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tasmanian hornworts

hornworts

Hornwort scene
1. Anthoceros or Phaeoceros sp.
Hornwort sporophyte
2. young sporophyte
Hornwort chloroplasts
3. single chloroplasts

 

These are thallose plants that look like some liverworts or lichens; sometimes all three are found growing together, as in this case.

We found this growing near the Crystal Bridge and initially identified it as a darker form of Aneura rodwayi. However we noticed from the images that the sporophytes had typical horn shape making this a hornwort.

Identification requires examination of spores as well as checking the number of chloroplasts at cellular level - our specimens had a single chloroplast per cell which made it either Anthoceros or Phaeoceros (but not the other possibility, Megaceros which has two chloroplasts per cell). Anthoceros has black spores and Phaeoceros has yellow but we were unable to discover any so this will remain unidentified for the present.

The first image is of hornworts growing under natural conditions. The second is of a newly emerging sporophyte which will elongate before dehiscing for spore dispersal. The third image depicts single chloroplast within each cell.

The prevailing wisdom is that the sperm is sufficiently motile to find its way to the archegonia under moist conditions. We were somewhat sceptical of this and suspected that the tiny worms (microarthropods) commonly found in this environment are largely responsible for transportation although we do not question the ability of the sperm to navigate the last few microns. The contours of the thallus probably help dispersal by water pooling on the surface.

Hornwort antheridium
4. hornwort antheridium
Hornwort antheridia
5. antheridia close-up
Hornwort antheridium
6. detached antheridia

 

2009/09/15: We were forced to revise our opinion some weeks after discovering the first specimen. We had collected a section of thallus which was kept alive with no greater effort than keeping it moist. The sporophytes kept developing and several weeks later we decided to take a closer look at the antheridium.

A minor miracle was taking place under our eyes when we realized that sperm was being expelled from the antheridium. This was no passive release - a strong stream of fluid was being forced out of what must have been a rupture in the antheridium covering and continued for several tens of minutes (we were too engrossed with what was going on to do time keeping!).

The image on left shows hornwort antheridium and that in the middle, at high magnification, shows sperm being released from roughly 120° position. Note how some of the container cells are now empty. The specks below the anteridium are sperm cells - we could see vigorous movement but the cells were too small to record on camera.

We suspect that the antheridia releases its contents over a long period, the timing dependent upon ambient conditions. We checked for activity several hours later and discovered more cells had released their contents and the visible area was teeming with life (possibly Brownian motion!).

We submitted this item to Bryonet, the list-server for bryologists, and were gratified to read shortly after that Zhang Li, a Chinese researcher, had observed the same earlier for a different hornwort, Folioceros glandulosus.

2009/09/25 - Six weeks after we collected the original specimen we discovered that the antheridium detached fairly easily from the thallus (pix 6 above), possibly because of maturity. We placed one under the microscope and observed it for several hours. Again we observed fluid was being expelled out of the antheridia although at a lower rate than on previous occasion.

We managed to record a 74 second segment of the activity on video which is a 137MB .avi file. These were converted to highly compressed (and pixellated) versions which depict the activity. (The videos and images are copyright of bluetier.org but we are placing them in the public domain; they may be used for any purpose with or without attribution.)

This is a 74 second compressed .flv video that works on Totem Player under Linux (1.3MB).

Here is the same compressed to .swf video that may work on Flash players under other operating systems.

 

Web References
- Australian National Botanic Gardens provides an introduction to hornworts
- Chapter 2-8 of Janice Glime's Bryophyte Ecology is devoted to Anthocerotophyta (pdf file, 650KB)
- A. Tomescu has a page on Phaeoceros species with links to many micrographs
- Southern Illinois University has a page of Phaeoceros carolinianus images which is probably typical of hornworts

Page URL: http://www.bluetier.org/nature/hornworts.htm

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