Friedman, W. (2013). One Genome, Two Ontogenies Science, 339 (6123), 1045-1046 DOI: 10.1126/science.1234992
All plants have two distinct life stages/generations. The gametophyte has one set of chromosomes per cell and the sporophyte has two sets per cell. In many plants, including the bryophytes, ferns, and seed plants these generations have wildly different forms, as distinct as the personalities of Dr. Jekyll & Mr. Hyde. The wild part is that the main genetic difference between the two generations is just dose. One has 1 set of chromosomes and the other has 2, but their sizes, shapes, and numbers of cells are amazingly different! Check out some examples below. (These pairs are not necessarily of the same species. I just wanted to pull together some examples for a visual.)
Size: Shorter than your pinky fingernail.
Size: As tall as your pinky finger.
Size: Fits on the end of your finger.
Size: Tips of the leaves at or below hip height.
I borrowed this photo from Emily's fern blog.
The evolution of this alternation of generations has long interested scientists. How is the difference between these two morphologies controlled? Well a piece of this puzzle has been recently figured out. Researchers report that they have discovered a gene, KNOX2, that suppresses gametophyte morphology. When this gene is turned off in a moss sporophyte the plant starts to grow but does not develop into a mature sporophyte, but instead grows into the shape of a leafy gametophyte plant. It is an elegant study and a great addition to our knowledge about the genetic control behind the transition between these two distinct generations!