Natural rubber latex (NRL) occurs in the latex vessels
of the bark outside the phloem and the milky latex is collected by tapping the rubber tree bark .
Sucrose from photosynthesis is translocated from leaves through vessels into latex vessels
. Salisbury and Ross  concluded that plants high level is limited by water availability.
Unlike hevea, which produces rubber particles in latex vessels
that are readily tapped, guayule accumulates rubber particles in its bark's parenchyma cells (Bonner and Arreguin, 1949).
In the book, Know Your Woods, by Albert Constantine Jr., the structure of rubber tree bark is discussed for its importance, "because it contains the latex vessels
which are the chief source of the world's rubber supply."
The serum constitutes the latex, which in turn is contained in specific latex vessels
in the tree.