The chloroplasts of Nitella may contract under natural conditions as well as under the influence of certain reagents. When a sufficient amount of water enters any part of the cell they contract in that region and they expand when the direction of the current is reversed. The current may be produced by placing water at one end of the cell and applying at the other end a solution which withdraws water from the cell. The contraction may be due to the washing out of substances from the chloroplast by the ingoing current. The outgoing current bearing dissolved materials from the sap may restore these substances and cause the chloroplasts to resume their normal shape. When blood or sodium dodecylsulfate is present in the ingoing current the contraction of the chloroplast usually fails to occur.