THE MICROBIOLOGY OF THE ATMOSPHERE 



{Phytophthora infestans) and onion downy mildew {Peronospora destructor)^ 

 n approaches 4. If n is small, local hygiene will soon be defeated by the 

 flow of infection from outside; but if n = 3 or more, 'a considerable 

 reduction of infection should be possible by a group of neighbours without 

 it being neutralized almost immediately from elsewhere'. 



TABLE XXVIII 



RE-COLONIZATION OF KRAKATOA 



The volcanic island of Krakatoa lies between Java and Sumatra and 

 is almost encircled by land. The eruption of August 1883 blew away 

 the mountainous two-thirds of the island, leaving a hole in the sea-bed 

 300 metres deep and covering the remainder of the island with lava and 

 ashes. A column of fine dust rose to a height estimated at 27 km., and was 

 carried westwards by the prevailing wind. Eventually this dust circled 

 the Earth repeatedly, spreading over the whole tropical and temperate 

 zones (Symons, 1888). Although this world circulation of dust is relevant 

 to long-distance dispersal of microbes, the story of re-colonization of the 

 island after the destruction of living things, which is usually considered 

 to have been complete, is of even greater significance. 



Three years after the eruption, the only flowering plants found by 

 Treub (1888) were two species of Compositae and two grasses. There were 

 also eleven species of ferns, and the volcanic deposit was colonized by a 

 film of blue-green algae (Cyanophyceae). All these were thought to have 

 been carried in by wind — the nearest land being 40 km, away. 



Records of subsequent visits are summarized by Ridley (1930), who 

 concluded that, of the 144 species of flowering plants then reported, 24 

 per cent were wind-distributed, 42 per cent were sea-borne on floating 

 tree-trunks, and most of the remainder had probably been carried by birds. 

 In addition there w^ere forty-eight species of Pteridophytes and nineteen 

 of Bryophytes — all potentially wind-borne. Boedijn (1940, and see 

 Leeuwen, 1936), who believed that most of the fungi present had been 

 carried to Krakatoa by wind, was impressed also by the paucity of lichens, 

 of which he found only thirteen species (o-i per cent of the world's list), 

 compared with sixty-one species of Pteridophytes and 263 of flowering 



184 



