These "magical corn plants" are reflective of a wider belief! Specifically, it's reflective of the belief in coessences, which are also referred to as Nagual, Tonal, Animal Alter Egos. The reason for the various names is that the concept wasn't properly understood understood until recently.
What are coessences/animal alter egos (I'm going to be switching between the two words in this answer)? Alter Egos are entities, usually animals, with whom a human shares consciousness with. The specifics of this belief vary from region to region, however, some common features include:
- When the human falls asleep, they will dream from the perspective of their animal alter ego.
- Accounts aren't consistent about whether the animal alter ego is "real", i.e. whether it is a physical animal.
- Some people can control their alter ego.
- Celestial phenomena, such as lightening, can serve as alter egos.
- If the human or the animal alter ego dies, the other will also die as well.
The last point--the connection between the human's life and the animal's life--is what plays a central role in the Popol Vuh. The corn plants are alter egos of the Hero Twins, which is why their lives are linked.
(If you are interested in learning more about coessences/alter egos, the paper "The Way Glyph: Evidence for "Co-essences" among the Classic Maya" provides an overview of the concept and is available online.)
Recent research suggests that coessences are a much wider phenomenon than I initially depicted. Specifically, in addition to these alter egos, people in Mesoamerica also form coessential relationships with practically anything they have a close relationship with. Examples include a farmer forming a coessential relationship with their tools, and a musician forming a coessential relationship with their musical instrument.
This is a weird belief from a western perspective. If you are interested, I wrote a blog post that explains why beliefs like coessences are actually rational beliefs to have.