Autotrophs are the vital base of food webs present on Earth today. In the ocean, photosynthetic prokaryotes and bacteria provide most of the energy. Today, photosynthetic plants support life on land. Plants evolved from aquatic species not much different from the primitive alge we see today.

Plants can be generally defined as multicellular photosynthetic eukaryotes, although there are a few obviously related species which rely instead on alternative modes of nutrition, such as Indian pipe (Monotropa).

Bryophytes were the first organisms to invade land. They developed certain adaptations which allowed them to cope with requirements of a terrestrial environment. They retained some of the characteristics of aquatic species, such as a soft body with the absence of root and vascular systems. The gametophyte is the predominat phase in the life cycle, and the sporophyte is totally dependent on the gametophate. They still need a water environment for the opening of the sex organs and fertilization to occur. Hence, bryophates are sometimes refferred to as the "amphibians of the plant kingdome".

A terrestrial environment entails an entirely different set of environmental conditions compared to an aquatic habitat. Water and nutrients are available only through soil. Oxygen and carbon dioxide needed for respiration and photosynthesis are available in gaseous form in the atmosphere. Aerial structures are exposed directly to atmosphereic conditions like air, light, temperature, wind, e.t.c. Unlike the aquatic environment, the plant does not get natural support or buoyancy from its environment. Thus, in order to survive on land, the terrestrial plants had to develop stratagies, including some of the following characteristics to address these environmental differences:

1. An extensive root system for efficient anchorage and absorption.

2. Leaves as specialized organs for photosynthesis.

3. Specialized vascular tissues such as xylem for conduction of water and minerals from roots to aerial parts, and phloem for the conduction of organic solutes from leaves and other green organs to non-photosynthetic portions of the plant.

4. Cuticle over aerial parts of the plant for protection and to prevent surface evaporation.

5. Stomata to regulate gaseous exchange with the environment.

The increasing complexity of the plant body was accompanied by "division of labor" among various organs according to the requirements of the plant body, including metabolic and functional adaptations.

Of the twelve phyla of the kingdome plantae, nine of them, known as tracheophytes,.display developed "plumbing" systems for transporting materials throughout the plant. The remaining three phyla contain a very simple or no vascular system and are referred to as non-tracheophates, which replaces the misnomer "nonvascular plants".

Another remarkable change that occurred in the evolution of land plants was the conversion of a predominant haploid, gametophytic generation, as in Bryophyta, to a prodominant diploid, sporophytic generation in the life cycle in pteridophytes, gymnosperms and angiosperms. In bryophytes, the gametophyte is very elaborate, less so in pteridophytes, and greatly reduced in gymnosperms and angiosperms. Fertilization in land plants occurs within the protection of the female sex organ.

Table of Contents

15.0 - Introduction
15.1 Pteridophyta : General Account
15.2 Gymnosperms
15.3 Angiosperms : Dicotyledons
15.4 Angiosperms : Monocotyledons
15.5 Vascularization
15.6 Development of seed habit
15.7 Development of Flower and Fruit

Chapter 16