CH. V] TUBGESCENT PITH. 128 



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 imperceptible. 



(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 

 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 ofpith^. 



Cut from theTresETurgescent pitB of the stem of the 

 Sunflower, Elder, and of Impatiens sultani, also from a 

 Ehubarb leaf stalk, 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 O'Ol 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 O'Ol mm. in 

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

 few readings, which usually indicate a slight shrinking, 



' See Miss Anna Bateson, AnnaU of Botany, Vol. iv. p. 117. A 

 drawing of the micrometer screw is given in Chapter VI. fig. 25. 



