HYDROGEN ION CONCENTRATION OF PLANT CELLS 79 



when the cytoplasm is actively streaming they may assume many different 

 forms. Cambium cells that are actively dividing may contain very large 

 vacuoles (Bailey, 1930). In cambium initials the vacuoles, like those in the 

 meristems of stem and root tips, are not uniform in size or shape, but may be 

 rod-shaped, thread-like or globular. They may coalesce into a large single 

 vacuole or they may divide into numerous smaller vacuoles. ^Mature cells, 

 however, whether they arise from primary meristems or from the cambium 

 cells typically contain one large central vacuole which arises by the increase in 

 size and coalescence of the numerous smaller vacuoles usually present in the 

 meristematic cells. 



There is no general agreement as to the method by which vacuoles origi- 

 nate. Three possibilities are recognized : ( i ) They may arise by the division 

 of pre-existing vacuoles, (2) they may originate de novo in the cytoplasm, 

 and (3) they may develop from organized units of the cytoplasm. There 

 is no convincing evidence, however, that the vacuoles of the cells of the vascu- 

 lar plants arise in any way except by the division of pre-existing vacuoles 

 (Zirkle, 1937)- 



Among the various substances present as solutes in the vacuole are sugars, 

 mineral salts, organic acids (oxalic acid, especially, seems to be of frequent 

 occurrence), amino acids, amides, alkaloids, glycosides, flavones, and anthocy- 

 anins. Fats and related compounds often occur in finely emulsified form. Pro- 

 teins, tannins, mucilages, lipids and other substances are commonly present in 

 the colloidal state. Aleurone grains develop from specialized vacuoles of cells in 

 storage tissues. Crystals of calcium oxalate are also of frequent occurrence in 

 the vacuoles of mature cells. 



Hydrogen Ion Concentration of Plant Cells. — The hydration and vis- 

 cosity of the protoplasm, the permeability of the cytoplasmic membranes, the 

 activity of enzymes, the chemical activity of various ions in the cell, and vari- 

 ous other physiological processes and conditions are all influenced more or less 

 by the hydrogen ion concentration of the protoplasm and the cell sap. 



As soon as its significance was appreciated attempts were made to deter- 

 mine the hydrogen ion concentration of plant cells. Most of the earlier 

 determinations were made on the juice pressed from plant tissues. Such a 

 crude method provides orily the roughest sort of an indication of the hydrogen 

 ion concentrations in individual plant cells. The death of the cells and the 

 mixing of the cell contents during the extraction process undoubtedly result 

 in marked changes in hydrogen ion concentration. Such determinations are 

 probably more nearly a measure of the pH of the cell sap than of the proto- 

 plasm. The values for expressed plant saps mostly fall within a range of pH 

 3.0 to pH 7.0 



