My Catasetum also has these bumps. It's physiological, and probably edema seeing how much water these plants are given. When a substrate is constantly (or very often), edema is much more likely to happen. Edema is caused by a combination of high root pressure (plant taking up lots of water) and periods of low transpiration rates : think cool gray day, night/day temperature fluctuations... This leads to water pressure building in the leaf cells, and this the water can't be shed fast enough through transpiration, it leaks out of the cells forming the bumps.
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Camille
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