Culture Techniques 135 



are known. Such sources of error can be reduced 

 by careful study but can never be completely 

 eliminated. 



Much of the variability, however, probably 

 arises from differences in age, state of nutrition, 

 degree of established dominance, extent of ex- 

 cision trauma, and similar characteristics of the 

 separate explants. The effects of these differ- 

 ences can be greatly reduced by careful selection 

 of the individual fragments used in each experi- 

 ment. The importance of adequate uniform 

 stocks from which to make this selection is thus 

 obvious. 



It is clear, for example, that, if a rapidly grow- 

 ing root culture containing one terminal growing 

 point and 18 branches none of which has yet ac- 

 quired an individual dominance is divided into 19 

 pieces each containing a single apical meristem, 

 these pieces will be far from alike. The terminal 

 meristem will be of much greater diameter than 

 the others and of quite a different past history. 

 It will suffer far less from the shock of excision, 

 since its traumatized surface will be far less in 

 proportion to its volume than for the laterals. It 

 will recover more quickly and will show a much 

 greater growth in the first few days after excision. 

 Experience has shown that this difference can be 

 somewhat reduced by cutting the main root above 



