Hello Rocky, Max and Jim, I missed Rocky's original post. Somehow it never made the digest form of the OGD. As far as why roots grow up or down I dug up this old post I wrote to the OGD a few years ago. You can read more about upward growing, negative gravitropic roots, if you search the OGD archives for "catch roots".
My hypothesis is that in upward aerial roots auxin is of a high enough level, plus with a phototropic response, to overcome statoliths banging on the cell’s membrane wall (positive gravitropism). Epithetic orchid roots occupy an intermediary between “classic” roots and upper growth because of photosynthesis. (There are a number of orchids that are mainly a mass of roots with little to no leaf.) Basically all epithetic orchid roots are aerial some just happen to come into contact with and adhere to a surface. In the case of what are being called “catch roots” the straight upward growth is a phototropic response. Both the catching of moisture or debris, and the presence of ants seem to be of more of coincidence then a reason for a negative gravitropic root growth. Root growth in search of moisture, debris, or a surface to adhere to is more hit and miss without a mechanism to cause that growth to move in a certain direction because it can sense a surface, moisture or more moisture at a certain spot. Interestingly one of the things I came upon was from Orchid Biology, Reviews and Perspectives, V, edited by Joesph Arditti, Water Relations in Orchids, Russell Sinclair, page 69: “Some orchids e.g., Ansellia, Catasetum, and Graphorkis produce dense clusters of fine, negatively geotropic roots. Johansson (1974) suggested that these may function by condensing water vapor at night which is then transferred to living tissues, allowing such species to survive in dry habitats. However, Barthlott and Capesius (1975) detected no uptake of tritiated water by these, in contrast to the thicker aerial roots of several other species. Dew on plant surfaces may be an important source of water in some plant communities such as cloud-forest. However, the importance of aerial roots in the absorption of dew is not yet clearly known.” So at least in the seventies what we are now calling “catch, trash, eagle nest roots” were called by some negative geotropic roots. The modern term would now be negative gravitropic roots. Moisture as a reason was proposed but somewhat ruled out. Mark Sullivan Hello Rocky, Regarding your question on the roots of catasetums. I think that this is natures way of ensuring that the plants get all the nourishment that they require when growing on trees etc. Whereas when they are growing in pots, they receive all the food and water from their grower. I have seen a similar thing happen with some of the Dendrobium speciosums. Please don't ask me where the trigger is that determines whether the roots grow up or down!! Cheers Max. _______________________________________________ the OrchidGuide Digest (OGD) orchids@orchidguide.com http://orchidguide.com/mailman/listinfo/orchids_orchidguide.com
