Primary vs Secondary Growth
Plant growth occurs at meristems. According to the origin they can be primary meristems or secondary meristems. Primary growth occurs at primary meristems, and secondary growth occurs at secondary meristems.
Primary meristems are apical meristem, intercalary meristem and intrafascicular cambium. Shoot apex is dome shaped with leaf primodia. There are axillary buds, nodes and internodes. The apex is divided into 3 regions. At the very top is the region of cell division where only cell division takes place. Behind this is the region of cell enlargement. Behind this region is the region of cell differentiation where each cell becomes fully specialized for its particular function.
Three types of basic merisematic tissues occur at the stem apex. They are the protoderm, procambium and ground meristem. The procambium is a series of longitudinally running strands. In a cross section, they appear in the form of a broken ring. From the procambium, primary vascular tissues are produced. The first formed cells are protoxylem to the inside and protophloem to the outside. The protoxylem typically has only annular and spiral thickenings of lignin allowing elongation to take place. Other thickenings occur only after elongation is completed. The cavities of the protoxylem are much smaller. Soon protoxylem and protophloem become inactive. Their function is taken over by later developing metaxylem and metaphloem.
After primary growth, lateral meristem becomes active and results in the formation of secondary permanent tissues. This is called the secondary growth. The lateral meristems are the lateral vascular cambium and cork cambium. They are formed only on dicots. In monocots, there is no cambium. Therefore, there is no secondary growth. As a result of secondary growth, there is an increase in thickness or girth in stems and roots. In the stem, the intrafascicular cambium becomes active and cut off cells to the outside and inside. The cells that cut off to the outside become secondary phloem. The cells on the inside become secondary xylem.
In the meantime, parenchyma cells between adjacent vascular bundles also become meristematic and form the interfascicular cambium. The intrafascicular cambium and the interfascicular cambium join to form a cambial ring which is the vascular cambium. The interfascicular cambium cuts off cells to the outside and inside. The outside cells become secondary phloem and inside cells become secondary xylem. The cambium contains fusiform initials and ray initials. Fusiform initials give rise to normal xylem and phloem. Ray initials give rise to parenchyma, which forms medullary rays. As the numbers of cell layers inside increase, the cells outside become compressed and this results in the formation of another lateral meristem in the outer layers of cortex. These become a ring of cork cambium. Cork cambium cuts off cells to the inside and outside. Cells that are cut off to the outside become suberized and form the cork. Cells that cut off to the inside form secondary cortex.
What is the difference between Primary Growth and Secondary Growth?
• Primary growth occurs at primary meristems, and secondary growth occurs at secondary meristems.
• Primary growth results in the increase of the height of the plant whereas, the secondary growth results in an increase, in the girth of the plant.