How a plant grows:
Primary growth
All plants grow from the tips of stems and roots. This type of growth causes stems and roots to elongate. This also causes new leaves to be produced at the tip of the stem as the plant grows. This is called primary growth.
[Advanced topic alert: you will not be explaining cells to K-5 students]: Terminal buds, axillary buds, and the tips of roots contain "apical meristems", balls of cells whose sole purpose is to divide to produce new cells. When these cells divide, some of the daugher cells remain in the meristem and others (the ones away from the tip), elongate and become new cells of the root or shoot. The apical meristems at the root tip are protected by a root cap; The apical meristems at the tips of stems and in the axils of leaves are often protected by small leaves. The terminal and axillary "buds" of the shoot include the apical merstems and their small leaves.
A time-lapse video of a shoot growing and producing new leaves can be found online at the University of Indiana's "Plants in Motion" website. A link to this video (http://plantsinmotion.bio.indiana.edu/plantmotion/vegetative/veg.html) will open in a new window. Select "rosette growth" to view stem growth and leaf production. If you have a very slow internet connection, you may not want to try this at home. This "Plants in Motion" website, by Dr. Roger Hangarter, has some excellent time-lapse videos showing growth, germination, and responses to the environment. It can be found at http://plantsinmotion.bio.indiana.edu.
Very early growth of a seedling will be discussed below, when we explore the parts of the seed and the process of seed germination.
Secondary growth
Some plants become thick and woody. These plants increase in girth, or thicken, by using a different type of meristem, a sheet of meristematic cells that exists between the bark and the wood. This layer is called the vascular cambium. It produces new xylem (water-conducting tissue, which is the wood) to the inside, and new phloem ("food"-conducting tissue, contained in the bark) to the outside. A labeled picture of these zones can be found at http://botit.botany.wisc.edu/Resources/Botany/Secondary%20Growth/Stem/Woody%20Section/Inner%20outer%20bark.jpg.html
If you strip the bark off a tree, the bark will peel off right at the vascular cambium. However, since the sugar-conducting (or "food"-conducting tissue) is in the inner bark, if you remove all of the bark around a tree, what will probably happen to the tree?