CELL-CONTENTS AND CELL-WALLS. 33 



(a) The leaves of most plants change but little in colour 

 when plunged into water at 60 C. or over, but they be- 

 come limp, owing to the cells losing their turgidity on 

 being killed, and cannot be restored to the normal condi- 

 tion of turgescence. However, some leaves, e.g. Oxalis, 

 Vine, Begonia (especially B. manicata), quickly become 

 discoloured by hot water, owing to the chlorophyll being 

 decomposed by the acid sap which, on the death of the 

 protoplasm, is allowed to come into direct contact with the 

 chloroplasts. 



(b) Tie an Oxalis leaf to the bulb of a thermometer and 

 hold it in water in a beaker above a Bunsen, or in a large 

 test-tube, and gradually heat the water. Try several 

 times, and carefully note the average temperature at which 

 the colour change occurs usually about 50 C. or a little 

 over. 



(c) Place Oxalis leaves, some entire and others cut into 

 pieces, in a bottle of 1 per cent, chloroform water ; note 

 the time taken for the colour change in each case. Try 

 other poisons instead of chloroform in the water, e.g. car- 

 bolic acid, formalin. 



(d) Cut out two pieces of living Begonia leaf -stalk, 

 rinse them in water, then place one in a beaker of cold 

 water (either distilled water, or water that has been boiled 

 and allowed to cool) labelled A. Kill the second piece 

 (B) by immersion in very hot water ; when it is dis- 

 coloured, put it in a second beaker of water. After half 

 an hour remove the two pieces, and pour into the water 

 in each beaker an equal quantity of strong calcium chloride 

 solution. In A the water remains clear ; that in B be- 

 comes turbid, owing to the formation of calcium oxalate 

 produced by the oxalic acid which has escaped from the 

 killed cells. 



(e) The effect of mechanical injury on the protoplasm 

 can readily be shown by firmly squeezing between the 

 fingers or a pair of forceps a Begonia leaf; the crushed 

 parts at once become brownish. Cut and mount in water 

 a tangential section of the injured part, and note that 

 the chloroplasts have lost their green colour and become 



P.B. 3 



