Question

What is lateral merstem? in book it is not explained well

Answer 1

In vascular plants, the lateral meristem is one of two meristems in which growth occurs in order to increase stem growth. A meristem is the region of plant tissue found on the tips of roots and shoots. It is where cell division occurs to produce new growth.
A lateral meristem helps the plant grow laterally. As humans grow, they periodically need to replace clothing in order to accommodate the growing body; a lateral meristem does this for a plant. In order for a plant to grow outward a couple of things need to happen. The outer layer needs to be replaced and a plant needs to able to provide nutrients to new tissue.
A plant does this by having two lateral meristem mechanisms. The cork cambium replaces the outside layer of a plant, while the vascular cambium transports the nutrients and water to the growing stem.