I'm a high school student from India and here in our prescribed biology book(NCERT), it is written that pollen grains of aquatic plants have a mucilagenous coating to protect them from wetting.
My doubt is that they already have the most resistant chemical, the sporopollenin, on their exine to protect them. It cannot be degraded by acids, enzymes, high temperature, alkalis. Then what is the need of another layer of mucus to protect even the most resistant of nature's chemical? Or is it that sporopollenin is absent in aquatic plants? I cannot find anything good to satisfy my doubt out there on Google.