BRYOPHYTES Spores of mosses and liverworts are formed in sporangia which are typically raised on stalks above the substratum, but the structure of the sporangium is quite different in the two groups. The moss sporangium is a firm `box' opening at the top, whereas the liverwort sporangium breaks open completely, exposing the spores in a mass of stiff threads (elaters).
In the simpler liverworts the spores may be blown away by wind from the mass of elaters, or the elaters may twist hygroscopically, actively throwing sports into the air. In most leafy liverworts, however, a spring mechanism released by water-rupture in the drying elaters throws the spores into the air (Ingold, while in Frullania the sporangium explodes by an efficient spiral spring mechanism which also is released on drying.
The mosses liberate spores from the stalked sporangium (capsule) by two principal methods. Sphagnum has an `air-gun' mechanism (Ingold, 1939). An air space below the spore mass is compressed by transverse contraction of the drying sporangium wall, internal pressure increases and, finally, the top of the sporangium breaks, ejecting a spore cloud to a height of 15 or more centimetres.
Most of the other mosses have flask-shaped sporangia, which open gently at the top when mature. In some genera the mouth of the spor angium is surrounded by one or more rows of triangular teeth which move hygroscopically, closing the mouth at high humidities. To what extent spore liberation in nature depends on shaking of the sporangium in the wind, and what role is played by hygroscopic movements of the teeth in actively throwing out the spores, is still a matter of controversy; but evidently spore liberation is checked by high humidities and low wind-speeds.