An example of commensalism - Spanish moss

Students learning to become teachers often come to the Phactor for assistance. They want examples, specimens, things they can take to classes and show younger students to help make a point or stimulate some thought and questions. And of course we have a greenhouse full of such examples, and for the right price. But this time they wanted an example of commensalism, an interaction between two organisms where one gains a benefit and the other is unaffected. That's what the textbooks say, but does commensalism really exist? Can one organism intimately associated with another organism gain a benefit while really having no affect at all? Perhaps it should say a negligible affect, a positive or negative affect, but so small it probably matters not at all, like that penny your dropped that rolled under the counter and you just decide to heck with it. A negligible sum not worth the effort of squating down and crawling around to find retrieve it. At what sum you would go to the effort says a great deal about you and your circumstances. At any rate the plant provided was Spanish moss (Tillandsia usneoides - a bromeliad, a member of the pineapple family), which is neither Spanish nor moss, but a rootless epiphyte that grows by draping itself upon the branches of trees, thus "stealing stature" to no great detriment to the tree unless it becomes too heavy or shades out too many tree leaves. Lincolnland is too far north for Spanish moss to survive the winter, but botanical friends used to go south each spring break to eat some Cajun food and pick up big bags of Spanish moss to hang from the spreading branches of the big oak in the front lawn of their big old house back up here in the north, thus affecting a sort of southern gentility while they sat on the veranda sipping julips along with their dog Beauregard, so the 3-way interaction now becomes a mutualism because their epiphyte decorated tree improved their attitude.