416 GROWTH OF PLANTS 



pH value at which no change in reaction is shown is the isoelectric point 

 of the tissue. 



"Furthermore, it is unlikely that the equilibrium point of a tissue in a 

 series of buffers represents the isoelectric point of the proteins of the tissue. 

 Thus the equilibrium point for potato tissue is about pH 6.4; but the iso- 

 electric point for tuberin, the principal protein of potato, is about pH 4.0 

 according to Cohn, Gross, and Johnson. Pearsall and Ewing find that 

 when the tissue is made as acid as, or more acid than, the isoelectric point 

 of the principal protein in the tissue there is a rapid exosmosis of ions, 

 indicating a serious injury to the tissue. The point for potato at which 

 rapid exosmosis of chlorids took place was not at pH 6.4 but at about pH 4.4. 



"Chibnall found that the pH of the cell contents and the isoelectric 

 point of the cytoplasmic proteins are not identical in any tissue he studied, 

 and points out the probability that any change in the reaction of the cell 

 Avhich brings the proteins to their isoelectric points Anil result in the death 

 of the cell. The buffer capacity of the cell contents protects the cell against 

 injury by tending to prevent the [H+] from reaching the isoelectric point of 

 the proteins of the cytoplasm." 



Rudolfs ^^ finds that when any given kind of seed is soaked in solutions 

 of various salts and organic and mineral acids, the solutions (except for 

 very dilute solutions) all change to the same pH, which he terms the iso- 

 electric point of the particular seed. He believes that the change is brought 

 about by ion absorption by the amphoteric substances, mainly proteins 

 of the living portions of the seeds, and not by substances leaching out of 

 the seeds. In grains it is chiefly due to the embryo rich in proteins rather 

 than to the endosperm, which is mostly carbohydrates. The pH equilib- 

 rium point for corn grains is at 3.9 to 4.1 pH, for Phaseolus vulgaris seed 

 at 5.5 pH, and for Lupinus albus seed 4.7 pH. Scott ''* finds similar results 

 by soaking living mycelium of Fusarium lycopersici in unbuffered and buff- 

 ered salt solutions with the equilibrium point at 5.4 pH. Dead mycelium 

 did not give a definite equilibrium point. He interprets this as the iso- 

 electric point of the main proteins of the mycelium. 



Denny and Youden -^ placed various plant tissues (thin slices of potato 

 tubers, carrot roots, and apple fruit, whole seeds of corn, rye, and wheat, 

 and corn seed-powder) in solutions of various salts ranging in concentra- 

 tions from O.lilf to O.OOlilf . Samples of the salt solutions were removed 

 after various periods of soaking and the pH determined. In all cases 

 where a change in pH occurs, the change is in the acid direction. Salts of 

 the monovalent cations, sodium and potassium, give slight or no shift in 

 acidity. Salts of the bivalent cations, calcium, magnesium, and strontium, 

 give decided shifts toward the acid; those of the bivalent cation, zinc, still 

 greater shifts; and finally those of the bivalent cation, copper, and of the 

 trivalent cation, lanthanum, give very large shifts toward the acid. The 

 amount of acidulation increases with the concentration of the solutions, 

 especially in the lower ranges of concentration. The final pH equilibrium 



