The plant's kitchen: where food is created
Leaves remove excess water from plants and provide structural support - but more importantly, the food-making process occurs in leaves. They're sort of your plant's kitchen.
Generally, leaves have two parts: the leaf blade and the leaf stalk, called the petiole, which attaches the leaf to the plant's stem.
Without petioles, there would be no way for the rest of the plant to obtain the food products produced within the leaves (through photosynthesis) and the leaves would not be able to restock on water and other nutrients. So the petioles are like the pantry doors between the kitchen and the dining room. You with us on the metaphors?
Green leaves are colored by the pigment chlorophyll. Leaves may also be red or purple and are colored by different pigments.
Leaves "breathe" as well. They take in oxygen and give off carbon dioxide as well as take in carbon dioxide and give off oxygen. Confused yet? Click to our "photosynthesis" section.
Does the phrase, "What's stomata you?" mean anything to you? Probably not, but to a plant, stomata are the pores that allow carbon dioxide, oxygen, and water vapor to pass either in or out of the leaves.
So, you'd think leaves would be needed year-round. In evergreens, the leaves (mostly needles) are around all year, serving food up from their kitchens, but in deciduous plants, the leaves tend to drop in the fall and winter because these trees go dormant during the colder part of the year.
When spring comes around, though, leaves form from buds - and the kitchen's open for another season of service!