So, the vertical shoots are the plant's effort to get a flower growing above the water, for sexual reproduction. Failing that, it just switches gears and reproduces without benefit of a partner? That is what appeared to be happening, but none of those vertical shoots ever made it all the way out of the water for me as best I remember.If it makes it out of the water, it will flower and may produce emersed small plants. If it stays submerged, small submersed plantlets will result.