Moss will grow anywhere it can
SeacoastOnline.com
The red arrow on the compass points north as moss is spotted growing on the north side of the tree. Despite common perception, this isn't always necessarily true.Sue Pike photo
SeacoastOnline.com
The red arrow on the compass points north as moss is spotted growing on the north side of the tree. Despite common perception, this isn't always necessarily true.Sue Pike photo
I just got back from a hike during which we spent a good deal of time discussing where moss grows on trees. The obvious answer is that moss is a shade-loving plant and so it grows on the shady side. But the myth that moss grows only on the north side of trees and can be used to navigate in the backwoods persists.
Since we had compasses with us we decided to test this idea and checked the compass bearings on all moss we could find growing on trees. Of the 36 trees we checked, 22 had moss growing on the north side (anywhere from northeast to northwest), 12 had moss growing on the south side and two trees had moss growing on all sides. These numbers are, of course, statistically meaningless since we checked so few trees and our approach lacked any sense of scientific rigor. However our haphazard sampling did confirm that moss can grow on any side of a tree.
Moss is going to choose any surface that is shady enough for it to thrive. Too much sun can actually inhibit growth in a moss, which is weird to think about since it is a plant that relies on the sun for photosynthesis.
There are two major reasons that most mosses need to grow in the shade, both have to do with water. Moss is in a group of primitive plants called bryophytes; this is a transitional group, somewhere between aquatic plants like algae and terrestrial (land) plants like flowers and trees. They lack a vascular system, so they cannot pull water out of the ground through roots or transfer it long distances in their bodies. This limits their size. They do not flower and therefore do not produce seeds; instead they rely upon water to carry their sperm (which can swim just like ours) to the eggs. So, moss will tend to grow in the shade, which reduces evaporation of water, and also low spots or depressions where water collects.
Many moss species have further adaptations that help them hold onto extra water: they usually grow in dense colonies that limit evaporation and encourage water retention. Some are very fuzzy, increasing the surface area of the leaf for additional contact with the surrounding water. Some species, like sphagnum moss, have both living and dead cells (called hyalocytes) that can absorb enormous amounts of water. Hyalocytes have cell walls that are strengthened with fibers that help keep the cell from either collapsing or ripping apart when filled with water. As a result, sphagnum can hold up to 20 times its dry weight!
Our study above, in addition to real scientific studies, indicates that moss does usually grow on the north side of trees. Due to the tilt of the Earth, the sun is always slightly to our south (in the Northern Hemisphere) and so the shady side is often the north side.
Which brings up the obvious question, does moss tend to grow on the south sides of trees in the Southern Hemisphere? Yes it does! However, I would not call this generality about moss growth habits a reliable navigation aid; I'd rather stick to a compass.
Sue Pike, a researcher and an environmental sciences and biology teacher at St. Thomas Aquinas High School, welcomes your ideas for future column topics. She may be reached at spike3116@gmail.com.
