Board Thread:Naruto Discussions/@comment-1411686-20140713052356/@comment-25065646-20140714120858

That is cause the plant-flesh/Zetsu mass works biologically different from humans: It doesn't need organs, bones, muscles, skin and stuff, all the cells just fulfill the same vital functions (they just 'live' for the sake of it) without cell specialization - thats why all body parts look exactly the same (white stuff), and the actual shape of the zetsu is irrelevant, the white thing can form anything from venus flytraps to underground roots, spare body parts for others or even merging many zetsu bodies...

As for their origin, i would say Banbutsu Sozo/ Yinyangjutsu: With chakra, a matter is created that possesses vital energy (Yang) in its most primitive form (sth like the concept of 'growing' or 'dividing/multiplying' when receiving energy), as well as a consciousness/soul (Yin) in its most basic form (primitive cognitive abilities - they don't seem to be that smart and mostly function as underlings/tools for others).