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.
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