SPORE LIBERATION 

 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 spores into the air. In most leafy liverworts, however, a spring 

 mechanism released by water-rupture in the dr\'ing elaters throws the 

 spores into the air (Ingold, 1939, 1956), while in FruUania 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 dr}'ing 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. 



PTERIDOPHYTES 



Spores of Pteridophytes (ferns and their allies) are formed on the 

 fronds m ithin 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 60 per cent, according to the species. 



Pollination of Phanerogams 



Insects and wind are the chief agents of cross-pollination in flowering 

 plants. Other pollinating agents that are effective in a far smaller number 

 of species include water and humming-birds. There are probably ten 

 times as many entomophilous (insect-pollinated) as anemophilous 

 (wind-pollinated) species of flowering plants in the world as a whole. 



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