Friday, April 26, 2013

Sex among the ferns

'Fiddleneck'  on my Woodwardia fern
Ferns have this way of emerging from winter that makes me feel happy.  Spring is the season for growth and reproduction with most fern varieties in California - due mostly to the fact that this is usually the wettest season and warm enough for active growth.  It's the time of year that the new 'fiddle neck' curls emerge and unfold their long graceful shapes.  This year I had to cut back and move my patch of giant Woodwardia ferns.  Woodwardia are found growing naturally along cool foggy Pacific coast canyons from Northern California up into Alaska... they form patches up to 6 feet tall... I grow my colony in the coolest, most moist microclimate that I have. A spot where full sunlight never falls. 

Newly transplanted 'Woodwardia'
I also have Southern California sword fern – This form has a more narrow frond than those found in the cool moist ocean valleys of Oregon and Washington.  They are very tough and can withstand periods of drought.  They only reproduce and grow during the season of spring time moisture, but are easy to replicate asexually.   I just dig up a small clump with roots and rhizomes, stick it into the soil where I want it to grow, and give it water.    This particular variety I got from my Uncle John who collected it from the mountain valleys above Pasadena CA.

Southern California 'sword fern'

Another favorite fern of mine is the 'maiden hair' ferns – they look so delicate – but are actually quite a tough plant – I transplant them with the same lack of finesse as I do the sword ferns – dig up a small clump, with roots and rhizomes, and plunk it into the ground... with a little water – they almost always “take”.
Young 'Maiden Hair' fern
There is a book about ferns that reads like an adventure story – I highly recommend it – its “Oaxaca Journey” by Oliver Sacks:
Arid fern

In the book he describes a group of New York fern enthusiasts (city folks)  who travel to Oaxaca Mexico to study ferns and to collect spores to grow back home in their New York greenhouses.  It is a surprisingly fun book to read!  The state of Oaxaca is a land noted for the largest diversity of ferns to be found in North America – some are adapted to the mountain-peak cloud forests, some to lower mountain valleys, and some adapted to arid desert setting.  Since reading the book I have been able to locate and add 3 varieties of arid ferns to my garden.
Spore cases on underside of frond - This is where cells with two sets of genetic information (2n) undergo a special cell division that will produce cells with a single set of genetic information (n) 

The life cycle of ferns has a surprise - Fern plants have two body forms that must alternate with each other!  What we call a 'fern' is the big showy green plant, but their offspring are tiny little inconspicuous things.  The offspring of those tiny plants are once more the big showy plant...  This is the sexual reproduction cycle of all ferns called 'alternation-of-generations'!  Simple life forms can tell us a lot about the development of life on our planet.  According to the fossil record, ferns developed long before there were any seed bearing plants on earth.  They even predate the time of dinosaurs!

Another variety of arid fern

Ferns reproduce by producing small reproductive 'spores' that form in certain seasons of the year on the underside of the fronds.  This is where it gets interesting!  Each cell in the tiny structures that form the spores must first undergoe a very special cell division that forms the spore cells with only one set of chromosomes (n)  from the parent plant – they are called haploid cells (n) .
Gametophyte has structures for producing both (n) sperm and  (n) egg - mature at different times
Countless millions of spores are released and a tiny few land in moist conditions and have a change to divide and grow into a tiny little heart shaped plant –usually not exceeding a quarter of an inch in length...these are so inconspicuous that we seldom see them or recognize them as special. This form of fern is called the “gametophyte” and it has within it genetic information for making a structure that will form sperm cells and another structure for forming reproductive eggs.  They generally mature at different times so they dont fertilize themselves.  Gametophytes are true independent plants necessary for the life cycle of the fern to be completed.
After fertilization, the fertilized egg stays attached to the gametophyte and it grows into a sporophyte plant
The 'n' number refers to whether the cells have one or two sets of genetic information
On a rainy day, the sperm are washed from onr tiny heart shaped gametophyte to another, from the sperm (n)  producing structure into the egg bearing structure,  and the egg (n)  becomes fertilized.   The new plant (2n cells)  grows directly out of the gametophyte.  They now have two sets of chromosomes in each cell, one from each reproductive cell.  (2n)  This new plant (what we call a 'fern plant') is called a “sporophyte” because it will grow into the ferns that we know and produce spores.

Arid cactus - Sporophyte individual

The amazing thing is that while each cell produced in the fern will have a full  2n set of genetic instructions copied from the fertilized egg.  An environmental cue is required to start a sequence of events that turn on a particular gene and allow it to express itself by forming a chemical response ( – like moisture, temperature, or light -).

All this taking place in the simple appearing little ferns in my back yard!

Broad leaf fern - the frond is never divided into leaflets

Spore cases on underside of Broad Leaf fern