What happens when flower seedlings are crowded in their ecosystem, or flower pot?

look up shade avoidance.  Crowded plants get too much far red light,
as do plants in the shade.  In daylight the ratio of far red to red
light is about equal but far red light predominates in the shade. 
this cause the seedlings to elongate, have strong apical dominace. 
this tactic allows plants to overgrow or find a better quality light
source.  In extreme cases of crowding or shade the plants will flower
early in a last ditch attempt to have any progeny at all.