The physiological mechanism that induces development of a structure one thing, and its ultimate, adaptive function is another (e.g., how fingers develop during human embryonic development does not explain how they are later used in adult life). Differential auxin levels might very well provide the physiological explanation for the upward root development, but it does not provide an explanation for their adaptive function.
Among orchid scientists, the consensus is that these erect, pointed roots function as impounders of debris falling from the canopy. In nature, the plants that produce them accumulate debris around their roots, which obviously provides an additional source of nutrients. From Robert Dressler's book (The Orchids: Natural History and Classification; 1981, page 92): "Grammatophyllum, Ansellia, Graphorchis, Cyrtopodium, some species of Catasetum, and some members of the subtribe Stanhopeinae, for example, produce thin, stiff roots that grow upward and outward from the substrate and serve very well as debris collectors (fig. 4.7). In other cases the stems or pseudobulbs themselves may accumulate some trash" The stiff, erect roots are non-absorbing, but retain the debris in place for the normal roots below to absorb the leached nutrients. -- Mario A. Blanco Herbarium FLAS Florida Museum of Natural History University of Florida 385 Dickinson Hall P.O. Box. 117800 Gainesville, FL 32611-7800 U.S.A. [EMAIL PROTECTED] wrote: > 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 > > > _______________________________________________ the OrchidGuide Digest (OGD) orchids@orchidguide.com http://orchidguide.com/mailman/listinfo/orchids_orchidguide.com
