Ill THE MEASUREMENT OF GROWTH 759 



pieces of the pith of tobacco stems {Nicotiana tabacum), for instance, grow well on a 

 sugar-salts agar medium with the addition of auxin, to form a mass of callus, from 

 which there soon differentiate roots, but no buds. If the auxin concentration is 

 not too high — about 2 mg indoleacetic acid per liter — then the addition of kinetin 

 produces a profusion of buds (see Fig. 2). Up to kinetin levels of 1-2 mg/1, 



Fig. 2. Effect of kinetin concentration (in o-io mg 1 range) on growth and organ forma- 

 tion of tobacco callus cultured on modified White's nutrient agar with 2.0 mg/1 indole- 

 acetic acid added to medium. Age of cultures 44 days. (From F. Skoog and C. O. Miller, 

 Symp. Soc. Exptl. Biol., Cambridge University Press, 1957.) 



the number of the buds appears to be roughly proportional to the logarithm of 

 the kinetin concentration. The absolute concentrations, 0.2-10 mg/1, are similar 

 to the effective auxin concentrations. Simple purines, especially adenine, have a 

 similar effect but quantitatively are only about i/iooo as active (Miller and Skoog, 

 1953). Increased auxin concentrations, in the presence of kinetin, suppress the 

 bud development and lead to still larger masses of more or less vmdifferentiated 

 callus. Thus the auxin and kinetin levels can be balanced against one another to 

 give quantitative control over the formation of roots, buds and callus {cf. p. 773). 

 In general where cell division is concerned, the requisite counts of cell number 

 are laborious and inconvenient to make. This difficulty has been circumvented 

 by the technique of maceration (Brown and Rickless, 1949). The tissue is shaken 

 in a mixture of 5% chromic and 5% hydrochloric acids, which separates the cells 

 from one another much as does strong NaOH. Drawing the macerate in and out 

 of a hypodermic needle increases the separation. Samples of the macerate are 

 then placed on a hemocytometer slide and the cells counted in the same way as 

 erythrocytes. A modification of this method has been extensively used in tissue 



Literature p. 8i6 



