THE MICROBIOLOGY OF THE ATMOSPHERE 



The properties of spores are not invariable, but may alter with external 

 conditions — sometimes enough to have a marked effect on their terminal 

 velocity. For instance, the spores of the toadstool Amanitopsis vaginata 

 were recorded by Buller (1922) as falling at 0-5 cm. per sec. when ob- 

 served immediately below the gill after liberation, but they became desic- 

 cated on continuing to fall through dry air and soon slowed down to one 

 third of their original speed. Durham (1943) gave laboratory determina- 

 tions of densities of pollens, and for some the probable outdoor values which 

 are shown in parentheses: Ambrosia elatior^ 0-63 (o'55); A. bidentata, 

 0-56 (0-50); Xanthiiim commune^ 0-52 (0-45); Iva xanthifolia, 079; 

 Salsola pestifer, i-o (0-90); Acnida tamariscina^ i-o; Zea mays, i-io (i-oo); 

 Phleum pratense, i-oo (0-90); Quercus imbricaria, 1-04; Juglans nigra, 

 0-93; Alnus glutinosa, 0-97; Fraxiniis americana, 0-90. 



Observed terminal velocities (vs) of spores and pollen grains are 

 collected in Table I. 



Flowering plants 

 Abies pectinata 

 Alnus viridls 

 Betula alba 

 Carpiiuis betiilus 

 Corylus avellana 

 Dactylis glomerata 

 Fagiis sylvatica 

 Larix decidua 

 Larix poJonica 

 Picea excelsa 

 Pinus cembra 

 Piiins sylvestris 

 Qiierciis robur 

 Salix caprea 

 Secak cereale 

 Tilia cordata 

 Tilia platypliylla 

 Ulniiis glabra 



Pteridophytes 



Lycopodiiim sp. 1-76-2-14 (2)5(5) 



Bryophytes 



Polytrichum sp. 0-23 (2) 



16 



