Question

Which meristem helps in increasing the girth?

Answer 1

Meristematic tissues are made up of undifferentiated mass of meristematic cells that are actively capable of dividing, thus forming plant structures and specialized tissues. Based on the location and position of meristematic tissues, meristem can be classified into two types:
Apical Meristem
Secondary/Lateral Meristem.

Complete answer:
The Apical Meristem produces the primary tissues of the plant body and are responsible for primary growth. The parts of these tissues occur at the apical region of the root and stem (Root Apical Meristem and Shoot Apical Meristem).
Sometimes, while a stem is elongating, some cells from the meristematic region are left behind. They occur between the matured tissues and help in regenerating the lost parts that get damaged due to wear and tear (e.g., in grasses). This type of meristem is known as Intercalary meristem.
Also, the left out meristematic tissues from the SAM or RAM region sometimes develop into an axillary bud. Such buds are present in the axils of leaves and are capable of forming a branch or a flower.
The Secondary or lateral meristems are a result of dedifferentiation of the permanent tissues, i.e., the tissues that get converted back to meristematic tissues. These tissues occur in the mature region of root and shoot; help in secondary growth like development of woody axis, or increase in girth of the stem and trunk of the tree. These are also known as cylindrical meristems. E.g., Cambium.

Note:
A tissue may be defined as a group of structurally similar or dissimilar cells that aggregate together and perform a common function. Meristematic tissues are the ones that serve the common function of growth in a plant.
They are capable of dividing, occur at specific locations and participate in primary (done by apical meristem) and secondary growth (done by lateral meristem) of the plant.