Storage & Survival in the Palm Family

Palm trees have had to develop some creative strategies to survive drought and cool climates.

My recent work with Botany One writing news briefs for newly published botany research has had me reading a lot of scientific articles, and it just so happens that twice in the last couple of weeks, they’ve dealt with new research on palms. Having worked mostly on legumes as a researcher, I’d never given much thought to these fascinating plants, beyond the fact that they look good on a tropical beach. But there’s a lot to them, as I’ve been finding out lately, and I thought I’d write a little post to share what I’ve learned.

Palms are members of the Arecaceae family, which has around 2600 species spread through the world’s tropical and subtropical regions. They are monocots, like grasses or orchids. The arborescent, or tree-like, members of the palm family – what we’d call a palm tree – are unique among tall trees in that they have no vascular cambium. This is the cell layer in the trunk of a tree that allows it to widen year upon year, and is also responsible for tree growth rings. If you cut a palm tree down, there are no annual rings in its trunk, because that trunk didn’t continue to grow. (It’s also why their trunks look so cylindrical, as opposed to the usual tapering you see in a tree trunk.) This imposes some interesting restrictions on the tree. For instance, the tree’s vasculature cannot be renewed, as it is in other trees. The cells making up the tubes that transport water and nutrients through the trunk must last the entire life of the tree, which can be upwards of 100 years in some species.

While more than 90% of palms are restricted to tropical rainforests, some also occur in cool, high altitude regions and arid deserts. Unlike most of the plants that live in cool and dry habitats, palms lack dormancy mechanisms, such as dropping their leaves, that would help them to survive these conditions. What’s more, like all monocots, palms have no central tap root that will allow them to reach deeper reserves of soil water. So they’ve had to develop some creative survival strategies. Under drought conditions, which some palm trees endure regularly due to their arid habitat, the greatest danger to a plant is vascular embolism. This happens when the water column that runs through the plant breaks because there’s not enough water, and air bubbles form and expand through the xylem tubes. Once a certain amount of air is present in the tube, it will never function again and the tissue it feeds will die. To help counter this, palm trees store water in parenchyma cells adjacent to the xylem, so that when an embolism is imminent, more water can be shifted into the column. Their anatomy also encourages embolisms that do happen to happen closer to the tip of the leaf, as opposed to near or inside the trunk, where they would do greater damage. 

Palms have a neat survival trick to help their seeds germinate in the low temperatures. Most palms store oil in their seeds to provide sustenance for the seedling when it germinates. This is usually high in saturated fats, which aren’t liquid at low temperatures. This would mean that seeds either couldn’t germinate under cool conditions, or would risk starvation if they did. New research has found that palms growing in cooler climates have evolved their own oil blend rich in unsaturated fats, which are liquid at lower temperatures, to help their seeds thrive in those habitats.

Speaking of oil storage, palms have been hugely important to human beings since before the dawn of civilization, all thanks to those oils, which can occur in both the seed and the fruit, and provide a high calorie food source. The best known is coconut, Cocos nucifera, with its greasy, delicious seed, which we eat as a fruit. In fact, the fruit of a coconut isn’t a nut at all, it’s a drupe. But while coconut is perhaps the most familiar palm food, the most economically important is certainly the oil palm, genus Elaeis. The oil that comes from this palm is high in saturated fat, making it useful for deep-frying (and bio-fuel), if not the best for your health. The use of palm oil is controversial, because of the environmental and human rights abuses linked to its production, yet production is ongoing in regions of Africa, Asia, and the Americas. Outside of their oil production, palms are also the source of dates, palm syrup, carnauba wax, and wood.

Recent research has found that the seeds with the greatest oil storage are all grouped in the tribe Cocoseae, but that palms with oily fruits and moderately oily seeds abound throughout the family, suggesting there may yet be nutritionally and economically valuable species that haven’t been discovered, though whether the further exploitation of these resources is a welcome development is debatable. 

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