my question is, do plants have brain? if no then how can they do all their functions, if yes why can't we see it?

No, plants do not have brain. A plant can react and respond to stimuli, such as light, gravity and touch.

Plant shows two types of movements- one dependent on growth and the other independent of growth.

A growth movement of a plant part in response to an external stimulus in which the direction of a stimulus determines the direction of response is called tropism. Thus tropism is a directional movement of the part of a plant caused due to its growth.

In nastic movements the movement of plant part is neither towards the stimulus nor away from the stimulus. This movement is not a directional movement of the plant part with respect to the stimulus.

Different plant hormones help to coordinate growth, development and responses to the environment.