508 Journal oj Agricultural Research Vol. III, No. 6 
The relation of the time elapsed since the cessation of rain to the 
prevalence of ascospores in the air among diseased trees is shown in 
Table VII. 
TaBLE VII.—Relation of the time elapsed since the cessation of rain to the number of 
spores falling on an area of I square foot per minute in 1913 at West Chester, Pa. 
PLATES EXPOSED ON SEPTEMBER 18, 1913 
Number of 
spores of 
‘ Number of Endothi 
‘* ZA onan 
ee tion of rain. parasitica. aren of 
1 square foot 
per minute. 
He. m. 
4787 I 51 Ir 15.74 
pies I 54 16 22. 32 
4789 Bs 9 12. 55 
4790 2 19 4 4.29 
4791 2 35 5 6. 34 
4792 2 48 2 2. 53 
4793 2 54 2 2. 32 
4795 3° 25 2 2. 53 
4796 3 34 I 1.27 
4797 3 58 I +1. 36 
4798 4 4 ° ° 
4799 4 29 x) ° 
PLATES EXPOSED ON SEPTEMBER 21, 1913 
5071 15 62 43-24 
5072 17 24 17. 62 
5073 44 19 19. 64 
5074 56 I ©. gI 
5975 i st. 2 2. 32 
5076 I 20 Co) ° 
5°77 I 23 ° ° 
5078 2 5 ° fo} 
An examination of these tables shows that on September 18 the spore 
content of the air decreased more or less gradually during the third and 
fourth hours after the rain, while on September 21 the spore content 
decreased very abruptly and no spores were obtained after the first hour 
following the cessation of the rain. The duration and the abundance of 
the ascospore expulsion on these dates (Table XI) are seen to have 
differed likewise, and a comparison of the weather conditions gives the 
probable explanation, since it was calm and foggy on the 18th and hot 
and sunny with a brisk wind just following the rain of the 21st. Condi- 
tions following the rain on the 18th were such as to prevent rapid drying 
of the bark, so that spore expulsion continued during a much longer time 
than on the 21st, when the bark dried rapidly. Furthermore, the brisk 
wind of September 21 would tend to disperse the spores very rapidly, 
whereas the comparative calm of September 18 would be favorable to a 
more prolonged prevalence in the air near their source. In this regard 
