|Coconut palms commonly grow along tropical coastlines|
in a zone of salt-tolerant vegetation, but not directly in
saltwater. Coconuts may fall onto the beach and be carried
away by high tides, but not usually directly into the water.
While these factors may contribute somewhat to the ultimate shape of the mature palms, I'd like to point to a more fundamental factor: the phase of development that all palms go through after germination called establishment growth. This is something peculiar to tree-like monocots, which have neither a taproot system nor layered secondary growth. In dicotyledonous trees, stem thickness increases gradually throughout the plant, and the root system branches to keep up with it. (See The Root of the Root Problem)
|The production of s series of aerial stilt |
roots allows this palm to increase the
thickness of its stem while growing upward.
|The horizontal establishment growth of the coconut|
palm stem will proceed to the right in this example.
Photo by Vencel, CC attribution 3.0.
It appears that the coconut palm follows a third model by establishing its basal thickness along with a mass of adventitious roots, through a period of condensed horizontal growth, with the lower side of the trunk remaining in contact with the soil. Once it achieves full thickness, the trunk gradually curves upward to achieve a more-or-less upright growth, though it often continues to lean. Since a coconut seedling sprouts out of one end of the coconut, the direction of the horizontal growth phase and the eventual upward curve, will depend on which way the coconut is facing when it sprouts - not so much for any functional reason.