122 LONGITUDINAL TENSION. [CH. V 



of strain or tension' which exists in turgescent tissues 

 may be made in summer or spring on any rapidly growing 

 juicy shoot, e.g. elder, or with certain leaf stalks, e.g. that 

 of the^rHuBarb. In winter it is sometimes difficult to find 

 suitable material: if a green-house is available the leaf 

 stalks of "EicFajdia will answer well. It is best to get 

 fairly long shoots, i.e. not less than 25cm., so that measure- 

 ments 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 vascultm: in this case too, it is 

 worth while to take care that the vasculum is held verti- 

 cally, lest the shoots sEoifrd Take a geotropic curvatttte, 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 



1 See Sachs' Text-Booh, Sect. 14, 15. The whole discussion should 

 be studied. 



2 Sachs' Text-Book, p. 797. 



