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. (t) 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, 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. Buller (1909) estimated that one giant puffball (Calva-tia gigantea) produced 7 million million (7 x spores. The spore output of mosses and ferns is also potentially enormous.