Palm Trees
Slushy cold drink in hand, I took a deep breath and looked up from my book. Rows and rows of beach chairs stretched out in front of the one I had claimed, their geometric arrangement accented by regularly spaced palm trees. Through the trunks, past the white sand, and between brightly colored bathing suits, turquoise waves sparkled in the sun.
I was grateful for the shade of the palms (I think they were coconut palms), since the warm afternoon sun would have surely boiled my winter-thickened blood and crisped my pale skin to crimson. But the trees bothered me, too. Usually on vacation (usually in the Boundary Waters), I contemplate the tenacity of gnarled jack pines whose resolute roots find the ingredients for life wedged deep inside bedrock cracks. How did the resort ever get palms to grow in pure, salty sand?
While my vacation in Playa del Carmen, Mexico, was filled with all the usual activities, I also experienced a fair amount of frustration. With little prior knowledge of tropical natural history (I tend to head north for trips), and no nature guides (paper or human) on hand, I felt lost. Next time I’ll do it differently, I resolved, but this time I still had to deal with the unusual experience of not understanding my environment.
Arriving home to internet access, I had many questions, but the problem of palm trees stood out. I knew they weren’t like maples or oaks, but why and how?
The essential difference is that maples, and many flowering plants that we’re used to, are dicots. When their seeds sprout, they produce a tiny pair of baby leaves, or cotyledons. Palms are monocots, and along with grasses, lilies, and asparagus, they only produce one cotyledon when their seeds sprout. The number of seed leaves is just the beginning—an easily visible character that actually indicates much deeper differences.
For one, the roots of palm trees don’t go deep. Instead of a sturdy taproot and woody lateral roots gripping deep into the bedrock crack, palm trees have a mass of small, non-woody roots that begin low on their trunk in the “root initiation zone” and extend to form a dense mat in the upper foot of soil. Instead of tapering toward the end, the roots of a palm stay the same diameter throughout their length. The roots visible around the lower trunk of many trees reminded me of a bowl of thick spaghetti noodles.
As it turns out, this root structure makes palm trees relatively easy to transplant, which explains how the resort could build itself a ready-made shade structure of mature trees. Not only are the root mats easy to manage, the trees have their own insurance against transplant shock.
Every year I cross my fingers when my tomatoes droop for their first days in the garden, as they struggle to get roots to water and restore turgor pressure to their cells. Palm trees can live off the water and carbohydrates stored in their trunks while the roots regrow.
That palm trunk arises from a single apical meristem, or cluster of undifferentiated cells where growth can take place. In a maple tree, meristems allow for branching, twig growth, and much more. A palm tree has no capacity for branching, and grows straight up, with its leaves also originating from the single meristem at the top of the stem. Old leaf bases cluster around the meristem in a bristly beard, which offers it some protection against cold, disease, trauma. If the apical meristem is damaged, the whole tree will die.
Maple trees also have a lateral meristem—the cambium—which grows new cells around the stem each year, thickening the trunk and forming the tree rings we know so well. Though stiff and sturdy, palm trunks aren’t truly made of wood, and the cells aren’t arranged in rings. Instead, its vascular system of xylem and phloem bundles is dispersed throughout the stem with no ring-like structure. The stem grows slightly wider and stronger, mostly through the thickening and strengthening of these cells as they age.
Because of the dispersed nature of palm trees’ vascular systems, they aren’t susceptible to girdling. In a maple tree, the vascular system is just under the bark, and if you cut it all the way around, water can’t flow, and the tree dies. With dispersed vascular bundles, water would simply continue to flow elsewhere inside the palm’s trunk. The tree might die of disease from such an injury, though. Palm trees don’t have the ability to grow scar tissue or repair damages, so any break in the bark-like cortex may allow in disease. In contrast, as maple sugaring operations prepare to pull their taps, the operators know that this year’s hole will soon fade under a healing scar.
The differences between sunny Mexico and spring in Wisconsin are extensive, and fun to think about. I’m glad to be home, but I do wish that the slush would stay placidly in my cup instead of falling from the sky.
For over 45 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI! Our new phenology exhibit: “Nature’s Calendar: Signs of the Seasons” will open May 1, 2016.
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