Well cyberstudents we have only one more major topic to cover. In our last four lectures together we will be studying plants. Springtime is the perfect time to be think about gardening, one of the most popular hobbies of all time. We will be learning about the common flowering plants that are likely familiar to you, as well as some of the more exotic types of plants that evolved millions of years earlier and have survived to the present day.

Evolutionary Trends

Millions of years ago the only photosynthesizers were cells living in the seas. The Earth's atmosphere was originally oxygen free and ultraviolet radiation bombarded the surface. Photosynthetic cells produced oxygen and allowed formation of a protective ozone layer.

The invasion of land began with the cyanobacteria spreading into and up freshwater streams. Later, green algae and fungi made the journey together. Every plant is descended from species of green algae.

Today, there is a rich diversity of green plants, making carbon compounds out of water and carbon dioxide using sunlight as the energy source and utilizing minerals dissolved in water. In general, plants are multicelled photosynthetic autotrophs, that are green in color and self sustaining.

You are probably most familiar with the more advanced seed-bearing plants, the gymnosperms (e.g. pine tress) and angiosperms (flowering plants), that have vascular tissues for transport of water and nutrients and possess root and shoot systems. In this lecture, we will mainly be discussing the more primitive plants and gymnosperms.

The ancestors of plants had evolved by 700 million years ago, but another 265 million years passed before simple stalked species appeared. Within another 60 million years, plants had radiated through much of the land.

Seedless vascular plants arose during the Devonian. They produce sprores, but no seeds. There are four main groups:

Of the seed-bearing vascular plants, the gymnosperms arose first. They include some groups with which you are familiar and others that are probably less familiar: Angiosperms arose later and there are two groups: Roots, stems, and leaves evolved as adaptations to land. Underground parts developed into root systems, specialized for absorption of water and minerals through extensive cylindrical tubes. Parts above ground developed into shoot systems, adapted for exploiting sunlight and absorbing carbon dioxide from the air. Vascular tissue became increasingly extensive with xylem for conducting water and minerals, and phloem for conducting products of photosynthesis.

Extensive growth of stems and branches became possible due to the strengthening of cell walls afforded by deposits of lignin. Stems and leaves were covered by cuticle to minimize water loss. Evaporation was controlled by opening and closing of stomata (openings in the leaves).

This animation (No Audio) describes the evolution of plants.

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