112 SPORE DISSEMINATION 



bersr (1935) in their aerobiologncal studies in the Arctic. Proctor 

 (1934) at the Massachusetts Institute of Technology used an 

 ingenious automatic device. 



Rate of fall of spores. Rate of fall of spores in still air has 

 been given consideration by a number of investigators, including 

 Buller (1909), Ukkleberg (1933), Stephanov ( 1935), and Gregory 

 (1945). In general the terminal velocity has been found to be of 

 the order expected from Stokes' law. According to this law, 



where V = terminal velocity, p = density of the spore, a = den- 

 sity of the medium, g = acceleration due to gravity, r = radius of 

 the spore, and /x = viscosity of the medium. Deviations from the 

 expected rate may be ascribed to the following factors: ( 1 ) shape 

 of spore; that is, they are seldom ideal spheres; (2) irregularities 

 in outer surface of spore wall; (3) rapid desiccation during 

 falling; and (4) inaccuracy in determining the density of the 

 spores. 



Buller (1909) found that the rate of fall of basidiospores of 

 Colly bia dryophila is 0.49 mm per second and of Coprinus plica- 

 tilis 4.29 mm per second. Ukkleberg (1933) determined that the 

 rate of fall of urediniospores of Pnccinia graminis tritici is 11.57 

 mm, of P. graminis secalis 10.58 mm, of P. coronata avenue 10.00 

 mm, and of P. triticina 12.62 mm per second. He found that the 

 rate of fall of aeciospores of P. graminis tritici is 10.56 mm, and 

 of P. graminis secalis 10.20 mm per second. 



Insects as vectors of fungi. It is well known that such in- 

 sects and arachnids as ticks, fleas, flies, mosquitoes, lice, bees, wasps, 

 beetles, and mites are capable of transmitting microorganisms, 

 especially species responsible for important diseases of man and 

 various animals. Much less is known, however, regarding the role 

 that animals play in the transmission of species pathogenic to 

 plants. Although the presence of certain virus diseases of plants 

 appears to require the presence of specific insects as vectors, for 

 instance, aster yellows, carried by Cicadula sexnotata, curly top 

 of beets, carried by Eutetix tenella, and kroepoek of tobacco, car- 

 ried by Bemesia gossipiperda, no portion of the life cycle of the 

 pathogen, whether virus, bacterium, or fungus, appears to develop 

 within the body of the vector. Instead the infective agent is 



