Studies on the Downy Mildew of Onions 29 



Germ tubes developed at or near the optimum temperature for germina- 

 tion were the longest and appeared to be the most nearly normal in 

 shape (figure 8). Those produced at the high temperatures were short 

 and tended to be misshapen (figure 9), while those at the low tem- 

 peratures were longer than those at the high temperatures but shorter 



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W 





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w 



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ft 



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Figure 9. germination or conidia of peronospora destructor at high temperatures 



than those at the optimum. The peculiar type of germ tube produced 

 at low temperatures is illustrated in figure 10. 



Normal germination is by a single germ tube, or occasionally two 

 tubes, produced from the side of the conidium. Less frequently, a 

 single germ tube may be produced from the apical end of the spore. 

 The germ tubes are uniform in diameter, measuring from 5.76 to 7.68 

 microns, and have been observed to attain a length of 940 microns. 

 As a rule they remain unbranched, but occasionally they undergo 

 branching. The latter condition is found at times in rain water, in 



