Ferns ( Pteridopsida )
Water ferns, or heterosporous ferns ( Hydropteridales )
Family Azollaceae ( Azollaceae )
Fern-like plants.
Among aquarium plants, fern-like plants make up a significant group. These include floating plants, such as *Salvinia auriculata*, and rooted ones, such as *Bolbitis heudelotii*. Ferns belong to vascular plants – class Polypodiopsida; unlike bryophytes and algae, they have vascular tissues, which allows them to form cylindrical strands (trunks) inside their stems, i.e., to take on tree-like forms. They have reached the highest morphological and anatomical level of development, adapting to a variety of habitats.
In addition to ferns, vascular plants include horsetails and clubmosses, which could be kept in paludariums as they grow in flooded meadows and marshes. Their appearance is so characteristic that people usually call them all the same – “ferns”, not suspecting that this is the largest group of spore-producing plants. The variety of leaf forms, amazing ecological plasticity, resistance to waterlogging, and a huge number of spores produced have contributed to the wide distribution of ferns across the globe.
Ferns do not yet have true leaves; they are replaced by a system of branches located in a single plane. What resembles a leaf in a fern is not a leaf at all. It is called a cladode, or frond, or another name, a pre-shoot. Despite the absence of a leaf, ferns do have a leaf blade. This paradox is easily explained: their cladodes and pre-shoot underwent flattening, resulting in the appearance of a future leaf blade – almost indistinguishable from that of a true leaf. However, evolutionarily, ferns have not yet managed to divide their fronds into a stem and a leaf.
Looking at a frond, it is difficult to understand where the “stem” ends, at what branching level, and where the “leaf” begins. Fronds are usually complex, their blade divided into leaflets attached to a rachis (an extension of the leaf stalk, or petiole). In almost all ferns, young leaflets in the bud are coiled like a snail, a phenomenon known as circinate vernation. This is due to the fact that the lower surface of the frond grows faster at an early stage of development than the upper surface. Ferns do not have true roots; they are replaced by rhizoids, just like in bryophytes.
Genus *Azolla* — Azolla.
A floating plant. The plant does not exceed 2.5 cm, leaflets are 0.5–1 mm. Upon careful examination, it can be seen that the leaflets of *Azolla* are arranged in two rows on the upper side of the stem. Each leaf consists of two lobes. The upper lobe is larger, light green, covered with protective hairs, located on the surface, and provides buoyancy to the plant. The lower lobe is submerged in water, significantly paler in color, and very fragile in structure. Unlike *Salvinia*, *Azolla* has numerous roots extending from the stem in two rows. *Azolla* helps to saturate the water with oxygen, and a large number of infusoria develop around it. It serves as a shelter and provides shading.
Carolina mosquito fern, water fern — Azolla caroliniana, caroliana.
“1” Synonym: *Azolla cristata*; *Salvinia azolla*. (English: Mosquito Fern, Carolina Mosquitofern, Water Velvet, Duckweed Fern, Koi Candy). North America, Mexico. An aquatic plant floating on the water surface, on the leaves of which blue-green algae grow, capable of absorbing nitrogen from the air, which in turn is used by the plant. The leaves are very thick and wrinkled, making them velvety to the touch. This is a fern that forms very beautiful green islands floating on the water surface. The stem is thin, fragile, and heavily branched. Just like other surface-floating plants, it has small leaflets arranged in pairs like tiles on a branched stem.
The coloration varies greatly depending on the amount of light and temperature. The upper part of the plant ranges from a beautiful bluish-green to reddish-brown, and the lower part is pinkish. Each leaf is divided into 2 lobes: the upper lobe is located above the water and participates in assimilation, thick, almost pointed, with a narrow, colorless edge, green or intensely brown (autumn coloration). The lower lobe is submerged in water, thin, and colorless. Spore clusters are found on the lower lobes of older plants. The leaf descending into the water is pinnately dissected, resembling a thread-like root. It has a pronounced seasonal growth pattern with a resting period in winter. With the onset of autumn, the leaves begin to rot, and the spores fall to the bottom and overwinter. In spring, young plants begin to develop from the spores.
“2” The aquarium water temperature can fluctuate within a fairly wide range. The plant feels comfortable both in moderately warm water at a temperature of about 20°C and in a tropical aquarium at 28°C. At very low temperatures, *Azolla* may sink to the bottom. If the temperature drops to 16°C and below, this leads to a halt in growth; after some time, the plant’s leaflets begin to rot, and it sinks into the water. Spores preserved in the bottom silt give life to new plants in the spring. The plant loves clean water. The water for *Azolla* should be soft, with a neutral or slightly acidic reaction. Hardness is preferably no higher than 10°, pH – less than 7.0. The plant requires very bright light. The daylight duration must be at least 12 hours.
“4” It loves nutrient-rich water, growing much more vigorously in it, particularly absorbing nitrates (NO3) and iron (Fe); in the absence of these essential nutrients, growth rates decrease significantly. Prolonged absence of phosphorus (P) can suppress reproduction and may even lead to the death of the fern.
“5” This species can serve as a decoration for cold-water and tropical aquariums with top lighting. The plant is very delicate and requires careful handling. Under favorable conditions, *Azolla* rapidly spreads, covering the entire surface of the aquarium and shading other plants, so its excess must be periodically removed from the aquarium. The die-off of *Azolla* in the winter period is usually associated with a decrease in lighting, so the plant can be preserved with very bright lighting and a sufficiently high water temperature. In winter, *Azolla* is also preserved in another way. The plant should be removed from the aquarium and placed in a shallow dish with damp moss. The wintering temperature should be no higher than 12°C with moderate lighting. In late March – April, the plant should be transferred back to the aquarium.
“6” Under favorable conditions, it reproduces by forming shoots as well as by spores.
Identification: Carolina mosquito fern, water fern — Azolla caroliniana.
Pacific mosquitofern — Azolla filiculoides.
Synonym: *Azolla magellanica*; *Azolla squamosa*; *Azolla arbuscula*. South America. A floating fern. Larger than *Azolla caroliniana*, but the plants can be distinguished by the following features: the stem branches pinnately. The leaves are more elongated with a pointed apex. The plant does not exceed 2.5 centimeters, leaflets – 0.5-1 millimeter. Upon careful examination, it can be seen that the leaflets of *Azolla* are arranged in two rows on the upper side of the stem. Each leaf consists of two lobes. The upper lobe is larger, bright green with a pinkish tint, covered with protective hairs, located on the surface, and provides buoyancy to the plant. The upper leaf lobe measures 2.5 x 0.9 mm, blunt, with a wide, colorless margin, ranging from blue-green to deep red-brown. The lower lobe is submerged in water, significantly paler in color, and very fragile in structure.
The plants are suitable for a tropical-type aquarium with intense top lighting. *Azolla* has numerous roots extending from the stem in two rows. Stem growth is accompanied by leaf death. The stem grows at the expense of dying leaves and serves to increase the plant’s buoyancy and equilibrium. It grows well at T 24°C and bright lighting. For optimal development, they need nutrient-rich, slow-moving water and a bright, sunny spot. In autumn, when the temperature drops, the plants turn brown due to the formation of anthocyanins. The breeding process in tropical aquariums is quite complex.
Identification: Pacific mosquitofern — Azolla filiculoides.
Green velvet fern — Azolla pinnata.
Synonym: *Azolla imbricata*; *Azolla africana*. Africa, Madagascar, Asia, Australia, New Guinea. An aquatic plant, 1.5-2.5 cm in size. The spreading leaflets have triangular outlines. This species has bright red-brown foliage. The stem has pinnate side branches that shorten toward the apex, with triangular contours. Leaflets are small, 1-2 mm long, floating on the surface, of two forms – acute or rounded. The upper lobe has single-celled villi giving them a velvety appearance. These villi make the leaf surface water-repellent. The leaf color varies in different shades – from neutral green to intense brownish-red. The rhizome is branched, pinnate.
*Azolla* creates favorable conditions for fish fry in aquariums, as it serves as a good shelter for them. An aquatic environment rich in oxygen is created near these plants. A huge number of infusoria breed in it, which serve as good food for fish. It is a more heat-loving plant, suitable for an aquarium with tropical environmental conditions. Wintering is possible only in a very bright, warm place.
Kept in a tropical aquarium. Optimum temperature 24-30°C. At temperatures below 5°C and above 45°C, the plants die. The presence of phosphorus is a crucial factor for growth. Plants survive at pH 3-10, but the optimum is pH 4.5-7.0. Reproduction is vegetative (division of branches) or sexual (by spores). With sufficient lighting, it multiplies rapidly, covering the surface of the aquarium.
Identification: Green velvet fern — Azolla pinnata.
Bolbitis heudelotii, Bolbitis heteroclita, Bolbitis.
Salvinia natans, Salvinia auriculata, Salvinia. 
