Tucked deep inside every tree's seed is another tiny tree waiting to sprout. This tree-to-be is actually called an embryo, and it is surrounded by tissue that contains rich nutrients, including carbohydrates and proteins. The exact blend of nutrients varies depending on the type of seed. The embryo has tiny leaves, called cotyledons, that absorb the nutrients as it needs them to develop. The embryo and nutritive tissue are both surrounded by the seed's shell, which protects the seed until the time is right for it to sprout, or germinate.
Once a seed has been dispersed from the tree (either by the wind or animals), it waits for the right time to start growing. Light-sensitive chemicals in the seed's coat can sense if there is enough light for the seed to grow. If there is, the seed's coat will start to absorb water, which triggers the process of germination. The seed will continue to absorb water until the coat bursts open. Thus, seeds need both water and light in order to germinate. Many tree seeds do not get enough light to ever germinate, as they drop directly under the mother tree where it is too dark and shady for growth.
The first piece of the tree to emerge from the seed is actually an early root. It is called a radicle and it will eventually become the tree's roots. It grows down from the seed, looking for minerals and more water. Second, a sprout emerges from the top of the seed, reaching for the sunlight above the surface of the ground. Once it pokes its head above the ground, it will develop early leaves, which are often called seed leaves. This is a seedling tree, and it is very fragile. Often they are crushed by animals, or fail to thrive due to lack of sunlight. If the seedling does survive, it becomes a sapling, or a young tree. At this point, the roots of the tree are spreading rapidly, and the taller the tree, the wider the roots will be spreading. Saplings are trees that are around 3 to 5 years old. After that, the tree continues to grow until it becomes an adult tree.