Tt- represents tall plant. This is because T is dominant so it is expressed and since t is recessive, it gets suppressed. (Where T=tall plant; t= dwarf plant)

tt- represents dwarf plant. This is because both the genes for determining the height of the plant are recessive and none of the dominating genes are present.

TT- represents tall plant. This is because T is dominant gene for height and both the dominant genes are present.