Question

are the hairy seeds light and flat . why

Answer 1

Seed dispersal is the movement or transport of seeds away from the parent plant.The seeds are dispersed by mechanisms like gravity, explosion or by agents like wind, water and animals. 
1) Gravity: Fruits of plants like mango, apple, etc. fall down due to gravity and later the seed inside the fruit germinates into a new plant.
2) Explosion: Fruits of some plants like castor and balsam explode releasing the seeds. These seeds disperse to other places.
3) Wind: Light seeds of grasses, hairy seeds of aak (Madar) and hairy fruit of sunflower get blown off with the wind to far away places.
4) Water: Seeds or fruits of some plants have spongy or fibrous outer coat. This helps them to float on water. Thus, they can be dispersed by water to far away places. For example, coconut. 
5) Animals: Animals eat fruits and the seed is thrown out of their body through their excreta. This helps in dispersion of seed. Also, seeds of some plants like Xanthium and Urena are spiny. Such seeds get attached to the bodies of animals and are carried to distant places.

Answer 2

Some seeds are hair and light because that they can easily disperse by wind, water animals