My school textbook briefly mentions a certain "Histogen Theory" (von Hanstein) of plant growth/development. However, I can't find any authoritative source on this matter online.
According to my book, as well as this site, the Histogen Theory is only used to model the development of the shoot (stem) apex.
But according to the definition on Merriam-Webster, the Histogen Theory is applicable to both the shoot and root systems.
Is the Histogen Theory applicable to only shoot apices (as some sources suggest), or does it instead, apply to both shoot and root apices?