PTERIDOPHYTES Spores of Pteridophytes (ferns and their allies) are formed on the fronds within a closed sporangium, from which they are dispersed into the air by a `sling' mechanism depending on water-rupture under great tension as the maturing sporangial wall dries (see Ingold, 1939). Pettersson (1940), in Finland, found that effective scattering of fern spores takes place out of-doors only when the relative humidity of the air falls to 76 per cent, or even to 6o per cent, according to the species.