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PRESERVATION OF SPECIMENS IN PLASTICS H 



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The preserving fluid requirements for the principal types of plant 

 pigments are described in greater detail for each of the five pigment 

 groups as follows: 



1. The anthocyanin pigments require preserving fluids of the non- 

 reducing type only, because they are bleached to a colorless form when 



veil acted upon by reducing agents. In order to insure that these pigments 

 ■wing will retain their original colors without change, it is necessary to employ 



a fluid that has a pH value that corresponds closely to the specific pH 

 ^requirement of the particular pigment that is present. These require- 

 made ments vary according to the chemical structure of the pigment, and the 



specific pH values range from below 3.5 for the scarlets and bright reds 

 l in to 7.5 for the bluest shades. Either sorbitol borate or sorbitol citrate 



preserving fluids may be employed for the anthocyanins, but the borate 



types are usually preferable except where the presence of flavone or 



flavonol copigments requires the citrate types. 



2. The anthoxanthin pigments are subject to oxidative changes, and 

 are, therefore, preserved more satisfactorily by the use of the reducing 

 type fluids. Nonreducing fluids, which are required whenever antho- 

 cyanin copigments are also present, will, however, give acceptable 

 results in most instances except where pH values above 6.5 prevail. 

 The optimum pH range for the preservation of the anthoxanthins is 

 between pH 5.5 and pH 6.5 but somewhat lower values may often be 

 employed satisfactorily when the presence of other pigments necessitates 

 their use. Since the flavones and the flavonols, which comprise the 

 greater part of this group of pigments, are discolored by boric com- 

 pounds, only sorbitol citrate (or other borate-free fluids) should be 

 employed for preserving plant tissue containing them. 



3. The tannins and other oxidizable color-producing substances, 

 which are naturally colorless but tend to cause browning or discoloration 

 of tissue during preservation, are treated most advantageously with 

 preserving fluids of the reducing type having pH values as low as the 

 accompanying pigments will permit. In many instances, however, 

 nonreducing fluids with pH values below 6.5 may be used for them if 

 anthocyanin pigments are also present. Where pH values higher than 

 6.5 are required by the presence of chlorophyll or carotenoid pigments, 

 only a reducing type fluid should be employed in order to minimize the 

 tendency of the tissue to darken during treatment. Either sorbitol 

 borate or sorbitol citrate preserving fluids may be used for the tannins 

 and other oxidizable color-producing substances, although the borates 

 are somewhat more satisfactory except where the presence of flavone 

 or flavonol pigments necessitates use of the citrates. 



4. The carotenoid pigments are preserved most satisfactorily by fluids 

 of the reducing type, because oxidation, if allowed to take place in 

 pigments of this class, results in eventual loss of color. When the 

 presence of anthocyanin copigments restricts the choice of preserving 

 fluids to the nonreducing types, acceptable results may still be obtained 

 in many instances where such fluids are used, provided their pH values 

 are not below 5.5. In general, however, it is desirable to employ reduc- 

 ing-type fluids with pH values between 6.5 and 7.5 wherever possible 

 for all tissue in which carotenoid pigments occur. Either sorbitol borate 

 or sorbitol citrate preserving fluids may be employed for these pigments, 

 but the former are usually preferable unless the presence of flavones or 

 flavonols precludes their use. 



