136 Plant Tissue Culture 



and below each branch instead of severing the 

 branch itself, in such a way that a "heel" of older 

 tissue is left attached to each branch. The differ- 

 ence is still more reduced if only branches which 

 have already acquired their full individuality or 

 "dominance" are used. With tomato this domi- 

 nance usually develops after the branch has 

 reached a length of about 20 mm. If branches of 

 this sort are severed, they will generally show no 

 more retardation on excision than will apical 

 meristems and can be considered comparable 

 thereto. It is, therefore, advantageous in prepar- 

 ing a stock from which to select root tips, to have 

 as high a proportion of well established branches 

 as possible. Experience has shown that, if a 

 piece of old tissue carrying several branch initials 

 but from which the apex has been removed is cul- 

 tured for a time, a considerable proportion of 

 these initials will establish themselves with a com- 

 parable degree of dominance. Acquaintance with 

 these facts has given rise to a standard experi- 

 mental procedure. 



Experience has shown that, in root cultures in 

 flasks, satisfactorily reliable results cannot be 

 counted on if a given experiment contains less 

 than 20 replications under each experimental con- 

 dition complex (see later). If, for example, it is 

 desired to compare four sets of conditions among 



