Plant Diversity II: The Evolution of Seed Plants

Introduction

This section explores the evolutionary history and biological features of seed plants, focusing on their derived characteristics, major clades, and significance in the plant kingdom. Seed plants are a major group within vascular plants and include both gymnosperms and angiosperms.

Phyla of Extant Plants

Major Groups of Plants

• Nonvascular Plants (Bryophytes): Lack vascular tissue; include mosses, liverworts, and hornworts.

• Vascular Plants: Possess vascular tissue (xylem and phloem).

  • Seedless Vascular Plants: Reproduce via spores (e.g., ferns, club mosses).

  • Seed-Producing Vascular Plants: Reproduce via seeds; include gymnosperms and angiosperms.

Two Sister Clades of Seed Plants

1. Gymnosperms: Seed plants with "naked" seeds not enclosed in fruit (e.g., pines, firs).

2. Angiosperms: Seed plants with seeds enclosed in fruits (flowering plants).

Life Cycles and Dominance

Sporophyte and Gametophyte Generations

• Sporophyte: Diploid (2n) generation; produces spores by meiosis.

• Gametophyte: Haploid (n) generation; produces gametes by mitosis.

In seed plants, the sporophyte is the dominant, conspicuous generation, while the gametophyte is reduced and often microscopic.

Comparison Table: Generational Dominance and Gametophyte Dependence

Derived Characteristics of Seed 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 allow for fertilization without water.

• Seeds: Structures containing the plant embryo, food supply, and protective coat; enable survival in diverse environments.

Homosporous vs. Heterosporous Spore Production

Definitions and Processes

• Homosporous Plants: Produce one type of spore that develops into a bisexual gametophyte (common in seedless vascular plants).

• Heterosporous Plants: Produce two types of spores:

  • Megasporangium (on megasporophyll): Produces megaspores → female gametophytes → eggs.

  • Microsporangium (on microsporophyll): Produces microspores → male gametophytes → sperm.

Ovules and Production of Eggs

Structure and Function

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

• Gymnosperm ovules: Usually have one integument.

• Angiosperm ovules: Usually have two integuments.

• The megaspore develops into the female gametophyte, which produces eggs.

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: The transfer of pollen to the part of a seed plant containing the ovules (e.g., ovulate cone or stigma).

• 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 Seeds

• Protection: Seeds protect the embryo with a tough seed coat.

• Food Supply: Seeds contain stored food to support the embryo during germination.

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

• Dispersal: Seeds can be transported long distances by wind or animals, aiding in colonization of new habitats.

Summary Table: Key Features of Seed Plants

Conclusion

Seed plants represent a major evolutionary advancement in the plant kingdom, with adaptations that allow them to thrive in diverse terrestrial environments. Their reproductive innovations—such as seeds, pollen, and reduced gametophytes—have contributed to their ecological success and importance to humans.