SPORE LIBERATION 



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, 1956^; Ingold & 

 Hadland, 1959). 



Four clearly-distinct t}-pes of liberation are recognized in the Asco- 

 mycetes by Ingold (1953), as follows: 



'i. 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 Pyrenornycete 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 cleistocarp. This is 

 rather like a perithecium but is completely closed; there is no ostiole. 

 In this t}'pe the swelling asci must first burst the wall of the cleistocarp 

 before they can emerge and discharge their spores. 



'4. In the Myriangium t}'pe, 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 Chaetomium 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) Roundifig-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 

 aecidia 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 



37 



