Dec.9. i9i8 Seedling Diseases of Conifers 529 



to apply. The relative number of seedlings appearing in the inoculated 

 pots and in the controls is an important element in the results, as it is a 

 measure of the seriousness of the germination losses occasioned by the 

 inoculations. It is, of course, not an exact measure because of the 

 unavoidable differences in the number of seeds viable in the different 

 pots at the time of sowing, but it is the only one available. 



The effect of the inoculations in causing normal damping-off is best 

 shown by the percentage of the germinated seedlings which damp-off. 

 The absolute number damping-off is not a figure on which conclusions 

 can be based, because of the great variations germi^iation loss may cause 

 in the initial stand. For example, if the pots inoculated with culture A 

 had a germination of only 15 seedlings, the subsequent damping-off of 

 10 seedlings will mean much more than the damping-off of 20 seedlings 

 in the pots inoculated with culture B, in which 100 seedlings may have 

 originally germinated. As a large proportion of the seedlings lost by 

 mechanical breakage are removed before the major part of the damp- 

 ing-off takes place, it is assumed that part of them would have damped-off 

 had they not been broken. In view of this, the percentage of damped-off 

 seedlings is obtained by the formula 



. rr D 



per cent damped-off = yy—-^ X 100, 



in which D represents the number of seedlings damped-off, and 5" the 

 number surviving at the time the experiment was closed. The broken 

 seedlings do not enter into this calculation. 



The germination and the percentage of damping-off are of value as 

 indicating the seriousness of the germination loss and of the normal 

 damping-off, respectively, but neither figure represents the whole effect 

 of the inoculation. The survival also is therefore given. If the limi- 

 tations in its exactness due to accidental variations in germination are 

 kept in mind, the comparative survival on inoculated pots and controls 

 can be used as evidence as to the total effect of the inoculations both on 

 germination loss and on normal damping-off. In order to free the sur- 

 vival figures, as far as possible, from the accidental variations due to the 

 different amounts of loss by mechanical breakage in the different pots, 

 it has been assumed that the broken seedlings in any series of pots, had 

 they not been broken and removed, would have lost the same proportion 

 of their number by damping-off as the unbroken seedlings. The actual 

 survival is therefore in all cases adjusted by adding to it the number of 

 the broken seedlings which the damping-off percentage indicates would 

 have sur\dved if they had not been broken. For example, in a set of 

 pots in which the damping-off percentage is 40, the number of broken 

 seedlings 15, and the surviving seedlings 56, the adjusted survdval is 

 obtained by adding to the actual survival 60 per cent of the number 

 broken; the adjusted survival is thus 56-^-9, or 65. In most cases the 



