Download Ferns: A Leafy Life Cycle

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 filixfemina 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 nonvascular, 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!