Like any plant, mosses grow where they can obtain adequate light and water for photosynthesis (and take up sufficient nutrients). For an epiphytic moss, the sunward side of a tree offers more light and therefore also more heat, which increases evaporation and need for moisture. In the right circumstances there's still enough light on the shadier side of a tree but not quite enough moisture on the sunnier side and a moss population occupying one side of a trunk but not the other results. As you might expect from this description, it's a fairly fragile circumstance and is easily and routinely overridden by variations in stemflow (downward flow of water from precipitation) along a trunk or branch based on its shape, angle, and what's above it.The old trick of finding your way in the wilderness using, the rule of, moss always grows on the north side of tree trunks, isn’t really a very good trick at all.
Thank you, they are individual cells. In the earlier 200% crop the larger squamous cells along the leaf margins---which P. insigne shares with a few other species---are also visible. I had one chance near the end of the last rainy season to try to acquire imagery at higher magnification but weather conditions proved unsuitable. I'm looking forward to trying again in the next rainy season, though the cells will be only about 20 pixels wide at the upper limit of what I can use noninvasively in the field. Some of Heribert's changes to Picolay since the rainy season should also improve the stacking results somewhat.Amazing! Are what we see individual cells?