THE MICROBIOLOGY OF THE ATMOSPHERE 



The discovery had to wait almost until the invention of the micro- 

 scope. 



For a long time after Aristotle and Theophrastus, the lower plants 

 lacking obvious seeds were believed to be generated spontaneously in 

 decaying animal or vegetable matter. The same view was held of the 

 origin of many of the lower animals. However, the minute 'seeds' or spores 

 of several kinds of plants were observed in the mass long before the 

 invention of the microscope allowed them to be identified and observed 

 individually. What was more natural than to suppose that these minute 

 particles were wafted about by the winds } 



The discovery of reproduction of ferns is attributed to Valerius Cordus 

 {b. 1515, d. 1564), and spores of the fungi seem to have been observed 

 soon after this by a Neapolitan botanist, J. B. Porta, although the rusty- 

 coloured spore deposits under bracket-fungi on beech trees must always 

 have been familiar to the countryman. 



It was P. A. Micheli {b. 1679, d. 1737), botanist to the public gardens 

 at Florence, who first illustrated the 'seeds' of many fungi, including 

 mushrooms, cup-fungi, truffles, moulds, and slime-moulds. Further, by 

 sowing spores on fresh-cut pieces of melon, quince, and pear, and repro- 

 ducing the parent mould for several generations, he showed that the 

 spores of some common moulds were, indeed, 'seeds' of the fungi. He 

 noted, however, that some of his control slices also became contaminated, 

 and he concluded that the spores of moulds are distributed through the 

 air {see Duller, 191 5). 



The hand-made lenses of Anton van Leeuwenhoek rendered visible 

 the world of minute organisms whose existence had only been guessed at 

 before, and whose significance in nature had scarcely even been imagined. 

 He could just see bacteria, and in his letters to the Royal Society in 1680 

 he described some yeasts, infusoria, and a mould. From his experiments 

 he came to doubt the current belief in spontaneous generation; it seemed 

 more plausible to him to suppose that his 'animalcules can be carried over 

 by the wind, along with the bits of dust floating in the air' (Dobell, 1932). 

 The controversy over spontaneous generation was to last for a couple of 

 centuries ; but, in the second half of the eighteenth century, ideas w^ere 

 developed by Nehemiah Grew and E. F. Geoffrey on the function of the 

 pollen of flowering plants. J. G. Koelreuter, in 1766, was perhaps the 

 first to recognize the importance of wind-pollination for some plants and 

 of insect-pollination for others. C. K. Sprengel in 1793 developed these 

 views and concluded that flowers lacking a corolla are usually pollinated 

 in a mechanical fashion by wind. Such flowers have to produce large 

 quantities of light and easily-transported pollen, much of which misses 

 its target or is washed out of the air by rain. Thomas A. Knight in 1799 

 reported that wind could transport pollen to great distances. 



By the beginning of the nineteenth century, therefore, it was recognized 

 that pollen of many, but by no means all, species of flowering plants, and 



