Complex sentence: This change occurred after discovery of a variety in the mid 20th century which ripened uniformly which was then widely cross-bred to produce attractive red fruit without the typical green ring surrounding the stem on uncross-bred varieties.
Split into simpler sentences: This change occurred after discovery of a variety in the mid 20th century that ripened uniformly. This was widely cross-bred to produce red fruit without the typical green ring around the stem on uncross-bred varieties.