THE ROYAL FERN

Osmunda regalisL.

For inquiries contact Ed Klekowski, University of Massachusetts, Amherst.

The royal fern is a common fern species found in bogs, swampy areas, and stream sides in the Connecticut River watershed. The generic name Osmunda is of Saxon origin and was one of the titles of the pagan deity Thor. The royal fern belongs to the family Osmundaceae; fossils belonging to this family have been found in rocks of Permian age (230,000,000 years before present), a time when the continents were consolidated into the supercontinent Pangea. Thus it is perhaps not surprising that Osmundaceae occur on all continents except Antarctica. The royal fern is one of the most widespread of all living species and is found on every continent except Australia. Osmundas have witnessed the rise of the reptiles, the flourishing and eventual extinction of the dinosaurs, the origin and adaptive radiation of the mammals, and the rise to dominance of an obscure genus of hominids named Homo.

The royal fern has many traits that appear maladaptive, or to put it another way, Osmunda regalis seems burdened with characteristics that handicap its survival - yet in spite of a variety of morphological and reproductive limitations, royal ferns have survived and flourished for hundreds of millions of years!

The royal fern life cycle is divided into two independent components, a diploid sporophyte and haploid gametophyte. The familiar fern plant with the stem, roots and leaves is the diploid sporophyte. The royal fern sporophyte is long-lived (+ 100 years). The stem (rhizome) is underground just below the soil surface and is very slow growing.

This characteristic prevents royal ferns from surviving in environments where sediments bury the stem. Thus royal ferns do not occur in the flood plain forests along the Connecticut River where plants must be adapted to frequent stem and root burial by the river's sediments.

The underground rhizome is clothed in roots which have two functions: they take up nutrients from the soil and also function as a protective armor for the living stem tissue (ferns have never evolved bark for stem protection).

. The above-ground parts are the leave (fronds); each leaf may be over three (3) feet in length and is twice subdivided before leaflets (pinnae) are developed. The terminal portion of the leaf forms masses of sporangia.

Sexual reproduction begins within the sporangium, where hundreds of cells differentiate that will undergo meiosis. These cells are called spore mother cells. Sex in the royal fern begins in late May or early June with the formation of sporangia on developing leaves and the initiation of meiosis by the spore mother cells. The diploid chromosome number is 44 (in humans it is 46), and each spore mother cell, after meiosis, will yield 4 haploid spores, each with 22 chromosomes; each sporangium forms over 500 spores; each sporophyte forms thousands of sporangia.

By mid-June, every sexually mature royal fern is releasing hundreds of thousands of haploid spores into the surrounding air, at this time the air is thick with spores; a large royal fern population will release literally millions of single-celled spores within a period of a week! Unfortunately, each spore has a major design flaw, royal fern spores have a very limited life span; they die after 2 days! In contrast, other fern species form spores that remain viable for years.

With luck and good weather, a few spores manage to find suitable environments and germinate, the rest (the vast majority) simply die. The reproductive waste in royal ferns, as in most plants, is incredible. Sexual reproduction in these organisms is so clumsy and ineffective that millions of spores must be formed for even the slightest chance of reproductive success. In most years, all of these efforts fail; there is no effective sexual reproduction.

The surviving spores germinate and form the gametophyte generation, a haploid sheet of cells smaller than a fingernail that develops the sex organs.

The majority of gametophytes succumb to drying or herbivory - those that survive, by midsummer, form eggs and motile spermatozoids. Fertilization is only possible after a rain shower and results in diploid zygotes that undergo cell division to form the embryonic sporophytes.

Just when one would think that success is at hand, more problems arise. The gametes (spermatozoids and eggs) of royal fern have a high frequency of defective genes (mutations), consequently the majority of young embryos exhibit various birth defects (no roots, no leaves) and die.

In spite of all of these reproductive hurdles, after a hundred years of trying, a royal fern sporophyte may be successful in reproducing itself. So much for the view that nature is the embodiment of perfection and efficiency, the royal fern seems a clumsy, inefficient organism that has somehow survived since the age of dinosaurs (who are, of course, extinct)! But perhaps royal fern only appears poorly designed from our viewpoint. Maybe it has evolved competitively superior characteristics that we yet know nothing about.

References:

Cobb, B. 1963. A Field Guide to the Ferns. The Peterson

Field Guide Series. Houghton Mifflin Company, Boston.

Tryon, A. F. And R. C. Moran. 1997. The Ferns and Allied Plants of New England.

Center for Biological Conservation, Massachusetts Audubon Society, Lincoln, MA.

Lellinger, D. B. 1985. A Field Manual of the Ferns & Fern-Allies fo the United States

& Canada. Smithsonian Institution Press, Washington, D. C.