The Wacky World of Whisk Ferns

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The whisk ferns (Psilotum spp.) are a peculiar group of plants. If you hang out in greenhouses long enough, you are most likely to encounter them as “weeds” growing in pots with other plants. Though they aren’t often put on display by themselves, the whisk ferns are certainly worth a closer look.

Psilotum comprises two species, the far more common Psilotum nudum and the lesser known P. complanatum. These two species will also hybridize, resulting in Psilotum × intermedium. Together, the whisk ferns make up one of only two genera in the family Psilotaceae (the other being Tmesipteris). They are strange plants to look at as there doesn’t appear to be much to them besides stems. Indeed, their peculiar morphology has earned them a fair share of taxonomic attention over the last century but before we get into that, it is a good idea to take a closer look at their anatomy.

Psilotum nudum  with yellow sporagia.

Psilotum nudum with yellow sporagia.

What we see when we are looking at a whisk fern is the sporophyte generation. Like all sporophytes, its job is to produce the spores that will go on to make new whisk ferns. This part of the whisk fern lifecycle is pretty much all stem. Though these are in fact vascular plants, they do not produce true leaves. Instead, the branching stem takes up all of the photosynthetic work. What looks like tiny leaf-like scales are actually referred to as ‘enations.’ These structures do not contain any vascular tissue of their own. Instead, they bear a type of fused sporangia that house the spores. When mature, these will turn a bright yellow.

Underground, things aren’t much different. Whisk ferns produce a branching rhizome that is covered in hair-like projections called rhizoids. These structures not only help anchor the plant in place, they also function in a similar way to roots. Rhizoids interface with the soil environment allowing the plant to absorb nutrients and water. However, they don’t do this alone. Like so many other plants, whisk ferns partner with mycorrhizal fungi, which vastly increases the amount of surface area these plants have for absorbing what they need. In return, whisk ferns provide the fungi with carbohydrates they produce through photosynthesis. As lovely as this mutualistic relationship sounds, it actually starts off as parasitism.

A  Psilotum  rhizome with hair-like rhizoids.

A Psilotum rhizome with hair-like rhizoids.

When the spores find a suitable place to germinate, they will grow into the other half of the whisk fern lifecycle, the gametophyte. These resemble tiny versions of the rhizome and contain male and female reproductive organs. Living underground, the gametophytes do not photosynthesize. Instead, they completely rely on mycorrhizal fungi for all of their nutritional needs. This can go on for some time until the gametophytes are fertilized and grow a new sporophyte. Then and only then will the plant actually start giving back to the fungi that their lives depend on.

Psilotum complanatum  with its flattened stems.

Psilotum complanatum with its flattened stems.

Because the overall form of the whisk ferns appears so “simplistic.,” many have hypothesized that the genus Psilotum is an evolutionary throwback to the early days of vascular plant evolution. On a superficial level, the whisk ferns do appear to have a lot in common with rhyniophytes, a group of plants that arose during the early Devonian, some 419 to 393 million years ago. A more detailed inspection of the anatomy of each group would reveal that there are some significant and fundamental differences between the two lineages, which I won’t go into here. Also, subsequent molecular work has shown that the whisk ferns reside quite comfortably within the fern lineage and likely represent a sister group to the order that gives us the adder’s tongue ferns (Ophioglossales). It would appear that whisk ferns more accurately represent a reduction in the more “traditional” fern form rather than a holdover from the early days of land plant evolution.

What the genus Psilotum lacks in number of species, it makes up for with its wide distribution. The whisk ferns seem to have conquered most of the tropical and subtropical landmasses on our planet. In fact, I found it incredibly difficult to discern much in the way of a native distribution for these plants. In some areas they are fairly common components of the local flora whereas in others they are considered rare or even threatened. I am sure that at least some of their expansive distribution can be attributed to human assistance as we move soils and plants around the world. To find them in nature, one must look in the cracks of rocks or on the trunks and branches of trees. Though both species can be found growing on trees, P. complanatum in particular seems to prefer an epiphytic lifestyle.

Psilotum complanatum  (left) and  Psilotum nudum  (right) growing epiphytically.

Psilotum complanatum (left) and Psilotum nudum (right) growing epiphytically.

Whether you grow them on purpose, fight them as a greenhouse “weed,” or track them down in the wild, I hope you take a moment to appreciate these oddball plants. The whisk ferns are intriguing to say the least and certainly offer up a unique conversation piece for anyone curious about the botanical world. They are a genus worth admiring.

Photo Credits: [1] [2] [3] [4] [5]

Further Reading: [1] [2] [3] [4]

Fern Ant Farm

An epiphytic lifestyle is no walk in the park. Baking sun, drying winds, and a lack of soil are the norm. As a result epiphytic plants exhibit numerous adaptations for retaining water and obtaining nutrients. One of the most interesting adaptations to this lifestyle can be seen in plants that have struct up a relationship with ants.

An amazing example of one such relationship can be seen in a genus of epiphytic ferns called Lecanopteris. Native to Southeast Asia and New Guinea, their unique look is equally matched by their unique ecology. Using a highly modified rhizome, they are able to latch on to the branch of a tree. In species such as Lecanopteris mirabilis (pictured here), it's as if the fronds are emerging from a strange green amoeba.

It's whats going on underneath their strange rhizomes that makes this group so fascinating. These ferns literally grow ant farms. Chambers and middens within the amorphous rhizome entice colonies of ants to set up shop. In return for lodging, the ants provide protection. Anything looking to take a bite out of a frond must contend with an army of angry ants. Moreover, the ants provide valuable nutrients in the form of waste and other detritus.

These are by no means the only plants to have evolved a relationship of this sort. Myriad plant species utilize ants for protection, nutrient acquisition, and seed dispersal. It has even been suggested that the unique morphology of Lecanopteris spores is an adaptation for ant dispersal. Certainly one can imagine how that would come into play. Interestingly enough, this group of ferns has attracted the attention of plant enthusiasts looking for a unique plant to grow in their home. As such, you can now find many different species of Lecanopteris being cultivated for the horticultural trade.

Photo Credit: Ch'ien C. Lee (www.wildborneo.com.my/photo.php?f=cld1505721.jpg)

Further Reading:
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The Evolution of Bulbs

Spring time is bulb time. As the winter gives way to warmer, longer days, some of our most beloved botanical neighbors begin their race for the sun. Functionally speaking, bulbs are storage organs. They are made up of a short stem surrounded by layers of fleshy leaves, which contain plenty of energy to fuel rapid growth. Their ability to maintain dormancy is something most of us will be familiar with.

They are incredibly hardy at this stage. As you might expect, bulbs are an adaptation for short growing seasons. Their ability to rapidly grow shoots gives them an advantage during short periods of time when conditions improve. Despite the energetic costs associated with supplying and maintaining such a relatively large storage organ, the ability to rapidly deploy leaves when conditions become favorable is nonetheless quite advantageous.

Contrast this with rhizomatous species, which are often associated with a life in the understory. Their ambling subterranean habit allows them to vegetatively "explore" for light and nutrients. What's more, the connected rhizomes allow the parent plant to provide nutrients to the developing clones until they grow large enough to support themselves. Under such conditions, bulbs would be at a disadvantage.

Bulbs have evolved independently throughout the angiosperm tree. Many instances of a switch from rhizomatous to bulbous growth habit occurred during the Miocene (23.03 to 5.332 million years ago) and has been associated with a global decrease in temperature and an increase in seasonality at higher latitudes. The decrease in growing season may have favored the evolution of bulbous plants such as those in the lily family. Today, we take advantage of this hardy habit, making bulbous species some of the most common plants used in gardens.

Photo Credit: Pixel Addict (Wikimedia Commons)

Further Reading: [1]