Spores

Although the average gardener looking for a few new ferns is rightfully happy with the season's crop at the local or mail-order nursery, the truly addicted eventually turn to growing their ferns from spores. Success will reinforce the addiction, causing it to grow exponentially, and can yield hundreds of plants, rapidly filling the garden (and on occasion leading to the establishment of a fern nursery). Despite its reputation, spore propagation is not particularly difficult, but it does take time to produce a mature plant.

Spores are dustlike and waft about like smoke. "Do you have an indoor fern that won't drop dust on my piano?" queried a customer. The spores are gathered in cases called sporangia, which in turn are in clusters called sori (singular, sorus) and are usually, but not always, on the underside of the fern frond. "I hate to tell you but you have

Capsule containing spores

Annulus i

SPORANGIUM BEFORE OPENING

MATURE SPOROPHYTE

True roots

YOUNG SPOROPHYTE

Spores

Annulus

SPORANGIUM AFTER OPENING

SPORES

GERMINATING SPORE AND EARLY PROTHALLIUM

Archegonium-

Antheridium

Sperm

MATURE PROTHALLIUM

The life cycle of ferns. Drawings by Richie Steffen, Miller Botanical Garden.

bugs all over the undersides of your ferns," observed another. Ah, yes, these are the sori with their spores, the fern's natural reproductive system.

With no flowers, and consequently no seeds, ferns have a unique life cycle consisting of alternating generations, the sporophyte and gametophyte. The familiar foliar form is the sporophyte, which when mature produces millions of spores. (Do the math. A single sporangium usually contains 64 spores. Multiply this by several dozen sporangia per sorus and dozens of sori per frond and we are talking serious numbers of spores. Moran [2004] calculated that a single 25-in. [63-cm] frond of Dryopteris carthusianaproduces 7,305,216 spores. Large and Braggins [2004] reported that a large specimen of Cyathea medularis may release up to one pound [2 kg] of spores per year.)

When spores germinate they create the gametophyte generation with small, sexual, usually cordate, tissue-thin structures known as prothalli (singular, prothallus), which are about V4 in. (6 mm) across. These contain an archegonium sheltering a single egg and antheridia housing the sperms. When mature and assisted by moisture, the sperms swim toward the egg and fertilization takes place. Shortly thereafter the "true" fern, or sporophyte, emerges, completing the cycle.

An odd bit of botanical magic takes place with some species, especially those from dry areas where fertilization is challenging, and this is a process called apogamy. Essentially the prothalli develop as described but with a nonfunctioning archegonium, the female egg receptacle. (Sperms perform normally and consequently are free to roam). In these juveniles a bud forms on the prothallus and develops directly into a sporophyte (frond-bearing generation) saving the propagator (and fern) lots of time and trouble. Approximately 5 to 10 percent of the world's ferns are apogamous, although research indicates that 13 percent of the Japanese natives are so inclined. Apog-amous species are noted in the individual plant descriptions.

Spores can be yellow, green, brown, or black and the soral arrangement is of easily observed, botanical significance in determining differences among fern genera. Primitive ferns, such as the osmundas, carry their (green) spores in specialized structures separated from the foliage. When ripe they are dispersed practically simultaneously. Over the millennia, as competition increased in the plant kingdom, ferns had to be more circumspect and the sori moved to the frond's underside for some protection, although still distributing spores immediately upon ripening. As they became more sophisticated the sori developed a fine membrane, known as an indusium (plural, in-dusia), which covers the sori and lifts up when the spores are ripe. Thus protected, these evolutionary sophisticates gradually release their spores from the pinnae at the bottom of the frond to those at the apex and from the rachis outwards, efficiently dispersing the spores over a longer period of time and optimizing their potential for finding a suitable site for germination. To accomplish this, these featherweights sometimes travel long distances. Research indicates, for example, that Asplenium adiantum-nigrum, a nonnative, arrived and survived in the Hawaiian Islands on at least three, and possibly as many as 17, separate occasions.

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