120 Plant Tissue Culture 



seeds can be placed in each receptacle since any 

 contaminant which may be present on one seed 

 will not easily spread to another. If placed di- 

 rectly in a nutrient solution, only a single seed 

 should be put in each container. The flasks, Petri 

 dishes, or test tubes containing the seeds should 

 be set aside in darkness until the seedling roots 

 have attained a length of 10 to 15 mm. The roots 

 are then severed with scissors or scalpel at a 

 length of 3 mm. or more and transferred to the 

 nutrient in which they are to be grown. 



Adventitious roots. While most cultures can 

 best be started from seedling roots, conditions 

 occasionally arise which cannot be fulfilled by such 

 materials. This has been notably true of certain 

 virus "cultures" in which no method of inocu- 

 lating virus into roots growing in vitro has yet 

 been found (White, 1934, 438). It is then neces- 

 sary to resort to adventitious roots coming from 

 cuttings of infected plants (see later). With 

 plants like tomato which root easily, this presents 

 no great difficulty. Cuttings are made from rap- 

 idly growing plants having a lush growth and the 

 leaves are removed, using a sharp clean scalpel. 

 The cuttings are then suspended in large flasks or 

 crocks containing a little water in the bottom and 

 lined with moist blotting paper so as to provide a 

 saturated atmosphere (Fig. 32). These are set in 



