Phytomers are functional units of a plant, continually produced by root and shoot meristems throughout a plant's vegetative life-cycle. The phytomer unit originates at the shoot (or root) apex, and a typical phytomer consists of a node to which a leaf is attached, a subtending internode, and an axillary bud at the base of the leaf. Each component of a phytomer can continue to differentiate and grow. Increases in a phytomer can be measured using the rate of phyllochron (rate of appearance of leaves on a shoot). Related to the phyllochron is the plastochron, which is the rate of leaf primordia initiation. Since many more leaf primordia are initiated than leaves develop, the plastochron develops at a much faster rate (sometimes as much as twice as quickly) as the phyllochron.