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Based on what Charles and AJ pointed out and a little more
reading, “catch roots” would be better called phototropic or negative
gravitropic roots. The reason phototropic roots would be correct is
that
epiphytic orchid roots photosynthesis. 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). Epiphytic 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 epiphytic 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." Mark Sullivan Thanks AJ, Charles, Dennis, and Tom. |
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