The Matrix
Pictured here are two similar sections of the wood
of an English oak tree (Quercus robur), fractured to
reveal a radial view of the xylem. You can see large
vessels running up and down both sections—rays
transiting left to right— and broken fibers. Viewed
collectively, one gets a better conception of the
matrix of connections and interactions that occur
between wood cells. Each cell type is multi-faceted
and attendant to the needs of adjacent cells.
Wood is a very successful 'invention' of plants, occur-
ring independently at least several times in evolutionary
history. However, you do come across some academic
texts which propose that wood is 'optimized'—and,
more specifically, 'mechanically optimized'. But due
to the many functions that wood performs (support,
conduction, storage, defense), a tree's wood cannot
be optimized for any one of these specific functions.
Rather, it is an elegant compromise on the many
demands upon it as a tissue. This is referred to in studies
as 'trade-offs in xylem', of which sap conductance vs.
mechanical strength is probably the most studied.
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