CH. V] TENSIONS OF TISSUES. 133 



blocks a and b is regulated by cylinders of paper acting as 

 moulds. When block b is set hard it may be removed 



/ 



from the root, trimmed with a knife and replaced : at 

 the same time the tissue-paper may be removed. Before 

 the flower-pot is placed in the supporting ring m the block 

 of gypsum b must be secured in its place by tying it with 

 a thread which will be cut when the arrangement is 

 complete. The block b is fixed by fluid gypsum to the 

 glass plate c which rests on the spring /. 



The plate I, forming part of the spring, is fixed by the 

 small screws k, k to the solid plate g } which can be raised 

 and lowered by means of the screws h, h, h. In this way 

 the desired amount of pressure can be applied at the 

 beginning of the experiment. The distance between the 

 needle-points is regulated by the screw i which moves the 

 lower needle. 



SECTION C. Tensions of tissues. 



(153) Longitudinal tensions. 



The fundamental experiment illustrating the condition 

 of strain or tension 1 which consists in turgescent tissues 

 may be made in summer or spring on any rapidly growing 

 juicy shoot, e.g. elder (Sambucus), or with certain leaf- 

 stalks, e.g. that of the rhubarb (Rheum). In winter it is 

 sometimes difficult to find suitable material : if a green- 

 house is available, the leaf stalks of Richardia will answer 

 well. It is best to get fairly long shoots, i.e. not less than 



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

 be studied. 



