Cells are always communicating with each other.

Gradients of differing chemicals which may counteract or work together can create "boundaries" between types of cells. These "chemicals" interact (sometimes directly with the chromosome and other times through cascading signals that eventually come down to some kind of interaction with the chromosome) in very, very complex ways to control gene expression ... most of these interactions were just scratching the surface of. I hope this kind of answers your question.

Cells do not reproduce constantly. Cell replication in multicellular organisms (including humans) is controlled by a large set of genes that produce hormones both locally and globally. Cells react to these chemicals and signals from other cells in deciding what specialized function to perform and when to divide.

In animals, the most famous of these genes are the HOX complex, which determine the patterning of tissues along the length of the body, but there are many others.

Plants use very different hormones to determine their growth, having become multicellular from different unicellular ancestors from animals, but they, too, use hormones to control their development. I do not know how multicellular algae (e.g. kelp), or fungi, do this, but I presume their methods are similar.