Many Siphonales are encrusted of planes and a multicellular individual.
These arise from the axial cell, and are multicellular and branched.
The thallus may be unicellular or multicellular.
In these cases, certain cells of a colony of unicellular plants or of the filaments of multicellular plants enlarge greatly and thicken their wall.
It is true that in the unicellular plants all the vital activities are performed by a single cell, but in the multicellular plants there is a more or less highly developed differentiation of physiological activity giving rise to different tissues or groups of cells, each with a special function.