Saturday, March 11, 2017

Mosses of Central Florida 20. Polytrichum commune

[For other mosses in this series, see the Table of Contents]

Polytrichum commune Hedwig (Polytrichaceae) is the giant among Florida mosses, with stems up to 10 cm. long. The stems are upright, with numerous stiff, narrow leaves.  It is not common in Central Florida, but when  found,  it is usually in extensive, dense colonies.
A colony of Polytrichum commune growing near Ft. Lonesome, Florida, in
Hillsborough County.  Many of the shoots in this picture bear clusters of sperm-producing antheridia at their tips. Photo by Steve Dickman.
There are seven species of Polytrichum found in North America, out of 70 worldwide.  P. commune is found throughout North America, Eurasia, New Zealand and Australia.  I recently found it in Taiwan.

The single specimen with sporangia at USF
shows the impressive dimensions of
Polytrichum commune. It comes from
northern Florida.
The leaves in the Polytrichaceae are unusual in consisting mostly of a massive midrib covered with vertical sheets of tissue arising from the upper surface.  This makes viewing the leaf anatomy more difficult, but the edges of the leaf in Polytrichum commune are lined with distinctive sharp teeth.  The bases of the leaves flare out into a broad sheath, and only here can one see the shapes of the cells.

 The upper surface of a Polytrichum leaf is covered with vertical,
blade-like sheets of tissue, here seen in cross section.
Photograph by Kristian Peters; Creative Commons
Attribution-Share Alike 3.0"
The midrib of Polytrichum commune is thick and fills the entire
blade.  Distinctive hard teeth line the edges.

The base of the leaf flares out into a thin
sheath, where the cells can be seen to be quite
narrow and elongate.

Evidently, Polytrichum commune rarely produces sporangia in Central Florida, as we have   We have o such specimens in the USF herbarium.  I can only speculate on the reasons for this.  In order for sporangia to form, sperm cells must swim from the tip of a stem where they are produced to the tip of a stem where eggs are being produced.  The large size of Polytrichum commune would make the conditions where this process could take place rather rare, particularly in the relatively hot and dry climate of Central Florida.  The species is at it's southern limit here. The one specimen with sporangia I've seen was collected in northern Florida.

 In addition, the colonies of Polytrichum are unisexual - they produce either sperm or egg, so two colonies of different sex must intermingle for sexual reproduction and spore-formation to take place.  The size of the plants would add to the difficulty of such mingling, and it is possible that some colonies in our area were established by a single spore, and hence unisexual.  In the photo  of living plants in Hillsborough County above, the colony appears to be all male.

Incidentally, in the process of researching this species, I found another excellent blog site for mosses from the University of British Columbia. Check it out for general information about the different groups of mosses.

No comments:

Post a Comment