66 MICROSCOPIC TECHNIQUES 



A means for the microscopic demonstration of ribonucleic acid 

 was developed by Diibos ( 1937) and Brachet ( 1940) , who employed 

 ribonuclease to break down the compound and thus destroy its 

 basophilic staining properties. The crystalline ribonuclease prepared 

 by Kunitz ( 1940) provided a more satisfactory reagent for carrying 

 out the procedure. Opie and Lavin ( 1946) demonstrated that ribo- 

 nucleic acid can be protected againct ribonuclease by precipitation 

 of the acid with lanthanum acetate. The basophilia of the precipitate 

 was retained even after treatment with ribonuclease. 



The danger of an uncritical acceptance of the localizations ob- 

 tained by the Feulgen reaction has been emphasized by Danielli 

 (1946a). He pointed out that it remains to be proved whether the 

 experimental treatment of the nucleic acid has rendered it diffusible 

 enough for this factor to become significant in the interpretation. In 

 addition, he stressed the point that the use of an enzyme to digest 

 away a particular substance is open to some question with reference 

 to the specificity of the enzyme and the degree to which a clear-cut 

 removal of the substrate is possible. On the other hand, Stowell 

 ( 1946) reviewed the evidence for and against the specificity of the 

 Feulgen technique for thymonucleic acid, and he concluded that 

 with the proper precautions it is one of the most specific histochem- 

 ical reactions. This does not mean that Stowell considers the tech- 

 nique beyond all criticism. No doubt he would agree with Danielli 

 that the interpretation of the results should be tempered with a 

 healthy awareness of the limitations involved, particularly the 

 diffusibility factor.* 



Turchini and co-workers (1943, 1944, 1945) reported the use of 

 9-phenyl (or methyl) -2,6,7-trihydroxy-3-fluorone for the differential 

 staining of ribo- and desoxyribonucleic acids, the former giving rise 

 to a yellow-pink color, and the latter to a blue-violet. It is necessary 

 to hydrolyze the nucleic acid, as it is the pentose, thus liberated, 

 which yields the color. The hexoses formed by the hydrolysis of 

 tannins produce an orange-yellow color in the staining reaction when 

 it is applied to plant tissues (Turchini and Gosselin de Beaumont, 

 1945). -^i 



* Other publications which have appeared subsequent to this writing are 

 given in the Bibliography Appendix, Refs. 15, 18 and 31. 



