Scientist categorise Palms in the Monocot family.
Scientists put palms in the monocot family (monocotyledoneae), which is related to grass, corn, and bamboo, not a tree. A palm uses a different system than trees for transporting water through its mass by using clusters of phloem and zylem packed in a sheath. Another argument is that palms do not produce bark or a cambium layer and do not have the same incredible healing process as trees. The palm’s trunk is spongy and hard as it expands but palms do not produce growth rings like trees.
Scientists determine types and families of plants based on how they operate on the inside and not on the appearance. Others argue that palms are trees for obvious reasons, they look like trees: palms and trees are tall with mainly brownish trunks and green foliage.
Even amongst trees, Palms are different!
Whether palms are considered a grass, tree, or stalk, makes no difference to many plant lovers because Palms are in a category all their own. Palms can be tall, elegant, fruitful and all with gorgeous palm fronds. Palms are a special species of heat loving and coastal air breathing organisms that provide food and materials to man and animal. For humans, these plants inspire strength and tropical nostalgia.
There are similar debates around the world for Bamboo. Is 'bamboo' a ‘tree’ or ‘grass’?
Here is an interesting article I found while browsing: