A SERIOUS LETTUCE DISEASE. 
107 
When mature the spores are ejected forcibly from the asci. Great 
numbers of them ripen at the same time and the slightest change in 
environmental conditions, as a slight draught produced by the breath 
causes myriads of them to be thrown into the air often to a distance of 
0.5 to 1.0 meter, their great number rendering them visible as a steain- 
like cloud which, caught by air currents, can often he followed by the 
eve to a distance of several meters. 
«/ 
M = 4.0880 ±0.0166 
0 = 0.2930±0.0117 
C. V. = 7.168 ±0.290 
n= 142 
3 3.5 4 4.5 5 
Fig. 19 .—Sclerotinia libertiana Fckl. 
Polygon of ascospores from middle-aged 
disk. 
M = 4.0393 ±0.0214 
o = 0.3743±0.0151 
C. V. =9.267 ±0.380 
n= 165 
3 3.5 4 4.5 5 
Fig. 20.— Sclerotinia libertiana Fckl. 
Polygon of spores from young disk. 
Several hundred measurements were made from disks of various 
ages to determine the normal variation in sizes of ascospores with the 
results shown in Figs. 19 and 20. 
It is seen that the spores are remarkably constant in size and that 
the age of the disk (Figs. 19 and 20) from which they are taken has 
no material effect upon their measurements. 
The spores as discharged from the asci are ready, if they fall in 
favorable places, to germinate at once. In germination they first en- 
