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adapt to these microenvironmental conditions, which may occur
randomly. Due to the non-specificity of molecules and thermal agi-
tation, dissociation and reassociation takes place continually in the
cells between different molecules. These events occur with certain
frequencies determined by the structure of the cells, the concentra-
tions of molecules and their speeds of diffusion (see earlier section
in ยง6.1). If one of the cells situated at the upper part of the colony
where growth has ceased (Fig. 16B) is subject to such stochastic
remodelling in its chromatin and this remodelling induces the
expression of new genes allowing it to use the resources which are
accessible to it (i.e. the metabolites and a minimal quantity of
nutrients), the growth of the colony can resume (light cells, Fig. 16C;
for an example of stochastic remodelling see Fig. 21). This 'proto-
organism' is then made up of two types of cell, corresponding to
two 'tissues' with different 'functions'. One type, made up of dark
cells, metabolises the nutrient and provides nourishment to the tis-
sue made of light cells, by supplying it with metabolites. Metabolic
cooperation is established between the cells which differentiate as a
function of their position in the colony.
Every time our proto-organism develops in an identical envi-
ronment the same structure is produced because it is subject the
same selective constraint. However, if that constraint changes, that
is to say, if the concentration of nutrient in the substrate changes,
the concentration gradients within the colony will likewise change.
The cells then have to adapt to these new conditions. The dark cells
will grow wherever there is an adequate quantity of nutrient.
The size of the two 'tissues' (dark and light) will therefore change
because the quantities of nutrient and metabolites available in the
colony will vary owing to modification of the gradients and, in
the end, the very structure of the heap of cells will be modified to
comply with these new gradients (Fig. 17).
This phenomenon of colony adaptation complies with the general
principle of ontophylogenesis in Fig. 15, except that it is produced
in the internal environment of the multicellular colony and concerns
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