390 VEGETABLE GEOWTH. 



salts of known strength in which the tissues are placed, and 

 which are then allowed to act upon the contents of the cells. 

 When the solutions are more dense than the fluids in the cavity 

 of the cell, an exosmotie action withdraws a certain amount 

 of the water from the cell, causing thereby a shrinking of its 

 contents which can be easilj- observed under the microscope, or 

 noted by curvature of the whole section. The method permits 

 the experimenter to ascertain within narrow limits the density 

 of the contents of a given cell, and to determine the relative 

 degree of turgidity in different cases. When a cell undergoes 

 no change of form upon being placed in a solution of a given 

 strength, that solution is taken as a measure of the density of 

 its contents.^ 



1022. Tensions in cell-wall. There may frequently be observed 

 a tension of different la3ers of the cell-wall. This can be easily 

 demonstrated by making thin sections of any succulent tissues 

 from which cells can be readily detached ; a curvature will be 

 detected at the moment of cutting. 



1023. Young cell-walls are elastic to a certain extent; but 

 their limit of elasticity is easily exceeded, and then they remain 

 in the stretched condition. When an internode is strongly, 

 stretched in the direction of its length, it undergoes permanent 

 elongation. This elongation may amount in some cases to three 

 or even five per cent ; whereas the temporarj- extension in the 

 same instances may range from seven to seventeen per cent. 

 The extensibility diminishes, while the elasticity increases, with 

 the age of the internode. 



1024. From his experiments Sachs draws the following con- 

 clusions regarding growing internodes : (1) After flexion they 

 do not completely recover their straightness ; (2) one vigorous 

 bending, and to a still greater extent repeated ones in opposite 

 directions, leave the internode flaccid, or deprive it of its rigid- 

 ity ; (3) when growing internodes are sharply struck, there is 

 a sudden curvature, the concavit}- of which lies towards the 

 direction of the blow.^ 



1025. Tension of tissues. Under the ordinary circumstances 

 of growth walls of young cells continue to be somewhat elastic 



1 Pkcsmolysis. For a full account of the qunntitative action of numerous 

 plasmolytic agents the student should consult De Vries's paper in Pringsheim's 

 Jahrbiicher for 1884, where the effect of potassic nitrate and other snhstances 

 upon the protoplasmic film is detailed at length. In the Laboratory at Cam- 

 bridge, Mr. Puffer has confirmed most of De Vries's observations. 



2 Sachs ; Text-hook, 2d Eng. ed., 1882, pp. 784-788. 



