Question

Which occurs in the roots of dicots: primary or secondary growth?

Answer 1

Hint: Most of the plants grow indefinitely through a combination of cell growth and cell division. This continuous growth throughout their lives is possible due to the presence of meristematic cells. Meristematic cells are undifferentiated cells that are capable of dividing into new cells or tissues. This type of growth by constantly adding new cells is known as an 'open form of growth'.

Complete answer:
On the basis of position, there are three types of meristematic tissues in plants, and they are apical meristem (tip of the root and shoots), intercalary meristem (intercalary position of leaves and internodes) and lateral meristem (lateral sides of stems and roots). Due to the meristematic activity of primary meristems such as apical and intercalary meristems, the plant undergoes primary growth and increases in length. In contrast, with the activity of secondary meristems, such as lateral meristems, the plant undergoes secondary growth and increases in girth. Secondary growth occurs as a result of the cambial ring formed in the stems and roots of gymnosperms and some angiosperms (dicotyledons).
Vascular cambium is the meristematic tissue that is responsible for the formation of secondary phloem and xylem tissues.
 In dicotyledon stems, intrafascicular cambium between the primary xylem and phloem together with cells of medullary rays form interfascicular cambium, which continues as a cambial ring. The ring of cambium divides and cuts off new cells both towards the pith and towards the periphery. In contrast, the vascular cambium in the root of dicotyledon (dicot root) is completely secondary in origin. It is formed from the parenchyma cells located just below the primary phloem,cells of single-layered pericycle tissue lying above the protoxylem.
This doesn't mean that stems and roots of dicotyledons only have secondary growth. Because in plants, secondary growth occurs in addition to primary growth. Primary growth is responsible for the primary plant body formation, and it takes place in the early stage of the lifecycle of all plants. Monocotyledons or monocots lack lateral meristems, so they display only primary growth and do not undergo secondary growth.

Note:
In plants, the lateral meristem is generally not present from the very beginning of life. They appear later in primary meristems. Fascicular vascular cambium, cork cambium, interfascicular cambium are examples of lateral meristems (secondary meristems). The cambial ring formed from vascular cambium becomes meristematic to form a secondary xylem on its inner side (towards the pith) and secondary phloem towards the outside.