128 SPLIT STEMS. [CH. V 



which weights may be hung. Having noted the position 

 of / on the scale, hang a small weight TT (a coil of 

 lead wire of 8 — 10 grams) on the loop, and read off the 

 position of the index. Remove the weight, and bend the 

 shoot two or three times backwards and forwards in the 

 vertical plane. When the weight is once more attached, 

 the index will move through a greater distance than that 

 at first recorded. 



(162) Increased length. 



Since the pith is in a state of compression, any in- 

 creased length of the cortical tissue must result in an 

 increase in the length of the whole shoot. Therefore 

 bending a turgescent shoot backwards and forwards as in 

 experiment 161, must lengthen it. The length must be 

 accurately measured, say to O'l mm., to make sure of a 

 result. 



(163) Splitting twrgescent tissues. 



The relation between the comp^-essed pith and the 

 stretched cortex can be demonstrated by dividing a shoot 

 longitudinally. It is best to prepare the shoot by cutting 

 it flat on two opposite sides. 



We now have a column of pith bounded on two sides 

 by strips of cortical tissue : this is placed on a glass plate 

 and bisected with a knife, when each half curves so that 

 the pith is on the convex, the cortex on the concave side. 

 The curvature can be greatly increased by putting the 

 half-shoots in water. This increase is strikingly seen if a 

 dandelion stalk is split into 4 or 5 longitudinal strips, 

 w^hich curl up in water into helices of many turns'. 

 1 This fact has already been utilised in experiment 148. 



