How plants grow
Some plants grow very large (like the 200-foot redwood trees of the Pacific Coast Ranges), while others are very tiny, like mosses. Even though plants vary a great deal in size, the basic principles of plant growth are the same.
Plant growth is confined to soft growing zones known as meristems, unspecialized cells that form many different plant parts. Within the growing cells, there are auxins, the hormones responsible for the regulation of plant growth.
Auxins affect the growth of plant parts such as stems, leaves, and roots by stimulating certain cells to grow in a certain manner.
The effect of auxins is seen in a plant's tropisms - the plant's reaction to external stimuli, like light (phototropism) and gravity (geotropism).
Phototropism is the movement of the plant body toward light (they grow toward light).
Geotropism is the growth of roots toward the ground and leaves away from the ground, away from the pull of Earth's gravity.
Photoperiodism is the system that plants use to figure out what season they are in; they use the uninterrupted periods of darkness to determine the season. Longer days = growing season. Smart little buggers, aren't they?