Friday, March 7, 2008

Petiole Vasculature

One fern feature that is useful in identifying a specimen is the pattern of its vascular tissue at the base of the petiole. We talked a lot about this on the fern course in Costa Rica, because it is particularly useful in determining which major group a fern belongs to, which will narrow down the possible families it could be in. 

The petiole of a fern is the narrow, stem-looking tissue (that's not actually the stem-the stem is the rhizome) to which the leafy blade is attached. If you clip this stalk right at the base, and look at it through a hand lens or with the naked eye if the fern is large enough, you should be able to see a pattern of lines or circles. These are the vascular bundles, composed of xylem and phloem, which conduct water and sugar through the plant. Certain patterns of the bundles are characteristic of the group of fern you are looking at; the omega shape in the first three photos above can be found in Eupolypods I and a few other groups, while the last image is of Angiopteris, a fern in the more basal family Marattiaceae.

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