162 " Damping off" etc., of Tomato Seedlings 



per box were sown and covered with a thin layer of sterile soil. In certain 

 instances, however, sand, lime or charcoal was used in place of the 

 sterilised soil. In other cases a layer of sand, lime or charcoal was put 

 on over the sterile soil, and in still other cases the soil was mixed with 

 different proportions of sand, lime or charcoal. A parallel set of experi- 

 ments was set up at the same time, using sterilised soil. Diseased seedlings 

 were removed from the boxes as soon as they were attacked in order to 

 eliminate the factor of superficial spreading of the disease. The results 

 of these experiments are shown in Table II. 



In the above experiments the soil sample, its weight, the sterility of 

 the seed-boxes, the seeds, the temperature, the barometric pressure, 

 the quality and quantity of the light, and the quantity of sterile water 

 given to each box were constant factors. The limiting factors were 

 solely those indicated in the above table. An examination of the per- 

 centage germination columns shows that the difference between the 

 lowest percentage germination and the highest is fairly constant and 

 the remainder of the results are symmetrically placed about the mean. 

 The percentage diseased seedlings showed a similar arrangement. This 

 further indicates that the average percentage results can be taken as 

 accurate measure of the experiment in question. 



A covering of sand, charcoal or lime either alone or above a covering 

 of sterile soil produces only a small increase or decrease in the percentage 

 of diseased seedlings per box. Charcoal has no effect, when put on as a 

 covering to the seeds. Sand reduced the percentage diseased by 20 per 

 cent., while lime increased it by 25 per cent. Five per cent, of charcoal 

 added to the soil has a distinctly beneficial effect, for, besides reducing 

 the percentage of diseased seedlings by 25 per cent., it produced a fine 

 crop of sturdy dark green seedlings. A further increase in the amount of 

 charcoal added is not advisable, for it only increased the difficulty of 

 keeping the soil at an even degree of moisture. Certainly a decrease in 

 the percentage of diseased seedlings is induced but this is obviously 

 caused by the increased proportion of sterile particles in the soil. In the 

 case of lime, whether it is added as a covering to the seeds or mixed with 

 the soil itself it increased the percentage of diseased seedlings by nearly 

 50 per cent., and appears actually harmful. This is in agreement with the 

 fact that the parasitic organisms grow best in a neutral medium. 



The relation of the closeness of sowing to the spread of the disease. 



In order to ascertain the correct closeness to sow the seeds, and 

 the effect of closeness of sowing upon the incidence of the disease the 



