A CRITIQUE OF CYTOCHEMICAL METHODS 



221 



.shows only the total light loss. In ultraviolet light of wave length near 

 that of nucleic acid absorption a large part of the light lo.ss, particularly 

 in fixed preparations, may he of nonspecific character mainly as a result 



Table (i-3. Beer's Law for Nucleic Acid 

 (After Thorell, 1947.) 



The extinction {E) and absorption constant (A-) of RNA and DNA at concentra- 

 tions of 0.005-2.5 per cent. Preparation according to Hammarsten (1924). 



Table 6-4. Ultraviolet Absorption of Maize Nucleoli 

 (After Polli.ster and Leuchtenberger, 1949b.) 



The reproducibility of the effect of the enzymatic and chemical (trichloroacetic 

 acid, TCA) removal of nucleic acid upon the extinction {E-za) of whole nucleoli of 

 pollen mother cells of Zea mays. (In experiment 1428E-3 the extinction is lower 

 because a very large central cylinder was measured; in 1428E-5 the higher extinction 

 results from using a smaller cylinder.) In each experiment the percentage reduction 

 of extinction indicates the proportion of the light loss (specific and nonspecific) due 

 to ribose polynucleotide. The residual light loss (46-50 per cent) is largely non- 

 specific, owing to high protein concentration. 



of scattering by the dense protein mass (see Table 6-4 and Fig. 6-56). 

 Specific nucleic acid absorption may be recognized from the shape of the 

 absorption curve (Caspersson, 1950, and Fig. 6-3). Also, the specific 

 polynucleotide absorption can readily be dissociated from the absorption 



