Alternation of generations in ferns


Transverse sections through a fern frond (Move the mouse over the pictures for labels)




As the indusium dries, it shrinks and the spore carrying structures, the sporangia, are exposed. Each sporangium can be seen to be surrounded by a ring (the annulus) of cells. As drying out continues, the sporangia lose water; this causes a tension to build up in the annular cells. Ultimately, the annulus ruptures and the spores inside are "shot" out.


If you leave a ripe fern frond, with its lower side down on a piece of paper for a couple of hours and then carefully remove the frond, you will often see the scattered spores in a ghostly fern frond pattern on the paper.

Alternatively, view the ripe sporangia under a stereo microscope and watch them explode, an exciting experience!



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