Anatomy
of
Palm Trees
Palm trees are pretty
sneaky. They have a
number of tree-like
characteristics. They’re tall
and have a main stem
resembling a trunk. The
stem is woody and tough.
Put enough palms
together and you can have
lovely palm forests. If you
look down at the roots or
take a close look at a
palm’s leaf-like fronds or
cut at a palm with a sharp
object you’ll notice some
distinct differences.
Look up at a palm tree and you’ll see that the
growth isn’t like other trees. The canopy of
the palm is restricted to the crown of frond
leaves surrounding the apical meristem.
Lateral growth and meristems don’t emerge.
If a second apical meristem emerges it leads
another trunk with another crown of fronds.
This gives palms a distinctive, paint-brush
shape. The single growing, meristem is
surrounded by overlapping leaf bases.
When you poke at the needles of your Christmas tree, you’ll reveal the green, cambium layer under the bark. If you did
this with a palm tree you wouldn’t find a green layer. Palm trees do not produce cambium. In fact, palms are incapable of
the ring-shaped secondary growth seen in other trees. Cut a palm tree down and you’ll see tiny circular vessels distributed
evenly throughout the trunk. These are vascular tissues, xylem and phloem. Stem cells lining these vessels produce
“anomalous secondary growth” to thicken young trunks but once the trunk reaches its maximum diameter this no longer
occurs. This keeps palm trunks narrow and stiff, perfect for supporting their frond crowns.
The drawback is that injuries to palm tree trunks
can never heal. Without a cambium to direct
closure over a wound, fungi, insects and other
pests can easily penetrate wounded trees.
The “bark” of the palm tree is not bark at all; it is
made of “sclerified” (hardened) cells left over from
the bases of previously shed fronds. This makes a
palm not unlike a column of reinforced concrete
with the vessels acting as rebar.
Pictured: A piece of rebar driven through a palm tree in a hurricane. Palm
trees are strong enough to withstand winds that would snap most trees during
hurricanes, but their distributed vascular tissue means this tree will never
close the wound.
Palm leaves are also different. They emerge as
primary growth from the meristem. The
youngest leaves are at the top of the leaf
crown. As the palm grows taller and older more
leaves are added to the top and older leaves
grow larger until they hit their maximum size.
When the leaves reach the base of the crown
they are cut off from the vascular system,
“abscising” their bases into new bark. Some
palms don’t drop their leaves cleanly, resulting
in “skirts” of dead leaves that dangle below the
crown. Growing like this means that you can
predict which leaves will drop from a palm tree,
prune them, and collect them before they fall.
The beautiful strangeness of
palms can be traced back to
the way palm seedlings grow.
When a palm first starts
growing it doesn’t have a full
set of leaves. Roots grow out
from the tiny, seedling
trunk. Each root adds new
vascular bundles, widening
the trunk and providing more
vascular tissue to support
leaves. These roots form a
shallow, fibrous network, not
unlike the roots of grasses,
onions and bamboo.
Unlike other trees which grow the root system and the shoot system at the same rate, a palm tree rapidly
expands the root system so that it can expand its trunk and begin to grow upward. In essence, a palm tree
grows in the same way that you might build a house. It establishes the foundation and then builds the
upper stories.
Why are palms this weird? As it turns out, palm trees
belong to a family of group of plants called monocots.
Monocots are one of the eight main groups of plants
within the angiosperms, the flowering plants. In an
earlier blog post I briefly mentioned that angiosperms
began to dominate the earth’s plant species at the end of
the age of the dinosaurs, replacing conifers as the largest
plant group.
Monocots are the second largest group by species,
containing 70,000 members including grasses, orchids
and palms. The largest group, eudicot, contains 175,000
species including oaks, apples, maples, sunflowers and
nightshades. Most “true” trees are eudicots, the rest are
conifers ( ie: pines and their cousins).
Palm trees are preserved in some of the earliest
flowering plant fossils that we have discovered. Palm
trees emerged 94 million years ago when dinosaurs
walked the earth. Some species, such as Nypa fruticans
and Acrocomia aculeata have been identified in fossil
pollen, making the modern species living fossils. This
means that palm trees diverged early from other
monocot species. Their global spread can be attributed
to the breakup of the continent of Gondwana into
Africa, South America, Antarctica, India and Australia.
The distinctive strangeness of palm trees is tied up in
their early divergence from other flowering plants and
their rapid (by the standards of geologic time) spread
over newly formed continents. Hopefully the next time
you pass a palm tree you give it a nod to honor its
pioneering, almost-grassy heritage.