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:
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.