Another answer is: They're not - at least in some plants: https://en.wikipedia.org/wiki/Valonia_ventricosa https://en.wikipedia.org/wiki/Valonia_ventricosa ht

Another answer is: They're not - at least in some plants: https://en.wikipedia.org/wiki/Valonia_ventricosa https://en.wikipedia.org/wiki/Valonia_ventricosa https://en.wikipedia.org/wiki/Acetabularia https://en.wikipedia.org/wiki/Acetabularia