AQUATIC HORTICULTURE		BY KAREN RANDALL

Aquatic Ferns — Part 1

As plants continued their evolutionary progression from the sea, they colonized dry areas and increased in size. It became advantageous to have roots for the transport of water and minerals. Some of these early land dwellers began to produce cutin and suberin, waxy coatings that slow water loss. They also developed a vascular system for the transport of water, minerals and photosynthetic products through the tissues of the larger plant body.

The earliest record of primitive vascular plants is found in fossils from the Silurian period, 400 million years ago. In these earliest vascular plants there was no differentiation between the vascular tissues in the upper and lower parts of the plant body.

KAREN RANDALL

“Thread leaf” water sprite covered with oxygen bubbles. While the scientific name of this Ceratopteris sp. is in question, it is still a lovely and useful aquarium plant.

In time, there was a reduction of the gametophyte generation in these new vascular plants in comparison to the Bryophyta discussed last month. The sporophyte generation became the one with a complex plant body made up of differentiated tissues and organs. The gametophyte was now reduced both in size and complexity.

Leaves

In more advanced plants, the vascular branch system flattened to increase surface area. In these plants there is a definite vegetative axis, with branching clusters along the axis. Tissue filled in, incorporating the whole branch system to form large, multi-veined leaves called megaphylls. In ferns, sporangia have been incorporated into the megaphylls.

In prehistory, megaphylls were often large, but in modern species, megaphylls are often found to be as small or smaller than microphylls. Still, the differences in structure help to determine the evolutionary history of a specific plant.

KAREN RANDALL

Reproduction

Pteridophyta

Azolla

This genus includes six species. Azolla has become cosmopolitan through introduction. They are small, floating ferns with pinnately branched stems. The leaves are alternate, overlapping and cover the stem. The roots are simple. The color ranges from bluish green in low light to bronzy red in full sun.

Azolla lives in symbiosis with a specific cyanobacteria, Anabaena azollae. The cyanobacteria is capable of fixing atmospheric nitrogen, which is then used by the Azolla. This gives the Azolla a competitive edge over other floating plants that must depend on the nitrogen available in the water column. Because of its nitrogen-fixing properties, Azolla and its symbiont are used as “green manure” to fertilize rice fields in parts of Asia.

KAREN RANDALL

Azolla is a very pretty little floater and is particularly useful in tanks with herbivorous fish, because it seems to be much less palatable than duckweed or Salvinia. In fact, pond experts warn that Azolla can blanket the surface of ornamental ponds thoroughly enough to inhibit oxygen exchange. For this reason and because it will also block light to rooted plant species it is good to harvest the Azolla regularly, whether in an ornamental pond or an aquarium. Azolla also makes a very attractive ground cover in moist terrariums, where it grows in small decorative mounds.

Bolbitis

There are approximately 85 described species of Bolbitis. The genus is found in both Africa and Asia. At least four species are aquatic and, more than likely, there are even more. The four currently known aquatic species are B. fluviatilis and B. heudelottii from tropical Africa, and B. heteroclita and B. hydrophylla from tropical Asia.

Bolbitis rhizomes are creeping and usually attach to rocks or fallen logs. The numerous roots are covered with brown triangular scales. Leaves are erect and pinnate, and evenly spaced along the rhizome. The leaves are variable in size and texture, both from species to species and depending on growth conditions. Submerged leaves are sterile. Spores only develop on emersed leaves and it is presumed that they are dispersed by the wind. Under cultivation, propagation is accomplished vegetatively, by division of the rhizome.

Of the four known aquatic species, only two are regularly offered for sale in the hobby. The first, B. heudelottiifrom Africa, makes an excellent aquarium specimen. It will tolerate fairly low light levels, but usually does better with CO₂ added to the water. It is one of many plants that seems to need to reach “critical mass” before it really gets going. A small piece of rhizome can go for months or even years without increasing appreciably. Then, when it finally does reach a good size, the plant will take off and grow quite vigorously. I have a stand that takes up about a quarter of a 70-gallon tank. This has to be divided in half at least a couple of times a year or it threatens to take over the entire tank.

It is important that the rhizomes of Bolbitis not be burried in the substrate. In fact, the roots need to have no contact with the substrate at all. The plant does best when attached to a piece of driftwood or stone. This can be accomplished by either tying it in place with thread or fishing line, rubber banding it in place or stapling the roots (not the rhizome) to driftwood with a staple gun.

Like many slower-growing plants, B. heudelottii is prone to algae infestation of the older leaves if it is not kept in a tank with clean water and good growing conditions. It can benefit from water movement around it, and is often happiest near the outflow of a filter or powerhead. In the absence of sufficient levels of CO₂, the leaves will turn black and crusty.

There appear to be two plants that are both being sold under the name B. heudelottii at this time. One has a shorter petiole, curlier leaves and tolerates soft water quite well. The other has petioles almost as long as the leaf itself. The leaves are flatter, with less curl to the edges and the plant does better in water of moderate hardness. In my tanks, the smaller variety grows to approximately 12 inches, while the larger easily reaches 18 to 20 inches in height. There is some speculation that while the larger plant is clearly a Bolbitis, it might actually be a different species.

The other species you will see on sales lists is B. heteroclita from Asia. Experience with this plant in the aquarium seems to be quite variable. I’ve never heard of anyone doing really well with it, but it can be kept submersed indefinitely. I have a piece that has been growing in an aquarium for at least five years now, but it has never increased in size in that period and seems to hang on more than thrive.

Most of the time when B. heteroclita is offered for sale, it arrives in the emersed form, looking very much like a piece of poison ivy. The leaves are comprised of three large, flat leaflets. It is hard to imagine it is even a fern! Grown underwater, the leaves become very crinkled and much smaller in size. The best way to use this plant, by far, is in a humid terrarium or paludarium where it can grow up out of the water.

Ceratopteris

Ah, the ubiquitous water sprite! The morphology of this standby of the aquarium trade is actually much more complex than you might guess at a casual glance. According to most literature, there are either three or four species in this confusing genus. All are aquatic and all are useful aquarium plants. But depending on the source you look at, you will find different names assigned to the various species. Some sources combine two types into one species, but not always the same two. Whether the genus can ever really be straightened out is in question, because the plants have been spread worldwide. So even if you find a wild population, you can’t know for certain whether it is native or not. Are you confused yet? So am I! And I don’t think we’re alone — the scientists are not in agreement either.

As a group, all Ceratopteris species have a number of characteristics in common. The leaves are all more or less pinnate and are held in a rosette. The lower leaves are sterile and less finely divided than the upper leaves. Juveniles are free floating, while adults can be free floating or rooted in the substrate. In the wild, these rooted plants are generally at least partially emergent, but most types will tolerate complete submersion in the aquarium.

I will describe the types I know of in the hobby and give you my best guess at the name. When in doubt, you can always resort to “Ceratopteris sp.!”

Ceratopteris cornuta is the plant usually traded among hobbyists as “broad leaf” water sprite. It is also sometimes called “Indian floating fern.” It is said to be found in Africa, Southeast Asia and northern Australia. C. cornuta is happiest as a floating plant. It can be planted in the substrate, but before very long, it will produce large numbers of offsets that pop to the surface, while the parent plant begins to slowly decline in vigor.

Ceratopteris pteridoides is not, as far as I know, available commercially. It is found in South and Central America. In deep water areas, the plant floats free with large cabbage-like leaves forming a rosette, and finely divided fertile leaves are held above. It differs from all the other water sprites by having large pontoon-shaped swellings in the stems just below the leaves, apparently for flotation. As the waters recede, the plant will root itself in the mud and produce the finely pinnate needle-like fertile leaves almost exclusively.

The specimen I have was collected wild in South America and has not yet reproduced. I have been told by the person who collected mine that it will reproduce vegetatively, as do the other Ceratopteris sp.

Ceratopteris thalictroides is the typical water sprite that people start out with in the hobby. It is also the water sprite most likely to be found offered for sale in pet shops. At club auctions in my area, it is often called “fine leaf” water sprite to distinguish it from the other types. C. thalictroides is found in the wild in the tropical and subtropical areas of North and South America, Africa, Asia and Australia. The leaves are mid way in shape between C. cornuta and C. siliquosa. It grows well either floating or planted in the substrate, tolerates a wide range of conditions and is quite attractive. Its long roots are excellent fry cover in a breeding tank. If the plant has a fault, it is that it grows so fast that it can take over a small tank in no time.

Ceratopteris siliquosa is the name most often questioned for validity. In more current books, it is usually listed as a synonym for C. thalictroides. The problem is, it is a different enough plant in looks and habit that, as aquarists, we need a way to distinguish it from “normal” C. thalictroides. I have begun to use the designation “thread leaf” water sprite when I put this plant in auctions.

Thread leaf water sprite, as my name for it implies, is the most finely divided of any of the water sprites. Like “normal” C. thalictroides, it is happy to remain rooted and submerged for extended periods. When grown in this manner it will produce thick, woody stems and become large enough to dominate a good section of a 4-foot tank. It easily reaches the surface of a 24-inch deep aquarium. Plantlets seem to develop most readily at the leaf axis and can easily be removed without damage to the leaf itself. A vigorous grower like its brethren, it is useful even in a smaller tank. As the older plant outgrows its spot, it can be removed and replaced by an offset which will fill in the empty space quite quickly. We will continue our overview of aquatic ferns next month.