8o PLANT CELLS 



Direct measurements of the hydrogen ion concentration of the protoplasm 

 and cell sap have been made by introducing indicator dyes directly into cells. 

 By careful manipulation of a micropipette the dyes can be injected into the 

 cytoplasm without penetrating into the vacuole or injected into the vacuole 

 without penetrating into the cytoplasm. Only non-toxic dyes should be injected 

 into living cytoplasm for pH determinations, else the results may be invali- 

 dated by injury or death of the cytoplasm. Dyes are considered to be non- 

 toxic or essentially so if protoplasmic streaming continues in the same way as 

 before the injection. In this manner it is possible to determine the reaction of 

 the cell sap and that of the cytoplasm independently. Results of the micro- 

 injection method when applied to the root hairs of the water plant Lirnnobiiun 

 spongia indicated a pH value for the cytoplasm of 6.9 ± 0.2 (Chambers and 

 Kerr, 1932). The cell sap of the same cells was found to be more acid, 

 having a pH of 5.2 ± 0.2. 



As indicated by the results cited above the pH of the cytoplasm and the 

 cell sap of a plant cell may be very different. The pH of the cytoplasm of 

 plant cells appears to be fairly constant, usually falling between 6.8 and 7.0 

 (Seifriz, 1936). The pH of the cell sap is usually lower than that of the 

 cytoplasm, values between pH 5.2 and 6.2 seeming typical for most plant cells. 

 The cell sap of some cells, however, shows a considerably more acid reaction 

 than this, values as low as pH 0.9 being reported for species of Begonia 

 (Smith and Quirk, 1926). On the other hand alkaline values have also 

 been found in the vacuoles of some species (Haas, 1916). The cell sap 

 appears to vary more widely in hydrogen ion concentration than the cyto- 

 plasm. 



Buffer Action in Plant Cells. — Both the cytoplasm and cell sap of plant 

 cells are usually buffered. The former, however, as its lesser variability in 

 pH indicates, is usually more strongly buffered than the latter. All the actual 

 information available regarding buft'er action in plants is based, however, 

 upon determinations made upon expressed plant saps. 



The most important buffer systems in living organisms are those com- 

 posed of a weak acid and one (or more) of its salts (Chap. II). The 

 principal acids which are components of plant buffer systems are carbonic, 

 phosphoric, citric, malic, tartaric, and oxalic. The first two of these acids are 

 chemical progeny of compounds which enter the plant from its environment 

 — carbon dioxide and the phosphates, respectively. The others are products 

 of the metabolic activity of the cells. The principal base-forming elements 

 present in the salts which may serve as components of buffer systems are 

 sodium, potassium, calcium, and magnesium. 



The buffer action of some types of plant cells is apparently due predomi- 



