CH. V] TURGESCENT PITH. 185 



cells\ To show this, place the pith from experiment 153 

 in water, and measure it again after an hour. 



(155) Change in the transverse dimensions of jnth'^ 



Cut from the fresh turgescent pith of the stem of the 

 Helianthus, Sambucus and of Impatiens sidtani, also from a 

 rhubarb leafstalk, parallel-sided pieces about 10 — 15 mm. 

 in length and 5 mm. in width, taking especial care that 

 they are free from all cortical tissue. Place a piece on its 

 side (i.e. with the 5 mm. dimension vertical) in a small 

 flat-bottomed glass vessel, and lay on the pith an ebonite 

 vessel measuring 4 — 5 mm. in diameter by 2 — 3 in depth, 

 and containing oil. By means of the following arrange- 

 ment the oil is made to serve as a delicate index of any 

 shrinking or swelling of the pith. A vertical micrometer 

 screw graduated to 0*01 mm. carries at its lower end a 

 vertical needle, which can be lowered until it dimples the 

 polished surface of the oil ; the moment of contact is 

 sharply defined, and in this way changes of 001 mm. in 

 the diameter of the pith are easily read. After taking a 

 few readings, which usually indicate a slight shrinking, 

 water should be added. The results of the increased 

 turgescence so produced vary with the material employed ; 

 in the case of Sambucus and Helianthus the pith begins to 

 shrink, i.e. diminish in transverse diameter ; Rhubarb-pith 

 increases and afterwards diminishes ; while Impatiens 

 increases but does not diminish. 



1 See A. Bateson and F. Darwin " On the Effect of Stimulation on 

 Turgescent Vegetable Tissues," Linnean Society\-i Journal, xxiv. 1889. 



- See Miss Anna Bateson, Annals of Botany, Vol. iv. p. 117. A 

 drawing of the micrometer screw is given in Chapter vi. fig. 27. 



