. Plant physiology. Plant physiology. io6 PHYSIOLOGY OF NUTRITION The crystalloids, saccharose and potassium nitrate, produced lower pressures than did the colloids, gum arable and gelatine, when plant or animal membranes were used. This seems to be in disagreement with statement 3, above, but it is explained by the fact that these two crystalloids readily pass through such membranes, while the precipitation membranes are almost impermeable to them. Pfeffer's experiments indicated that, other conditions remaining the same, the magnitude of the osmotic pressure differed according to the nature
. Plant physiology. Plant physiology. io6 PHYSIOLOGY OF NUTRITION The crystalloids, saccharose and potassium nitrate, produced lower pressures than did the colloids, gum arable and gelatine, when plant or animal membranes were used. This seems to be in disagreement with statement 3, above, but it is explained by the fact that these two crystalloids readily pass through such membranes, while the precipitation membranes are almost impermeable to them. Pfeffer's experiments indicated that, other conditions remaining the same, the magnitude of the osmotic pressure differed according to the nature of the dissolved'substance, and the question arose whether this phenomenon obeyed any law. This question was answered by deVries,^ who used living plant cells instead of the artificial cells employed by Pfeffer. He determined the isosmotic (or isotonic) coefl&cients of various substances by means of the plasmolytic method. As is well known, plasmolysis occurs when a living plant cell is placed in a sufficiently strong (lo-per cent.) solution of such substances as cane sugar, sodium. 12 3 4 Fig. 68.—Successive stages of plasmolysis. N, nucleus; V, vacuole. (.After deVries.) chloride, etc. At first there is a decrease in cell volume, to a certain point, after which the protoplasm separates from the cell wall and withdraws inward (Fig. 68). The cell gradually regains its earlier form if the salt solution is replaced by water. Cells with colored sap are very good for plasmolytic ex- periments, since the coloring matter is retained within the shrinking vacuole, leaving the space between the protoplasm and the cell wall filled with colorless solution. By the use of such cells plasmolysis may be readily detected, even in its incipient stages. DeVries used mature cells with colored sap and determined the concentration of the plasmolyzing solution when the latter was just strong enough to cause separation of the protoplasm from the wall at the corners of the cell (Fig. 68, 3). If
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