Sunday, November 29, 2015

Filmy Ferns - Not everything in the Moss Forest is moss!

This Japanese cypress in Shiratani
sports a glowing crown of filmy ferns.
The spot called the "Moss Forest,"
wouldn't look so mossy without
its filmy ferns .
Leaves so thing the light shines
through are supported by
a network of viens.

Several species can
be found in Yakushima.
While hiking through Yakushima's old-growth forests, look up at the spreading branches of ancient trees, caked with layers of spongy green moss. But look again. Is moss the only thing up there?

From towering tree ferns reminiscent of a prehistoric jungle to large forked sprays fit for New Years decorations, ferns abound in Yakushima from the coasts to the mountains. However, when hiking through the dark and damp, lush and mossy forests, one of the most prolific, visible, and very beautiful families often goes overlooked:

The Filmy Ferns

I suppose it really shouldn't be a surprise that this family is often clumped together with unrelated mosses by the casual hiker. Many filmy ferns have leaves that are roughly the same size and shape as a sprig of moss such as Plagiochila pulcherrima (ウツクシハネゴケ, literally "beautiful feather moss"). And filmy ferns have similar growing habits: They thrive in moist places. On humid days, the leaves fan out, and on dryer days, they shrivel up.

If you look up at the sky through the leaves of a filmy fern or moss, you can see sunlight seeping through. Beautiful. Both mosses and filmy ferns seem to glow in the sunlight, especially after a good rain. The body of these translucent leaves is (with few exceptions) only one layer of cells thick.  They lack a waxy epidermis and pores (stomata) that allow many so many plants to breathe without drying out.

There are about fifteen species of filmy ferns recognized in Yakushima. Of these, seven species belong to the genus Hymenophyllum. They are easily confused with each other, and I can't yet keep them straight, but let's not confuse them with mosses!

If you are not sure what is moss and what is a filmy fern, you are in good company, but if you consider the size of the leaves, it becomes immediately obvious: This simple vascular systems of mosses they can (usually) only support leaves on the scale of millimeters, or less. One leaf of the filmy fern Hymenophyllum barbatum (コウヤコケシノブ) is roughly the size of a whole specimen of P. pulcherrima, consisting of hundreds and hundreds of leaves.

Another difference is the stems. Again, because of lack of a vascular system, mosses don't tend to have long dangly stems between leaves, but the leaves of filmy ferns are connected by creeping, threadlike stems, and one plant can cover quite a large area.

 No matter how much area they cover and how dense they grow, both mosses and ferns readily dry out and shrivel when a dry spell comes through, but will perk up again with a little rain. Many people feel that the Moss Forest is most beautiful when the sun comes out right after a shower and shines through all those refreshed layers of green, but after a couple days of sun, the effect is lost.

Close up of part of a leaf. The body of the leaf
is one cell-layer thick.
The forest comes alive when the
filmy ferns catch the first rays
of morning light.
Like moss, filmy ferns rely on moist