How Cultures Are Started 119 



bromine-water. They are then spread out on 

 sterile filter paper in Petri dishes. The bromine 

 quickly penetrates all crevices in the seed coats, 

 killing most if not all contaminants, yet evaporates 

 completely leaving no traces to affect the seed- 

 lings. In spite of its obvious advantages, bromine 

 must be handled with great care, under a hood, so 

 that some workers prefer to use other reagents. 

 A solution of calcium hypochlorite containing 1 

 per cent chlorine will give fairly satisfactory re- 

 sults (Wilson, 1915, 455; White, 1932, 63) if seeds 

 are shaken therein for 15 to 30 minutes, then 

 washed in sterile distilled water and placed in 

 Petri dishes. The time required for complete 

 sterilization will vary considerably with the 

 species of seed, as will the amount of injury to the 

 seedling which usually results. Each species must 

 be studied specially. Dakin solution (Parker, 

 1938, 26) is also good. Mercuric chloride, 85 per 

 cent alcohol, "Uspulin," and other disinfectants 

 have been used by particular workers. The meth- 

 ods of using these reagents are standard biological 

 procedures and need not be presented in detail. 



Once sterilized, the seeds may be placed for 

 germination either directly in flasks or tubes of 

 nutrient or in Petri dishes on filter paper mois- 

 tened with sterile water or on agar. If sown on 

 a semi-solid substratum such as agar, several 



