Ever wondered why moss only grows on the north side of trees? Didn’t think so…

****Actually, moss doesn’t only grow on the north side of trees, it just MOSTLY grows on the north side. Also, that rule only applies in the northern hemisphere–in the southern hemisphere, moss mostly grows on the south sides of trees. The reason is that in the northern hemisphere, because of the tilt of the Earth on its axis, the sun almost always appears to be a little south of directly overhead. That’s why rooms with windows facing south are brighter than rooms with windows facing north. This is important for the mosses because the north sides of trees (in the northern hemisphere) are shadier and therefore moister. The south sides of trees get more sunlight, so water evaporates faster there.

Mosses need a lot of water for two reasons. One is that they’re not “vascular” plants–that means that they don’t have the plant version of a circulatory system, and they can’t move water around inside their bodies. All cells in a moss’ body need to have easy access to water from the environment. The second reason they need water is because their male reproductive cells can only survive by swimming in droplets of water. The only way these cells can get from one plant to another is to hope that a raindrop will cause the water they’re swimming in to be splashed onto a neighboring plant. If the surface that they live on (like a tree trunk) dries out, the moss will be unable to reproduce, and it will dry out. That’s much more likely to happen on the sunnier side of a tree trunk than on the shadier side.

The same rule applies to rocks, fallen logs, or anything else that mosses might grown on. If it has a sunny side and a shady side, the moss will mostly grow on the shady side. In the northern hemisphere, that’s usually the northern side, and in the southern hemisphere, it’s usually the southern side. If you look closely enough, though, you’ll see exceptions.

The texture of mosses is usually (but not always!) fuzzy because their leaves have many little projections on them, like the finger-like projections on a maple leaf. The moss leaves and their projections are so small, though, that they seem fuzzy to us. These projections are probably to help the moss cells deep inside the leaves to be as close as possible to external water sources.

MP3: Cocteau Twins – Cherry-Coloured Funk


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  1. twoeightnine September 28, 2011 at 9:09 pm #

    Funny thing is, I was actually just wondering this. I was driving through Utah on my way back to NY and was trying to remember which side it grew on. I knew, sorta, why it grew on a certain side but couldn’t remember which side.

  2. Reigs September 29, 2011 at 6:04 am #

    Thanks for this, I freakin LOVE Moss and Lichen too – beautiful!

  3. Forest Uprising October 1, 2011 at 9:59 am #

    Yes, we also find it arousing.

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