Cell elongation in internodal regions of the green plants takes place due to :-

Gibberellins promote elongation of the internodes. They cause bolting (sudden elongation of the internodes just before flowering) in rosette plants such as cabbage and beet.
Spraying sugarcane with gibberellins increases the length of the stem and the sugar yield; genetically dwarf plants grow to normal height when treated with gibberellins.
Cytokinins mainly promote cell division; ethylene inhibits elongation and promotes ripening and senescence; auxins (IAA) promote cell elongation mainly near the apex and in coleoptiles, but internode elongation of the whole stem is the characteristic effect of gibberellins.
So cell elongation in the internodal regions takes place due to gibberellins.