512 Journal of Agricultural Research Vol. III, No. 6 
Obviously no exposures could be made during a rain, but ascospore- 
trap examinations have shown that abundant spore expulsion may 
occur during the actual fall of the rain. It is evident, however, that at 
such a time wind dissemination would be reduced to a minimum, because 
the spores upon expulsion would soon be washed to the ground or to 
near-by bark or foliage. 
Therefore, under the conditions of our tests it can. be said that, with the 
exception of the few stray ascospores loosened from temporary lodgment, 
wind dissemination of Endothia parasitica occurs only during certain 
periods after rains, when ascospore expulsion is in progress. 
ASCOSPORE-TRAP TESTS 
In order to detect ascospore expulsion whenever it occurred, use was 
made of what we have termed ‘“‘ascospore traps.’’ An ascospore trap 
consisted of a glass object slide held in place over perithecial pustules 
on the bark of a diseased tree by means of a wooden bracket either above 
or below the slide (Pl. LXIV, figs. 1 and 2). The slide was wedged firmly 
into a slot in the bracket so as to be suspended about one-eighth of an 
inch or less from the papille underneath. These traps were placed on 
lesions of various ages on trees more or less scattered throughout the 
experimental plot (fig. 1). 
As the ascospores of Endothia parasitica are expelled they adhere to 
the glass, and the spores expelled from each ostiole usually form a definite 
“spot,” so that the number of spots on the slide represents the number 
of perithecia in the area underneath which have expelled spores. 
During the progress of the work on wind dissemination, it was found 
possible by means of these traps not only to detect the occurrence of 
ascospore expulsion but to determine even with some degree of accuracy 
the exact duration of perithecial activity. 
As has been brought out in the discussion of the éxposure-plate tests, 
the occurrence of ascospores in the air in numbers is directly dependent 
upon the continuation of their expulsion after a rain has ceased. The 
duration of expulsion becomes, therefore, an essential factor in deter- 
mining the period during which wind dissemination may occur. 
In making this determination the method of procedure was as follows: 
Out of the total number of 69 ascospore traps usually about~-20 were 
selected, representing areas of vigorous perithecia where previous expe- 
rience indicated that abundant expulsion was most likely to occur. The 
slides from these traps were collected as soon as possible after the rain 
and were replaced with clean slides. Then, after a convenient interval, 
this second set of slides was collected and replaced with clean ones. This 
operation was repeated at intervals of several minutes to several hours 
until none of the slides bore spots of expelled ascospores. 
Several series of trap collections were usually made after each rain, 
and a subsequent examination of each slide revealed whether or not any 
expulsion had occurred under that.trap in the period during which that 
particular slide had been in place on the tree. Although usually visible 
to the unaided eye, an examination with a hand lens was often necessary 
to detect very faint or very diffuse spots of ascospores on the slides. 
The detailed results for September 20 are given to show the behavior 
of individual traps (Table X). The results given in the summary for 
the other dates were obtained in a similar manner and the individual 
records will therefore be omitted. 
