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In all cases observed by us, when plant tissue is placed in contact with a tenth-normal solution of calcium chlorid the solution soon becomes acidified. Within an hour, under suitable experimental conditions, the hydrogen-ion concentration may increase ten, twenty, or thirty fold. What is the cause of this development of acidity? Should this effect be interpreted as evidence that the tissue is absorbing cations faster than anions? And if so, is the process of ion absorption related to the amphoteric character of the proteins in the plant? This problem is important because it is related to the question of whether plant tissue in absorption of ions behaves like an ampholyte with an isoelectric point. Through this connection we first became interested in it. The effect of seeds upon the hydrogen-ion concentration of salt solutions was studied in detail by Rudolphs (6, 7, 8), who soaked the seeds of different species in various salt solutions of different concentrations, and then determined the pH of the solution surrounding the seeds. He was impressed by the tendency of the seeds to bring the pH of the various salt solutions to definite values, each final value being fairly constant for a given species of seed regardless of the original pH or concentration of the salt solution (except in the case of very dilute solutions). Thus the final value for corn (Zea mays) was between pH 3.9 and 4.2 when the seeds were soaked in such varied chemical solutions as K2SO4, KNO3, CaC12, H2SO4, HCI, KC1, BaCl2, oxalic acid, citric acid, etc. Rudolphs' conclusion was that "the chemical properties of the chief protein constituent of the seeds seem responsible for the changes in hydrogen-ion concentration." Scott (g) found that the mycelium of Fusarium lycopersici was able to change the reaction of several different dilute salt solutions to pH 5.4 which was regarded as the isoelectric point of the tissue. Thus, the behavior of the living mycelium was thought to be analogous to that of an amphoteric colloid. We have experimented upon the effect of plant tissue upon the pH of unbuffered salt solutions, using 6 different tissues, I5 different salt solutions, with 3 to 5 different concentrations of each salt. In our experiments we |
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