Ferns: A Leafy Life Cycle

As we have been illustrating over the past few months, leaves are a fascinating part of plants. Initially, tasked with the job of producing carbohydrates that in turn provide the building blocks for the growth of roots, stems and branches, the leaves of some plants took on additional challenges. An example is a common garden plant whose complicated, albeit very successful, life cycle is all about leaves: the ferns!

Ferns are vascular plants, meaning that unlike some of the other more primitive plants, they have the appropriate plumbing to move water and nutrients throughout the plant. Ferns originally started to develop around 350 Million Years Ago (MYA) during the early Carboniferous period, but the ferns we recognize today did not start to appear until around 290 MYA with the advent of the Osmundales family, represented in NJ gardens by Royal and Cinnamon Ferns (Osmunda regalis and Osmunda cinnamomea respectively, Osmunda regalis is picture at left). Interestingly, ferns did not stop developing at that point in time, but continued over the millennia. In fact, many ferns appeared after 145 MYA and are a contemporary of the true flowering plants or Angiosperms. Examples include the Lady and Maidenhair Ferns (Athyrium filix- femina and Adiantum pedatum respectively). For 250 million years ferns were slowly changing! Regardless of the changes, their life cycle was and remains highly dependent upon leaves!

Fern leaves, which are often called fronds, can be simple in form, meaning that they are unlobbed, much like the Birds Nest Fern (Asplenium nidus, pictured at right) or highly lobbed forms such as the aforementioned Royal Fern (Osmunda regalis). The individual leaflets of these highly lobbed or compound leaves are called pinnae. Similar to the more advanced leaves of the flowering plants or Angiosperms, fern leaves possess a mesophyll, which contains the chloroplasts and is the site of photosynthesis; they have an epidermal layer complete with stomata and guard cells on the lower surface, allowing the exchange of gasses within the leaf; and they have a waxy cuticle layer covering the epidermis, reducing water loss. The fronds emerge from buds, typically found at the tips of the stems. The stems typically lie horizontally just below the ground and are called a rhizome. However, members of the genus Cyathea or tree ferns have vertical stems. When the fronds make their initial appearance, they resemble the scroll at the tip of a fiddle’s fingerboard or the top of a bishop’s staff, affectionately giving rise to their commonly used names of fiddleheads or crosiers. For some ferns, such as the Ostrich Fern (Matteuccia struthiopteris), the fiddleheads are actually an edible delicacy and when lightly sautéed, provide a taste and texture much like asparagus. The uncurling and expansion of the fiddleheads is called circinate vernation, whereby the cells on the outer surface of the crosier expand faster than the cells on the inner surface, allowing the leaf to literally unroll!

What is not blatantly obvious from a casual study of a fern is how the life cycle is directly connected with three different types of leaves: namely the Trophophylls, Sporophylls and the leafy prothallus. The first two types of leaves appear during the sporophytic or spore producing portion of a fern’s life cycle, while the prothallus appears during the gamaphytic or gamete producing stage A trophophyll is the traditional fern frond, whose primary function is that of photosynthesis. The sporophyll is a modified leaf that produces spores. This leaf can assume the shape and photosynthetic function of a trophophyll, with the only perceivable difference being the appearance of very organized rows of dark brown specks beneath the frond (those of Asplenium nidus pictured at left). Many people mistake these specs for a very organized insect or scale infestation! In reality, these spots are actually sori (sorus for singular), which in turn contain clusters of sporangia, the organelles that actually produce the spores! Sporophylls can also appear uniquely different than a trophophyll – typically smaller and narrower – with the only function being to produce spores! Examples include the Ostrich Fern (Matteuccia struthiopteris, pictured at the right) and the Sensitive Fern (Onoclea sensiblis). Due to their shorter stature, they may not be particularly noticeable throughout the growing season. However, since they fail to collapse following autumn frosts and persist through the winter as bolt upright, dark brown structures, they provide a little winter interest and aid with winter plant identification! Regardless of the form assumed by the sporophylls, their primary function is that of spore production. In many aspects, spores serve a function similar to seeds, allowing the plant to spread to new locations that might be conducive for the plant to grow. What is different is how spores and seeds are created. Spores are produced through cell division within the sporangia. Each sporangium typically produces 64 spores, beginning with a diploid ‘mother’ cell which has the normal set of two chromosomes. This cell divides through the process of mitosis, which creates two identical ‘daughter’ cells. These two daughter cells continue to divide, until there are 16 daughter cells. These cells then each undergo the process of Meiosis, through which each cell divides twice to create 4 genetically unique or different haploid cells. A haploid cell only has 1 set of chromosomes. Since each of the original 16 spores underwent meiosis, a total of 64 haploid cells result, each of which will ultimately become a spore. By contrast, a seed is the result of the sexual merger of two haploid cells – one ‘male’ and one ‘female’ – along with the merger of the DNA information that each cell contains to create a diploid!

When the spores are mature, they are released from the sporangia. The time of year for spore release varies remarkably between the genera. Many are released in late summer, but for both the Ostrich and Sensitive Fern, with their sporophylls remaining bolt upright through winter, spore release is not until spring! Spores are very small, each measuring about 1/10 of a mm in diameter and resemble dust. As a fun experiment, once the spores are mature, the sporophylls can be cut, flattened atop a white sheet of paper such that the sori are facing down and then left overnight to dry. The next day, the red, brown, yellow or even green spores can be seen on the paper. For other ferns such as the Staghorn Ferns, simply tapping the tops of the sporophylls when the spores are ripe will release a cloud of spores that can be collected. When the moisture and light conditions are proper, the thick and protective cell walls surrounding the spore will split open and the spore will ‘germinate’. Spore germination involves the haploid cell dividing via the process of mitosis, yielding two identical haploid, cells, which in turn continue to divide mitotically until a haploid, leafy prothallus results. This marks the beginning of the gametophytic stage.

One might wonder what advantage there would be to a haploid gametophytic stage and such a seemingly complicated life cycle. In diploids, the dominate genes of one chromosome may mask a potentially hazardous recessive gene of the second chromosome, which would continue to be transferred to its offspring. When a portion of a life cycle is haploid, there is no possibility for a dangerous or life threatening gene to be masked and transmitted, since it would be overt from the beginning and the haploid organism would perish!

The prothallus is typically heart-shaped in appearance (as seen on the left, Source: Random Tree on Wikipedia [CC License]) and since it is non- vascular, it is small, roughly the size of a child’s fingernail. Most often they are free living organisms, being green in color and conduct photosynthesis. However, in some fern species the prothallus lack chloroplasts and the carbon and other nutrients are obtained from symbiotic relationships with fungi. The center of the prothallus is several cells thick, but along the periphery, it is often a mere one cell thick. Interestingly, as many gardeners in deer country have observed, very few insects or animals eat ferns, but nearly every critter eats the prothallus, which in part, explains why they are not seen more readily in the wild! The prothallus produces tiny rhizoids or root like structures on its lower surface (as can be seen above), allowing it to anchor itself to substrates. The lower surface is also the site where the male and female structures are produced. The antheridia (singular antheridium) are ball shaped organs that are the site of the male gametophyte or sperm production; they are typically located near the periphery of the prothallus. Unlike most mosses that have one flagella or ‘tail’, fern sperm are multiflagellated, with some having upwards of 100 flagella! The flask shaped archegonia (singular archegonium) are the site of the ovum production and occur along the central portion of the leaf. Most often, but not always, both the male and female structures are located on the same prothallus In the presence of water, the multiflagellated sperm literally swim to the ovum, apparently attracted by Malic Acid and other chemicals. HOWEVER, it is rare that the sperm fertilizes the ovum of the same prothallus; since all the cells of a prothallus are haploid and contain identical genetic information that would prevent the blending of genetic information and essentially create ‘inbreeding’. Ferns accomplish crossbreeding through the archegonia and antheridia maturing at different times on any given prothallus, a technique which has been mimicked in many of the early angiosperms (flowering plants) or failing to have both on the same prothallus. Given this fact, it becomes far more obvious as to why water and the large number of flagella are necessary since they have some significant ground to cover if the outcome is to be met with success! Once an ovum has been fertilized, the other archegonium essentially ‘shut down’, allowing only one fern to be produced per prothallus. The enlarging embryo initially receives all its nutrition from the prothallus. As with seeds, the first organelle to develop and emerge is the embryonic root, which allows the developing fern to begin the process of becoming self-sufficient. The fern or sporophytic stage grows rapidly and quickly becoming an independent plant, and since the prothallus is no longer needed, it degrades and vanishes.

Ferns are wonderful additions for every garden, providing a texture and color that many other plants simply cannot replicate. However, as you look at your ferns this coming year, marvel at the complexity of their leafy life cycle and how many have survived the changing climate of the Earth for millions upon millions of years!