gametophyte (produces eggs).

  • Microspore: Develops into a male gametophyte (produces sperm).

Ovules and Production of Eggs

Structure and Function

• An ovule consists of a megasporangium, megaspore, and one or more protective integuments.

• Gymnosperm ovules typically have one integument.

• Angiosperm ovules usually have two integuments.

Example: The megaspore turns into the female gametophyte.

Pollen and Production of Sperm

Structure and Pollination

• A microspore develops into a pollen grain, which contains the male gametophyte enclosed within a pollen wall.

• Pollination is the transfer of pollen to the part of the seed plant containing the ovules.

• After pollination, the pollen grain germinates, producing a pollen tube that discharges sperm into the female gametophyte within the ovule.

• Difference from sperm of spore-bearing plants: Pollen allows sperm to reach the egg without water, unlike flagellated sperm in seedless plants.

Evolutionary Advantages of Seeds

Benefits of Seed Adaptation

• Seeds can remain dormant for extended periods until conditions are favorable for germination.

• They contain a stored food supply to support the developing embryo.

• Seeds can be dispersed over long distances by wind or animals, aiding in colonization of new habitats.

Summary Table: Key Features of Major Plant Groups

Feature	Mosses (Bryophytes)	Ferns (Seedless Vascular)	Gymnosperms	Angiosperms
Dominant Generation	Gametophyte	Sporophyte	Sporophyte	Sporophyte
Vascular Tissue	No	Yes	Yes	Yes
Seeds	No	No	Yes	Yes
Flowers	No	No	No	Yes

Key Terms and Definitions

• Sporophyte: The diploid, spore-producing phase of the plant life cycle.

• Gametophyte: The haploid, gamete-producing phase of the plant life cycle.

• Heterospory: The production of two distinct types of spores by different structures.

• Ovule: The structure in seed plants that develops into a seed after fertilization.

• Pollen: The structure that contains the male gametophyte and is involved in the transfer of sperm to the ovule.

• Integument: Protective layer(s) surrounding the megasporangium in an ovule.

Summary

Seed plants represent a major evolutionary advancement in the plant kingdom, characterized by reduced gametophytes, heterospory, ovules, pollen, and seeds. These adaptations have enabled gymnosperms and angiosperms to dominate terrestrial ecosystems and diversify into a wide range of forms and habitats.