Meristems are cells that divide, which is how plants grow. An apical meristem is a cell responsible for the primary growth of a plant, the lengthening of shoots and roots. Apical meristem cells are responsible for germinating seeds sending down roots and the stems of seedlings getting taller. Secondary growth meristems are responsible for the thickening of stems, trunks and branches.
Primary vs Secondary Growth
The tip of each bud and each root contains an apical meristem that conducts primary growth. Not all plants have meristems capable of secondary growth. These plants can not become wood. Grasses are an example of plants that can only conduct primary growth. They can grow taller but not fatter. Fruit trees can conduct secondary growth, so they have trunks and thick branches.
New branches, primary growth, are sometimes pruned to give shape to a tree or shrub and to prevent tangled interiors and disease. The person doing the pruning allows some buds to grow into shoots and thicken through secondary growth. Primary growth never stops, so shrubs and trees have to be pruned on a regular basis, usually in the winter when the plant is dormant.
Monocots and Dicots
Primary growth in the stems differ patterns in flowering plants that are monocots and those that are dicots. Monocots have flower parts in multiples of three; dicots have flower parts in multiples of four. The veins in the leaves in monocots run parallel to one another; the veins in the leaves of dicots form a net or web. Most flowers and vegetables are monicots. Woody shrubs, some vines and trees are dicots.
Primary Stem Growth
Each bud contains a meristem. The primary growth in the stems of monocots results in random, scattered vascular bundles. Monocots do not have secondary growth, so they are limited in how tall they can grow.
The pith is the middle of a dicot stem; the cortex lies just beneath the epidermis.
Meristem cells grow vascular bundles in a ring between the pitch and the cortex. Dicots do have secondary growth and can become trees and other woody plants.
Primary Root Growth
As the roots get longer, meristem cells begin to different differentiate. Some epidermal cells form root hairs, cells that absorb water and minerals from the soil. The primary growth of roots is often encouraged by potting before they are planted outside in the spring. Growing tomato seedlings indoors before they are planted in the garden is an example.
Young plants are also mulched to prevent growing plants from damage by the cold and so the soil can hold more water while the new roots are branching and young stems are getting taller.
Primary growth in animals is determinate. The embryo grows according to a genetic pattern; the offspring of animals, carrying the genes of both parents, grows in a predictable manner. Although plants grow according to genetic rules, the primary growth of new roots and shoots continues as long as the plant lives. That means the overall shape of a plant is not determined in advance--primary growth can send branches in virtually any direction.