Re: Why do palms have sponge-like wood and shallow roots?

Palms are monocots, and monocots do not have a vascular cambium and so 
cannot produce wood, which by definition is secondary xylem meaning it was 
produced by a lateral meristem, the vascular cambium.  Woody plants with a 
vascular cambium produce new xylem and phloem annually, and the 
accumulation of annual increments of xylem produces "growth rings" in 
woody stems.  So palms have no wood, but their stems can still be hard and 
strong, which is sometimes called "woody".  In the stems of monocots the 
vascular tissues (xylem and phloem) are organized into discrete bundles 
embedded in a spongy ground tissue.  The bundles are described 
as "scattered", meaning not in a ring, but their pattern is not random.  
The bundles have an outer layer of tough fibers, and the vascular bundles 
of several monocots (agave, coconut, banana) produce fibers used for tough 
cordage.  The vascular bundles tend to be widely spaced in the center of 
the stems and ground tissue is more abundant.  Toward the periphery of the 
stem the vascular bundles become more closely spaced until their fibrous 
sheaths fuse leaving essentially no soft ground tissue.  This construction 
makes the trunks of palms flexible and very strong for their diameter.  
Fishing rods woven of fiber glass or carbon fibers exhibit similar 
construction and flexible strength.  Palm stems and bamboo are the 
strongest stems that occur using the monocot pattern of construction to 
which they are constrained by their ancestry.

Monocots also have diffuse, fibrous root systems.  Many of the roots are 
adventitious, meaning they arise from the stem.  Fibrous root systems tend 
to be shallow and spreading rather than deep rooting.  Shallow, spreading 
root systems are good anchorage for flexible stems, particularly in sandy 
soils, but their chief advantage is that they are very efficient at water 
and nutrient uptake making a layer just beneath or just at the soil 
surface.  Many tropical plants have dense, shallow roots systems because 
soluble nutrients are quickly washed away in high rain environments.  As a 
result most tropical soils are poor in nutrients.

Some palms produce rather massive prop roots, and the stems of tropical 
stilt palms end up a meter or more above ground level on a spreading base 
of big, tough stilt roots.  One of my rain forest ecology students from 
Illinois State University decided to investigate the function of palm 
stilt roots.  She found that stilt palms were taller and had larger crowns 
in proportion to their stem diameters than palms lacking stilt roots.  She 
concluded the stilt roots allowed greater crown height with less stem 
construction.  At ground level the stilt roots branch into fibrous roots 
for anchorage and absorption.