134 LONGITUDINAL TENSION. [CH. V 



20 cm., so that measurements to 1 mm. may give perceptible 

 results. The material must be as fresh as possible, and if 

 it has to be brought from any considerable distance must 

 be wrapped in a wet cloth and placed in a vasculum : in 

 this case too, it is worth while to take care that the 

 vasculum is held vertically, lest the shoots should take a 

 geotropic curvature, as they may do if kept horizontal for 

 an hour. 



Place the shoot on the table, cut the ends as square as 

 possible and measure its length with a millimeter scale 

 placed lengthwise on it. Remove a strip of cortical tissue 

 along the side measured ; it will be shorter than the 

 original shoot. Now remove the whole of the cortical 

 tissue, and measure the length of the cylinder of pith 

 remaining, which will be found to be longer than the 

 intact shoot. 



This experiment shows that the internal tissues are in 

 a state of compression, while the cortex is extended. It 

 is important to note that the amount of extension of 

 the freed pith need not by any means be the same as the 

 contraction of the cortex ^ If the experiment is repeated 

 with a scape of Fritillaria imperialis which has ceased to 

 grow, it will be found that the pith lengthens considerably 

 while the contraction of the cortex is very slight. 



(154) Extension of pith in water. 



When pith is placed in water it increases greatly in 

 length in consequence of the increased turgescence of its 



1 Sachs' Text-hook, p. 797. 



