Adder's-tongue (Ophioglossum vulgatum) is a small plant you can walk by and not know it is there. It is little more than an inch or so high and it looks like the leaves of a flower that has just come up above the ground. It is really a case of getting down on your hands and knees for a good look at this. It is a bit like wild arum with a sheath enclosing a central spike. It is, however, a spleenwort (part of the fern family structure) and not an flower at all. I only found this thanks to help from a ranger at Durlston which is the only place I have knowingly seen it.
The plant is visible from May through until November; the central stem that is supposed to resemble a snakes tongue is not a flower spike in the conventional sense but it is the organ that emits the spores. This stem is not alwys present.
My book describes it as widespread and frequent in lowlands, especially in damp meadows, scrub and open woodlands on limestone soils and so I guess that is why it grows at Durlston! I have never seen it anywhere else and I understood this to be a rare plant, after all limestone is not that common a rock or soil type.