While leaves soak up the Sun to convert light energy in chemical power, roots forage for water and soil nutrients. As root cells take in water, pressure builds.
Cells must contain these pressures to avoid bursting. Hence, root cells are held within a surprisingly strong wall based upon cellulose chains only a few nanometers in length.
Individual cellulose chains are not especially robust, but they are woven into microfibrils that are as strong as steel. Microfibrils are then embedded into a matrix of other sugar-based molecules to form a crystalline polymer that can contain intense pressures and enlarge at a controlled rate.
(Mechanical properties do not control cells. Plants would be out of control if cellular mechanical stresses dictated their behavior. Control is effected through distributed energetic intelligence.)
“Growth and form in plants are controlled by the precisely oriented expansion of the walls of individual cells.” ~ English physiochemist Lynne Thomas et al
“The cell has ample regulatory mechanisms to control wall formation.” ~ Dutch cytologist Anne Mie Emons & Dutch physicist Bela Mulder