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 laminar-air layers, at the interface between the ground or other surface and the atmosphere, in order to enter the freely moving air of the tur bulent boundary layer, where it stands a chance of being carried into higher layers of the troposphere.
Many species that are distributed as spores have not solved this problem, but instead have become adapted for dispersal by some other agency such as water, insects, or other animals. There are more insect pollinated (entomophilous) species of flowering plants than wind-pollin ated (anemophilous) species, though in the temperate regions at least there are more wind-pollinated individuals because of the preponderance of grasses and anemophilous trees. We may wonder how important in practice is the occasional dispersal of a spore by some agency other than that to which it is adapted. However, it is a fundamental principle that the better a species is adapted to dispersal by one agency, the poorer are its chances of dispersal by another agency—unless, like many fungi, it produces spores of several distinct types that are specialized for different dispersal mechanisms.
If we wish to control the dispersal process, a precise knowledge of the mechanisms involved is preferable to the vague idea that the spores will get there somehow anyway! Success in colonization or fertilization de pends on logistics—on getting enough material to the right place at the right time.
Energy is required to detach spores from their source. It may be an active process through which, by some explosive or hygroscopic mechan ism, spores are discharged by energy operating through the parent struc ture. Or it may be passive, by the energy of an external agent—usually wind or the kinetic energy of falling raindrops. Seasonal development of the parent structure and maturation of the spores commonly determine what organisms are in the air at a particular time, but other factors modify this pattern. The working of the various discharge mechanisms is more or less affected by external conditions, and the result is that the output of spores of a particular species varies greatly from time to time. Conversely, all the individuals of one species in an area may behave in unison, so that the composition of the air-spora differs vastly on different occasions.