A hypothetical explanation of how the sporophyte came to be the more prominent form in the life cycles of most vascular plants. It postulates that early vascular plants had gametophyte and sporophyte generations that were very similar in appearance, with prominent upright branching forms. Over time, the gametophytes became smaller and simpler, whereas the sporophyte got progressively more elaborate in form. Eventually, the gametophytes became retained within the sporophyte—the situation in modern seed plants. Hence, the modern sporophyte evolved through the transformation of an existing form. Compare interpolation hypothesis.