Primary growth within a plant refers to the vertical extension of the plant from the roots locked in the soil to the highest tips. This provides stability and support against gravity for the plant. Secondary growth occurs only in woody plants and is the thickening of stems and branches as they continue to age.
As a plant ages, it increases its length at its meristems, where cell division takes place throughout maturity. All plants grow in this fashion in one form or another -- such as shooting apical stems at the plant roots or adding meristems throughout height of the plant body.
Some plants are able to increase their stem width through what are called lateral meristems. These extensions allow for increased transportation of water and sugar throughout plant cells. As the tree reaches maturity it will create a second meristem layer that will eventually develop into the plant's bark.
Primary Growth Plants
Although plants that fall under the category of monocotyledon can exhibit secondary growth, this is not very common and is relatively short. Since the seed leaf that develops is singular, monocots most often never produce the woody cell growth of secondary plants.
Secondary Growth Plants
Most commonly monocots, dicots and gymnosperm plants will develop the secondary growth traits found in plants. It is important to understand that although both are considered angiosperms or plants that fruit, dicotyledon trees grow their secondary cells at a much faster, larger rate than their monocotyledon counterparts. Meanwhile, gymnosperms which do not develop fruits will instead create seed distribution organs such as cones to ensure protection.
Atypical Secondary Growth
Monocotyledons that do have secondary growth are atypical. Rather than producing the vascular cambium within the plant to grow in width, they will thicken their cells around the stems and provide the plant with increased base support.