FUNGI Adaptations facilitating air dispersal show more diversity in the fungi than in any other group—except, perhaps, adaptations for seed dispersal among the flowering plants. They vary from the passive but quite effective processes in the Fungi Imperfecti, to the spectacular ballistic feats of the ascus gun. The various mechanisms have been summarized by Dobbs (1942, 1942a) and Ingold (1953, 196o), and they formed one of the main topics of the classical work of Buller (1909-5o).
In contrast, spores of many other species of fungi rarely get into the air but are carried by insects, on seeds, or in soil. Mere dispersal by in sects may be relatively unimportant; but, where the insect actively inoculates the substratum or host, it is a mechanism comparable in efficiency with insect pollination of flowering plants. Passive liberation by the action of external energy depends on `spore presentation' (Hirst, (i) Shedding of spores under gravity. Stepanov (1935) concluded that spores of some Cunninghantella species, and of some Fungi Imperfccti, including Botrytis cinerea, Monilia sitophila and Helininthosporiunt sativtnra, as well as the macroconidia of Fuserium, could be shed under gravity. However, as he also showed that minor air currents released spores of some of these fungi, the effect remains uncertain.
(ii) Shedding in convection currents. Stepanov (1935) placed open Petri-dish cultures at the bottom of glass cylinders io to 12 cm. high in which convection currents were induced by differential heating. Sticky slides or a surface of inverted sterile medium at the top of the cylinder trapped spores which might become detached and carried aloft by con vection. With temperature differences of the order of io°C., conidia of Monilia sitophila and Botrytis cinerea were freely transferred upwards, but CoI/etoirichurn lini was not. Smaller temperature differences, such as resulted from the slight heat produced by a mould culture or an electric lamp shining on the floor, were ineffective.
(iii) Blowing away (`deflation'). This occurs commonly with dry spored fungi including moulds, smuts, and rust uredospores. The spores arc often `presented' on an elevated sporophore, any stem or leaf patho gen usually being adequately raised on its host tissue. Quantitative studies so far are insufficient to lead to a theory of `deflation'. Little is known about the quantitative effect of wind-speed on liberation, but there is good evidence that the higher the wind-speed the more spores are carried away.
Stepanov (1935) was apparently the first to use a small wind-tunnel to blow spores at controlled wind-speeds. Using either cultures or plants infected with pathogenic fungi, he found that the minimum wind-speed required to remove spores varied according to the organism being tested: for Botrytis cinerea it was o•36—o•5o metres per sec.; for Monilia sitophila, Ustilago spp., uredospores of Puccinia triticina, and Helminthosporium sativum it was o•51—o•75 metres per sec.; for aecidiospores of Puccinia coroniftra and P. pringsheimia, o.76-2 -o metres per sec.; for Cunning hamella sp., 1•5-1.75 metres per sec. On the other hand, Phytophthora infestans and Fusarium culmorunl spores were not removed at any speed tested up to 3•37 metres per sec. More spores were removed in turbulent than in streamlined wind.
A special structure facilitating blowing away is the `wind-cup' described by Brodie & Gregory (1953)• Flow of air over a cup-shaped structure produces a double eddy system which can effectively remove dry spores contained in the cup, as shown by wind-tunnel experiments with smoke and Lycopodium spores. Soredia were also removed from the podetia of Cladonia at 1-5 to a•o metres per sec., and spores were removed from the cupulate sporangia of certain Myxomycetes at o•5 metres per sec.
Certain Gasteromycetes, including the puffballs Lycoperdon perlatunt and L. pyriforme, and the earth-stars (Geaster spp.), have a `bellows' mechanism consisting of a thin, flexible, waterproof wall covering the spore mass. Indenting this wall forces out a jet of air laden with spores. Contact with animals operates the bellows efficiently, but must be a relatively rare event in nature. Raindrops or run-off drops from trees also operate the bellows mechanism, and as one fruit-body would be hit many thousands of times in a season, rain is probably the most effective mech anism in the field (Gregory, 1949). In India W. H. Long & Ahmad (1947) find that the bellows mechanism of Tylostoma is operated by wind-blown sand grains in addition to raindrops.
(iv) Mist pick-up. This is a mechanism that has only recently been recognized. Dry, or even humid, wind fails to detach spores of some fungi which are nevertheless readily removed from their conidiophores by collision with minute droplets carried by mist-laden air. This method is known to function with two important crop pathogens, Cercosporella herpotrichoides (Glynne, 1953), and Verticillium albo-atrwn (R. R. Davies, and it may play a part also in the dispersal of Cladosporium.
(v) Splash dispersal. Spores of some species are `presented' in sticky masses to which they adhere tenaciously in wind. However, spores may become incorporated in splash droplets (Plate 3, and Fig. 4) which are thrown up from the impact of a falling raindrop, or a drip from a leaf, hitting a liquid film containing spores (Gregory et al., 1959). Rain splash is thus another passive mechanism, quite different from mist pick-up by which slime-spored fungi may become airborne in the smaller droplets. Experiments suggest, however, that the larger splash-droplets, over 5o or too /i in diameter, carry most of the spores which are dispersed in this fashion, and these droplets arc massive enough to follow definite tra jectories without being truly airborne.
(vi) The splash-cup mechanism. This is a device, studied particularly by Brodie (1951, 1957), which is widespread among lower as well as higher plants, by which the energy of falling raindrops throws relatively large bodies to distances of several feet. Examples are the peridioles of the birds-nest fungi (Nidulariaceae), the gemmae of Polyporus conchatus, and droplets bearing spermatozoids of the Bryophytes. However, the projectiles scarcely become airborne, for they follow a definite trajectory.
(vii) Hygroscopic movements of conidiophores, which may result in detachment of spores during violent twisting, occur in a number of Fungi Imperfecti and Phycomycetes. The effect depends on rapid changes in atmospheric humidity and is often most marked in the morning hours.
All active mechanisms for spore liberation depend on the fungus having sufficient water-supply. The more ephemeral fruit-bodies develop after rain and discharge spores for a short period only. More durable fungi can be dried but will discharge spores again when re-wetted; others again can draw on an extensive mycelium deep in the substratum and may be almost independent of the weather. Distances of ejection vary and have been compiled by Spector p. (viii) Squirt gun mechanism. This is found in many Ascomycctes in which the ascus, which contains the ascospores, typically swells at maturity and finally bursts at the tip, projecting the spores into the air to a distance varying from a fraction of a millimetre to several centimetres. The larger the projectile, the further it tends to be shot (Ingold, 1956a; Ingold & Hadland, 1959).
Four clearly-distinct types of liberation are recognized in the Asco mycetes by Ingold (1953), as follows: `1. In the Discomycete type the spore-producing surface, consisting of asci intermixed with parallel paraphyses, is more or less exposed, most often as a lining to a shallow cup-shaped apothecium. The extensive exposed hymenium allows opportunities for `puffing'—the simultaneous bursting of numerous asci.
`2. In the Pyrenomycete type the asci are contained in a small flask shaped structure (perithecium) which opens to the outside by a minute ostiole. Before each ascus can discharge the spores, its tip must reach the ostiole, and the canal of the neck is usually so narrow that normally only one ascus can emerge at a time.
`3. In the Erysiphales type the fruit-body is a cicistocarp. This is rather like a perithecium but is completely closed; there is no ostiole. In this type the swelling asci must first burst the wall of the cleistocarp before they can emerge and discharge their spores.
`4. In the Myriangium type, though the hymenium is exposed in a structure like a small apothecium, the spherical asci are embedded in a plectenchymatous tissue and are free to discharge only when this gradually undergoes gelatinization.' Some Ascomycetes which lack explosive asci may liberate spores in slime to be dispersed by rain-splash. Other species, again, may be either explosively or slime-dispersed, according to the conditions obtaining. With still others, such as Chaetonuum whose spores are regularly found in the air, the spore discharge mechanism is unknown.
(ix) Squirting mechanisms, which propel spores violently into the air, occur among Phycomycetes in Pilobolus, Basidiobolus, and Entomophthora muscae, as well as in the imperfect genus Nigrospora (Webster, 1952).
(x) Rounding-off of turgid cells acts as a discharge mechanism when the flattened double walls between two turgid cells suddenly separate. By this means spores of some Phycomycetes can be ejected up to a centimetre into the air. The same mechanism operates to eject aecidiospores when accidia of rusts become moistened. Discharge of all these types is favoured by high humidities, and indeed aecidiospores of the rust fungi are dis charged under conditions unlike those favouring dispersal of uredospores.
(xi) Basidiospore discharge. This is a highly characteristic process which is found with the same essential features almost throughout the Basidiomycetes. The basidium is a cell producing one or more sterig mata, at the end of each of which one basidiospore is formed asymmetri cally. Typically, when the spore is mature, a drop of water is excreted at the hilum end of the spore and almost immediately the spore is shot off to a distance of a fraction of a millimetre or more. In species which form the basidia on exposed surfaces, as in many lower Basidiomycetes, the spore after discharge has a chance of being picked up by an air current.
The higher Basidiomycetes often show great elaboration of a stalked fruit-body with the basidia lining the vertical surfaces of folds, gills, pores or spines. Here, in cavities protected from wind and adverse conditions, the basidiospores are discharged into still air and fall under the influence of gravity into the moving air-current below the cap-shaped or bracket-shaped fruit-body. Spore discharge in the higher Basidiomycetes often goes on continuously throughout almost the entire life of the fruit body—to all appearances little affected by wind, temperature, or humidity, though it must be emphasized that accurate quantitative studies on the effects of these factors are lacking. Just how a basidiospore is shot off the sterigma remains a major puzzle of mycology; several explanations have been advanced, but none seems entirely satisfactory. Nevertheless the process is highly efficient and basidiospores are a conspicuous com ponent of the air-spora.
The same mechanism occurs in the mirror-yeasts (Sporobolomy cetaceae), which may possibly have evolved from lower Basidiomycetes (unlike the Saccharomycetaceae, which are clearly Ascomycetes). To avoid prejudging the issue by calling the spores of the mirror-yeasts `basidiospores', the term `ballistospores' has been coined to include all spores showing the drop-excretion discharge mechanism. A moist sub stratum is necessary for spore discharge in the Sporobolomycetaceae.