546 



ILLUSTRATIVE EXPERIMENTS 



54. Conditions Affecting Growth, etc. 



To a test tube Jilled with a nutrient medium add 1 drop of yeast emul- 

 sion. (Nutrient mediimi contains the elements C, 0, H, N, S, and P, e.^r.Urea 

 CO(NH2)2, glucose CeHi^Oc, with traces of Na2HP04, and Ca3(P04)2.) Add 

 a similar drop to a test tube filled with distilled water. Shake the tubes 

 well and examine a drop of the mixtixre from each microscopically, counting 

 the number of cells in three fields ; take the average. Note the appearance 

 of the mixtures. 



Student at bench (1). Insert a cork fitted with a glass tube into the test 

 tubes, and place them in the incubator at 37° C. 



(2) As in (1), placing the tubes in ice. 



(3) Introduce a few drops of pure phenol into each of the tubes, insert the 

 corks as in (1) and place the tubes in the incubator. 



(4) Boil the mixtures, cool the tubes under the tap, insert the corks and 

 place the tubes in the incubator. 



The tubes are left under these conditions for 24 hours. 



Results. . Examine the tubes before and after shaking. A drop from each 

 tube is to be examined microscopically after shaking, counting the number 

 of cells in three fields ; take the average. 



Record the results on the sheet provided, using the signs -(- and — . 



Tubes witli sugar (l)\t 37° C. 

 Tubes with sugar (2) at 0° C. 

 Tubes with sugar (3) with phenol 

 Tubes with sugar (4) ^boiled . 

 Tubes with water (5)'at 37° C. 



Gas 



Opacity. 



dumber. 



Test (1) is also carried out in bulk, the yeast cells are removed by filtration. 

 Use some of the filtrate for the followmg tests. 



(a) Disappearance of sugar. To about h in. of the fluid in a test tube add 

 an equal volume of Folin's copper sohition and boil for one minute. Note 

 the extent of reduction. Repeat, using the original nutrient medium. Com- 

 pare the reduction obtained with that obtained in the preceding test. 



[h) Formation of alcohol. Some of the filtrate from (1) is distilled. Test 

 the distillate for alcohol. To about | in. of the distillate add a few drops of 

 Pot. bichromate and a little H2SO,, cone, warm. 



Note. (1) Pungent odour, aldehyde. (2) Green colour. 



Nature of the gas evolved. Fill a test tube with the nutrient medium, and, 

 covering the mouth of the tube with a coin, invert it into a dish containing 

 nutrient medium ; care must be taken to avoid entrance of air bubbles into 

 the tube. Introduce a piece of yeast about the size of a pea so that it comes 

 to lie in the mouth of the inverted test tube, which is now allowed to rest on 

 the dish. Su})port the tube by means of a burette stand and place in the 

 incubator at 37° C. for 24 hours. 



With the mouth of the tube still beneath the surface of the fluid in the dish, 

 introduce into the tube a few cubic centimetres of 40 per cent. NaOH using a 

 bent pipette. 



Conclusions. (1 ) What has happened to the yeast cells in each of the tubes ? 



(2) What conclusions do you come to as to the influence upon the yeast 

 })rotoplasm of the various conditions to which it has bean subjected ? 



