THE PLANT CELL 



ness and close contact with the wall; but the greater part of this 

 difl&culty vanishes when the protoplast is made to shrink away 

 from the wall. This we can do to advantage with the epidermal 



cells of Tradescantia zebrina 

 containing a colored cell-sap. 

 We strip a bit of epidermis 

 from the under side of a 

 midrib of a leaf and mount it 

 under a coverglass in a drop 

 of water. We bring some of 

 the colored cells under the 

 objective and run a 5 per 

 cent. NaCl solution under 

 the coverglass. The salt 

 solution draws water out of 

 the cells by osmosis and the 

 protoplast soon shrinks away 

 from the walls because it is 

 elastic and had been stretched 

 by the water within the cell. 

 We can now make out the 

 thin plasma membrane at the 

 surface of the shrunken pro- 

 toplast (Fig. I, C). If we 

 replace the salt solution under 

 the coverglass with fresh 

 water the protoplast quickly 

 swells up and presses against 

 the cell-wall all around as 

 before. 



In this experiment we can see 

 that while the plasma mem- 

 brane allows the water to be drawn from the protoplast by the 

 salt solution it does not permit the coloring mater or the osmotic 

 substances in solution in the cell-sap to escape, for the color 

 does not at all diminish in the protoplast as it shrinks, and the 



Fig. I. — A, embryonic cells from onion 

 root tip; d, plasmatic membrane; c, cyto- 

 plasm; a, nuclear membrane enclosing the 

 thread-like nuclear reticulum; b, nucleolus; 

 e, plastids (black dots scattered about). 

 B, older cells farther back from the root tip. 

 The cytoplasm is becoming vacuolate; /, 

 vacuole. C, a cell from the epidermis of the 

 midrib of Tradescantia zebrina, in its natural 

 condition on the right, and plasmolyzed by a 

 salt solution on the left; g, space left by the 

 recedence of the cytoplasm from the wall; 

 the plasma membrane can now be seen as a 

 deUcate membrane bounding the shrunken 

 protoplast. All highly magnified. 



