Folk Art 

Folk art encompasses art produced from an indigenous culture or by peasants or other laboring tradespeople. In contrast to fine art, folk art is primarily utilitarian and decorative rather than purely aesthetic.

In these three pictures you see mostly tress, houses, leafs, anything nature like. In the last picture it is a farmer selling goods or food to make a living, back then most things where made by hand and nature was really all we had. Thats why this type of art stands for our country and reminds us of simpler times with creative nature paintings.