Maybe some epigenetic mechanisms cause the significant morphological differencies in plants. They are affected by plants characteristics and corresponding environments.

Yes they do. In animals too. Check out some imprinted genes and there associated phenotypes.

Many mutations in epigenetic pathways produce striking developmental phenotypes. Check out mop1, mop2, rmrs, and ufo1 in maize and fwa, drm, cmt in Arabidopsis. These are only a few examples.