The seeds of European mistletoe pass safely through a bird's gut yet stick to branches where they germinate due to mechanical properties of the cellulosic filaments in their sticky coating.
"The only European mistletoe is the strange twin-leaved parasite that once played an important part in human fertility rites, perhaps because in winter its leaves remain green and visibly alive when those of the tree on which it grows have all fallen…Its white berries have flesh that is so extraordinarily sticky that when a bird such as a thrush or a blackbird tries to eat them, they often become stuck to its beak. The bird finds this so irritating that it tries to wipe the berry off by scraping it on to another branch and in doing to, rams it into a crevice. The seed then puts out a root which worms its way into the tree and eventually connects with the vessels within the branch that carry the tree's sap. And with that as food, it flourishes." (Attenborough 1995:229-230)
"The results presented in this study illustrate the remarkable characteristics of the cellulose located in the thick cell walls of the viscin tissue from V. album. Initially, the microfibrils of this cellulose are indeed tightly coiled perpendicularly to the viscin cell axes. Due to the hemicelluloses that are also present and when the hydration is sufficient, these cellulose microfibrils are free to move past each other in such a way that the wall of the viscin tissue is able to get deformed upon the slightest stretching action. This deformation can reach extraordinary values of several hundred folds without breakage, each viscin cell giving one tiny cellulose filament having no more than a few microns in width, as opposed to the initial viscin cells that had diameters of several tens of micron. As deduced from diffraction experiments (Figure 9(a)), the orientation of the cellulose in these stretched filaments is unusually high. The combination of this high orientation with the fairly large molecular weight of the corresponding cellulose indicates that these filaments should have high mechanical properties. Their strength must in fact be correlated to the biological function of the viscin tissue, which is to hold firmly the mistletoe seed through the bird guts. When expelled from the bird and dropped on the branch of a tree, the seeds will normally stick to the branch thanks to their hemicellulosic glue. Some of the seeds will even dangle down from the branch, held by the viscin cellulosic filaments. Under the action of the wind, these seeds will be brought back in contact with the branch to which they will adhere for further germination. This phenomenon explains why mistletoe sometimes germinates even at the underlying part of branches." (Azuma et al. 2000:16)
Learn more about this functional adaptation.
- Attenborough, D. 1995. The Private Life of Plants: A Natural History of Plant Behavior. London: BBC Books. 320 p.
- Azuma, J-i; Kim, N-H; Heux, L; Vuong, R ; Chanzy, H. 2000. The cellulose system in viscin from mistletoe berries. Cellulose. 7(1): 3-19.
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