Culture Techniques 137 



one another and with a control, the experiment 

 will require a minimum of 100 cultures. The 

 routine procedure is as follows : 



Ten rapidly growing, uniform cultures of clonal 

 ancestry and each about 100 mm. long are selected. 

 The terminal segment of each to a length of 3-4 

 mm. is removed and discarded. The remainder is 

 divided into 10 to 15 mm. segments and each seg- 

 ment is placed in a flask of fresh nutrient and 

 allowed to grow for a week. Each will develop 

 from 3 or 4 to a dozen branches, and of these an 

 average of 4 or 5 on each piece will have estab- 

 lished comparable growth rates and full domi- 

 nance. There should thus be available 300 or more 

 uniform, well established root tips. These are care- 

 fully severed at a length of 10-15 mm. and from 

 them 100 tips of as uniform character as possible 

 are chosen for use in the actual experiment (Fig. 

 34). There is thus available a supply of tips 

 which are all root apexes, of uniform size, age, 

 nutrition, and degree of trauma, and which have 

 all come from comparable root pieces of clonal 

 relationship. The individual variability is thus 

 reduced to a minimum. This does not mean that 

 all cultures will behave exactly alike, even under 

 as nearly like conditions as we are able to set up — 

 there still remains a considerable residual vari- 

 ability that is as yet unexplained. But when a 



