ABSTRACT
In the young and active tissues of trees and other plants (341, 342), the mechanical strength of their cell walls is provided by the cellulose-hemicellulose network (343), while the pore size is controlled by the pectin network. The two networks act together to provide a structure which is mechanically strong and resistant to penetration, yet retains the ability to stretch under controlled conditions and permit cell expansion. In the lignified fibres and tracheary
elements of the vascular system (344), the mechanical strength of their cell walls is greatly increased, while porosity is reduced further by the loss of water from the cell walls.
