IV 



SPORE LIBERATION 



Up to now we have considered only the physical properties of spores and 

 of their environment. Spores, however, are parts of living organisms whose 

 evolution has been extensively moulded by the environment. The air- 

 spora comes mainly from species which are highly adapted towards using 

 wind energy for their dispersal. The physical properties of the atmosphere 

 make dispersal possible, but also set problems to organisms using it. 

 Adaptations for wind transport have been evolved in many widely- 

 separated taxonomic groups. 



The process of wind dispersal of spores has three principal stages, 

 (i) Spore liberation. This chapter describes the processes by which pollen 

 grains or spores 'take-off' into the air from the structure where they have 

 been formed. (2) Dispersion. Chapter V describes the transport of spores 

 by gentle air currents or strong winds, and the diffusion of an airborne 

 spore cloud. (3) Deposition. Chapters VI and VII will deal with the pro- 

 cesses by which spores leave the air and land on a surface — a necessary 

 prelude to the germination of a pollen grain or a mould spore on its 

 substratum. 



The spore or pollen output of many species is notoriously large. For 

 instance, Pohl (1937) estimated, for the dominant species encountered 

 by pollen analysts, the pollen production per stamen, flower, inflores- 

 cence, and branch, revealing an annual production averaging many 

 millions per square metre of ground covered {see also Erdtman, 1943). 

 According to J. J. Christensen (1942), a field of wheat moderately affected 

 by Puccinia graminis would produce at least 25 million uredospores per 

 square metre. Duller (1909) estimated that one giant puff'ball {Calvatia 

 gigantea) produced 7 million million (7 x 10^^) spores. The spore output 

 of mosses and ferns is also potentially enormous. 



PROBLEMS OF 'tAKE-OFF' 



As described in Chapter III, the surface of the ground or plant is 

 covered by a thin layer of still air and by the laminar boundary layer of 

 slowly moving air ; a spore will fall through this composite zone under the 

 influence of gravity. To tap the energy of moving air for dispersal a spore 

 must overcome the adhesive forces which tend to keep it in contact with 

 neighbouring spores or with the substratum. It must cross the still- and 



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