VEGETABLE PARASITES AND WEEDS 



177 



pose in the greenhouse steam may be introduced through 

 perforated pipes buried in the soil. The method most 

 in favor, however, is to invert a large metal pan over the 

 bed into which live steam at high pressure is discharged 

 (Fig. 79). In this way the surface layer of the soil may 

 be so " cooked " in about one-half hour that not only are 

 the disease germs destroyed, but also the weed seeds. In 



Fig. 79. — Steam sterilization of greenhouse soil. 



tobacco seed-beds, the labor-saving as a result of so killing 

 the weed seeds pays for the expense of steaming. 



327. Seed treatments aim to disinfect seed without 

 injuring its germination. This can be accomplished in 

 certain cases, e.g. (barley smut), by immersing the seed 

 a short time in hot water. The simplest agent is the 

 chemical formaldehyde. Thus oat smut is prevented by 

 soaking the seed two hours in a solution of 1 pint of this 



