Plant Diversity II: The Evolution of Seed Plants

Plant Diversity II: The Evolution of Seed Plants

Introduction

This section explores the evolutionary innovations and diversity of seed plants, focusing on their distinguishing features, life cycles, and major groups. Seed plants represent a significant evolutionary advancement over earlier plant lineages, enabling greater adaptation to terrestrial environments.

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 (Spore-bearing): e.g., ferns, club mosses.

  • Seed-producing: Divided into two sister clades:

    1. Gymnosperms

    2. Angiosperms

Vascular Seed-Bearing Plants: Derived Characteristics

Key Innovations in Seed Plants

• 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 plant embryo, a food supply, and a protective coat, enabling survival in diverse environments.

Alternation of Generations in Plant Groups

Comparison of Life Cycles

Seed plants exhibit a dominant sporophyte generation, with highly reduced gametophytes. The following table summarizes the differences among major plant groups:

	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 sporophyte for nutrition
Examples	Sporophyte (2n), Gametophyte (n)	Sporophyte (2n), Gametophyte (n)	Gymnosperm: Microscopic female gametophytes (n) inside ovulate cone; Angiosperm: Microscopic female gametophytes (n) inside parts of flowers

Heterospory: Spore Production in Plants

Homospory vs. Heterospory

• Homospory: Most seedless vascular plants produce one type of spore, which typically develops into a bisexual gametophyte.

• Heterospory: Seed 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 of Ovules

• 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.

After fertilization, the ovule develops into a seed.

Pollen and Production of Sperm

Development and Role of Pollen

• 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 a 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 be transported without water, unlike the flagellated sperm of seedless plants.

Evolutionary Advantages of Seeds

Benefits of Seed Evolution

• 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, increasing the range of the species.

Summary Table: Key Features of Seed Plants

Feature	Gymnosperms	Angiosperms
Integuments per ovule	One	Two
Seed enclosure	Exposed on cones	Enclosed within fruit
Pollination	Mainly wind	Wind, animals, insects
Examples	Pine, fir, Ginkgo	Oak, maple, grasses, orchids

Conclusion

Seed plants, through the evolution of seeds, pollen, and reduced gametophytes, have become the dominant flora in most terrestrial ecosystems. Their reproductive adaptations have enabled them to colonize a wide range of environments and provide essential resources for human societies.