I am writing a story (well, developing one) in which a humanoid intelligently genetically engineered species is effectively immortal. The creature maintains its lifespan through a thick-shelled seed about the size of a child's fist which rests below the heart. Within the seed there are exact copies of the creature's DNA and a brain-like neural structure which preserves some memories.
Upon the creature's death, the seed sprouts in a large bulb (about 3m tall and 2m in diameter) which encases the new "resurrected" form of the creature with some of its previous memories preserved. The bulb opens when the humanoid is sufficiently developed and the life-cycle continues with a new seed forming during puberty.
(NOTE: The bulb begins growing when a number of factors - circulation, brain activity, temperature - convince it that the creature is in fact dead. It also releases a chemical which breaks down the corpse of the creature into more useful components which the plant can use to sustain itself.)
The seed, of course, is vulnerable to disease while it is inside the creature and it can be broken or damaged with sufficient force, which would probably kill the creature due to seed fragments causing internal bleeding. One disease causes the seed to germinate while the creature is still alive. Another uncommon mutation causes the seed to not develop at all.
I don't really care if the creature is actually two different life forms that sustain each other in some mutually endosymbiotic way.
How realistic is it for a complex organism to have two stages of life where one is a plant performing photosynthesis and the other being an animal similar in size and energy requirements to a human? Or how can it be made more realistic to achieve these qualities?