What Are Plants?

Plants are more complex than animals and fungi: in addition to nuclei and mitochondria, plant cells contain plastids (organelles for tapping the sun's energy). Plants evolved from protoctist algae that had already incorporated cyanobacteria that became chloroplasts.

Significant challenges face the earliest plants as they confront the demands of surviving on dry land. Accustomed to an aquatic lifestyle, early plants lay on the surface, unable to support their weight against gravity. Watery conjugation is no longer a viable survival strategy. The most crucial innovation for the first true plant is the ability to develop a fertile egg into an embryo, a multicellular young plant, within moist, protective maternal tissue.

Life history of an oak tree. Great oaks from little acorns grow. Sexual coupling of the female and male flower (left) results in the embryo. (Illustration by Christie Lyons)


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