Think of the daily activities of a wild animal---a songbird, for example. It pecks the ground for worms. It calls for a mate. It flies to a birdbath for a drink of water. Plants undertake the same life functions as animals---nutrition, respiration, reproduction---but do so while completely immobilized. Biologists often quip that form explains function, and a closer look at plant structure reveals how plants conduct everyday life functions.
Early in their evolution, plants began their life in the water, where they were bathed with a constant supply of water, dissolved gases and nutrients. Life on land required drastic changes in structure to cope with the new challenges of a land-based existence, and tracing the evolution of the plant kingdom, you can observe how successive species developed structural improvements that let plants survive in even the most hostile terrestrial environments.
The earliest terrestrial plants, the mosses, don't have roots. They have structures that help them grab hold of a growing surface, but they require almost constant contact with moisture, a vestige of their aquatic origins. Plant roots plunge into the earth, accessing water supplies slow to dry up to periods of low rainfall. They consist of special cortex cells that absorb water and nutrients and move them into the plant's vascular tissue for transport throughout the plant. Tiny root hairs increase the surface area available for water absorption.
Mosses also lack vascular tissue, accounting for their small size, as all parts of the plant must be near a water source. Ferns first developed vascular tissue, which has persisted throughout plant evolution. Vascular tissue carries water from the roots and sugars manufactured in the leaves and distributes them throughout the plant. Vascular tissue lets plants grow beyond where each cell has direct contact with a water source. Plants have two types of vascular tissue. Xylem conducts water and nutrients upward from the roots, and phloem transports sugars produced in the leaves.
Plants cannot wander about to hunt or forage for food, so all of their energy is produced in their leaves. Inside each cell are tiny structures called chloroplasts that, when heated by sunlight, combine that energy with water and carbon dioxide to produce sugars that power growth and metabolic processes. Leaves also contain closable pores called stomata that let in carbon dioxide for photosynthesis, a spongy layer where sugars are stored and a waxy cuticle that limits water evaporation from the top of the leaf.
The three most obvious structures of the plant---the roots, stem and leaves---work together to keep the plant alive. The roots take in water, and the leaves produce chemical energy. The vascular tissue found in the stem carries these up and down the plant, distributing them where they're needed. Each structure helps assure the plant's survival.