WORK 169 



slices do not give up their pigment. If boiled, so as to kill the cells, 

 the water becomes red. 



To show that the living cells are also impermeable to sugar a 

 chemical test must be used. Owing to its aldehyde group, glucose 

 reduces copper salts in alkaline solution. The sugar in beet is, 

 however, cane sugar, which has no free aldehyde group. It must 

 be hydrolysed by boiling with acid. The test may be made with 

 the water in which living beet has lain, and also with that in which 

 it has been boiled. Add to a sample in a test-tube a drop or 

 two of strong hydrochloric acid, boil for a minute, or immerse the 

 end of the test-tube in boiling water for a few minutes. Cool. Add 

 a few drops of copper sulphate solution, a crystal of Rochelle salt, 

 and then sufficient sodium hydroxide to make a deep blue solution. 

 On boiling again, a red precipitate of cuprous oxide is formed if 

 a reducing sugar is present. 



The effect of other methods of killing the cells of the red 

 beet may be tested by adding chloroform, formaldehyde, or acid. 

 Substances which lower surface energy, such as saponin, bile salts, 

 or amyl alcohol, also have the effect of allowing the pigment to 

 escape. In some specimens, it may be possible to observe the 

 reversible increase of permeability produced by sodium chloride. 

 Take, a i per cent, solution in distilled water, change several times, 

 and the pigment may be seen to escape slowly from the cells. 

 The normal state is reproduced in such cases by the addition of 

 a calcium salt in small amount, most simply by changing to hard 

 tap water. 



Dilute acid and alkali may also be tested, say, o.oi per cent, 

 hydrochloric acid and a similar strength of sodium hydroxide. 



Osmotic Pressure 



Blood Corpuscles. Make a preparation of blood from the finger 

 as described above (p. 166). Measure the diameter of a number of 

 corpuscles and take the average. The measurement is done by a 

 scale in the eyepiece, the value of its divisions having previously 

 been determined by the aid of a scale divided into o.ooi mm. 

 placed on the stage and observed with the same objective as that 

 used for the blood preparation. 



On allowing water to run under the cover-slip, the corpuscles 

 rapidly swell up and their contents escape, leaving the framework 

 as a nearly invisible disc. This action of water is, however, so 

 vigorous that it is difficult to follow the series of changes. It 

 is better to take a solution of sodium chloride of about 0.3 to 

 0.4 per cent. 



Make next solutions of cane-sugar of 15, 10, and 5 per cent. 



