Primary Growth Versus Secondary Growth
For many plants, including most monocots, actively dividing cells occur only at the apical meristems producing growth that increases the length of a shoot or root. This kind of growth is called primary growth, and the tissues that develop from this growth are primary tissues. Thus, primary xylem and primary phloem refer to vascular tissues originating from apical meristem growth.
Other plants, like conifers and the woody dicots, undergo secondary growth in addition to primary growth. Whereas primary growth extends the length of plant parts, secondary growth increases their girth, or lateral dimension (to the side) and is the origin of woody plant tissues. Secondary growth occurs at two lateral meristems, the vascular cambium and the cork cambium. These cells are meristematic, capable of dividing and producing new cells throughout the lifetime of the plant. The tissues that originate from the vascular cambium are the secondary xylem and the secondary phloem. The cork cambium gives rise to periderm, the protective material that lines the outside of woody plants.