ERG AST I C SUBSTANCES 97 



cambium of woody plants and other tissues Bailey and Zirkle^ find two 

 kinds of vacuoles in the same cell: relatively alkaline ones staining 

 reddish orange in neutral red (B-type), and markedly acid ones staining 

 bluish magenta with the same dye (A-type). Those of the latter type 

 contain phenolic 'compounds and tend to form copious precipitates. 

 It is observed that vacuoles in the cambium of conifers and many dicoty- 

 ledons are alkaline when stainable and become acid in the xylem and 

 phloem; whereas, in some dicotyledons they are acid and show changes 

 to alkalinity and vice versa in the differentiating cells. Seasonal varia- 

 tions also occur. Although the accumulation of basic dyes appears to 

 be correlated with the presence of specific substances in the vacuoles, 

 these workers do not favor Dangeard's hypothesis that a substance called 

 "metachromatin" is chiefly responsible for their staining reactions and 

 other activities. It is further found that colorimetric methods are very 

 unreliable for determining the pH of B-type vacuoles. It is this type 

 which occurs in Nitella and Valonia, which are so widely used in physio- 

 logical studies. 



Nearly all of the red, blue, and purple colors of flowers, fruits, and 

 other plant parts are due to anthocyanin pigments in the cell sap. These 

 pigments are usually reddish in an acid medium and bluish in a neutral 

 or alkaline one, though these colors may be masked by other pigments in 

 the sap or plastids. The yellow flavone and flavonol pigments are widely 

 distributed in the cell sap of plants, but they occur in such dilute solution 

 that they do not give a noticeable color to tissues except in rare cases 

 (e.g., Antirrhinum), Yellow color in plants is usually due to plastid 

 pigments.'^ 



Frequently the cell sap contains visible solid or fluid particles in 

 suspension; these may be so numerous as to give the sap a milky appear- 

 ance. The latex found in special cells or vessel systems in certain plants 

 is a sap which contains a great variety of substances in solution, together 

 with suspended droplets or granules of oils, tannins, gum, starch, resin, 

 caoutchouc, and other compounds. As familiar examples may be cited 

 the juices of the rubber tree, dandelion, milkweed, and poppy.^ 



Under certain circumstances organic and inorganic substances dis- 

 solved in the cell sap may crystallize or precipitate. This may result 

 from an increase in the concentration of such substances beyond the 

 saturation point, often because of a decrease in the amount of solvent, as 

 in drying seeds; it may also be due to the appearance of some other com- 

 pound which precipitates them. As examples may be cited the formation 



6 Bailey (1930), Bailey and Zirkle (1931), Zirkle (1932a). See also Went (1888) 

 and Mangenot (1927, 19296) on the presence of two vacuole types. 



^ For accounts of plant pigments, see Onslow (1923), Mobius (1927), Meyer (1926), 

 Howard (1925), and Gortner (1929). 



^ For accounts of such saps, see Czapek (1913), Meyer (1920), and Went (1926). 



