Plant Diversity II: The Evolution of Seed Plants

Introduction

This chapter explores the evolutionary history and biological significance of seed plants, focusing on their derived characteristics, major clades, and adaptations that have contributed to their success on land. Seed plants include both gymnosperms and angiosperms, which together dominate modern terrestrial ecosystems.

Phyla of Extant Plants

Major Groups of Land Plants

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

• Vascular Plants: Possess specialized tissues (xylem and phloem) for transport.

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

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

Seed plants are divided into two sister clades:

1. Gymnosperms: Seed plants with "naked" seeds not enclosed in fruit.

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

Life Cycles and Dominant Generations

Sporophyte and Gametophyte Generations

• Sporophyte: The diploid (2n) generation that produces spores by meiosis.

• Gametophyte: The haploid (n) generation that produces gametes (egg and sperm) by mitosis.

In seedless vascular plants like ferns, the visible frond is the sporophyte generation.

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 carry the male gametophyte and allow fertilization without water.

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

Comparative Life Cycles of Plant Groups

Table: Comparison of Major Plant Groups

Heterospory: Spore Production in Plants

Homospory vs. Heterospory

• Homospory: One type of spore gives rise to a bisexual gametophyte (common in most seedless vascular plants).

• Heterospory: Two types of spores are produced:

  • 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.

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

Pollen and Production of Sperm

Structure and Function of Pollen

• 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.

• 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 non-seed plants.

Evolutionary Advantages of Seeds

Benefits of Seed Adaptation

• Protection: Seeds protect the developing 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.

Major Clades of Seed Plants

Gymnosperms

• Four phyla: Cycadophyta (cycads), Ginkgophyta (Ginkgo biloba), Gnetophyta (Gnetum, Ephedra, Welwitschia), Coniferophyta (conifers such as pine, fir, redwood).

• Most gymnosperms have woody cones and are evergreen.

Angiosperms

• Classified in a single phylum: Anthophyta.

• Key adaptations: flowers (specialized for sexual reproduction) and fruits (aid in seed dispersal).

• Flowers facilitate pollination, which can occur via wind, animals, or self-pollination.

• Fruits develop from the ovary wall and protect seeds, aiding in their dispersal.

Angiosperm Diversity: Monocots vs. Eudicots

Table: Comparison of Monocots and Eudicots

Importance of Seed Plants to Humans

Economic and Ecological Significance

• Food: Most human food comes from angiosperms. Six crops (corn, rice, wheat, potatoes, sweet potatoes, cassava) provide 80% of human-consumed calories.

• Wood: All wood comes from seed-bearing plants, used for fuel, paper, and construction.

• Medicines: Many plants produce compounds used in pharmaceuticals; about 25% of current medicines are plant-derived.

Key Terms and Definitions

• Sporophyte: Diploid, spore-producing generation in plants.

• Gametophyte: Haploid, gamete-producing generation in plants.

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

• Ovule: Structure containing the female gametophyte, which develops into a seed after fertilization.

• Pollen: Structure containing the male gametophyte, enabling fertilization without water.

• Seed: Fertilized ovule containing the plant embryo, food supply, and protective coat.