It turns out that some angiosperms felt homesick after a couple hundred million years of being away and decided to go back home to the ocean (I guess they felt like Moana). For a type of plant so specialized for life on land (e.g. sophisticated water transport system, seeds, negating the need for water for reproduction to occur via the development of pollen) it seems strange that they would return to the environment from which they differentiated so greatly from. This of course poses many challenges as many of the adaptations to life on land would prove detrimental to life in the sea, being submerged in water among other things.
I particularly wondered how they overcame the challenge to reproduce underwater - how did the plants undergo pollination in the water? The concept seemed a bit strange, especially considering how water is a lot less dynamic than air, particularly in stagnant or low flow water conditions - the pollen either never being disrupted for dispersal or just sinking to the ocean floor. The answer though is quite simple.
Negating the more complex fluid dynamics description which I do not comprehend very well, the underwater plants pollinate basically by... drum roll please... water pollination (aka hydrophily). Initially, I was expecting something a little more involved. Water pollination is somewhat analogous to the wind pollination that occurs on land between plants. In particular, grasses tend to wind pollinate. Can you guess what type of plants tend to water pollinate? Sea grasses.
Water, however, transports pollen grains somewhat differently than air. This has led the sea grasses to evolve some particular mechanisms to help maximize pollen and stigma contact events. Some pollen grains of sea grasses have lost their outer thick layer known as the exine which the absence of may help the pollen grains stick to and wrap around the stigma more often and efficiently. The plants may also grow in particular patterns which maximize pollen distribution and encounter events, some literature describing regular spacing between the plants that essentially creates a concentration gradient of pollen down the row of plants. The sea grasses have also been observed to occupy different areas relative to one another, shallow and deep sites, which can also help maximize pollination.
Something really cool also happens on the microscopic level. As current flows through the sea grasses, carrying pollen, there is a reduction in the speed of the current carrying the pollen as it approaches the flower arrangement (which is postulated to be because of the extension of the stigma outwards), disrupting the flow. This reduction in speed is thought to help direct pollen grains to the stigma, acting similar to a filtration device. The reduction in speed may also make it easier for the pollen grain to stick to the the stigma.
Of course, other aspects of these plants have changed as well, many have lost their UV protecting pigments and adapted their light capturing systems to be more efficient for the differential availability of light underwater as opposed to that on land. Sea grasses have also lost stomatal genes and regained some lost abilities/functions such as the ability to live submerged and in a high salinity environment.
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