116 nobbe's experiment. [ch. V 



(134) StijKt: salt-solution. 



An awn which has come to rest in water can be made 

 to twist in the dry direction by transferring it to 10 per 

 cent. NaCl solution. 



(135) Stipa : mechanism of the movement. 



The twisting power of the awn depends on the hygro- 

 scopic torsion of its individual cells. To show this it is 

 necessary to isolate some of the elements. Prepare 

 Schulze's macerating fluid by dissolving in 50 c.c. of nitric 

 acid, 1 grm. of potassium chlorate ; add to this half its 

 volume of water, and boil a ripe awn cut into two or three 

 pieces in a test-tube half full of the diluted liquid. It is 

 best not to boil it too much ; as soon as the awn is clearly 

 beginning to disintegrate it must be removed, thoroughly 

 washed in w^ater\ and teased out with needles. A small 

 portion^ is now dried on a glass slip without a cover-glass 

 over a flame and examined under the microscope with a 

 low power. Cells will be found which are obviously 

 t wasted on their axes, and which at once untwist when 

 water is added. Or the dried fragments of awn can be 

 seen to exhibit torsion if an assistant breathes cautiously 

 on the preparation w^hilst under observation. 



(136) Nohhes experiment"^ 



Take two stoppered bottles of about 400 c.c. capacity : 

 fit each with a rubber cork through which passes a narrow 



1 Because the fumes of Schulze's fluid are bad for the lenses of 

 microscopes. , 



2 The colourless pieces should be selected. 



3 Handbuch der Sainenkunde, 1876, p. 126. 



