PRACTICAL WORK III 



(7) With a knife split a length of fresh bean stem longitudinally 

 into quarters. Note that the ends of the strips curve outwards. 

 The harder tissues of the outside of the stem are in a state of 

 elastic tension in the intact stem, i.e. they are tending to contract 

 in the longitudinal direction, but cannot do so against the swelling 

 force of the turgid cells of the centre (pith) of the stem, which 

 are similarly prevented from actually swelling. When the 

 stem is divided longitudinally the inner cells swell and the outer 

 ones contract, so that each strip of stem bends outwards. 



(8) Strip ofE a fragment of the coloured surface layer of 

 Tradescantia leaf as free as possible from the green tissue below. 

 Mount in water and examine under the high power. The vacuoles 

 of the purple cells are filled with coloured cell sap. Place a drop 

 of 10 per cent. CaCl2 solution on the slide just touching the edge 

 of the coverslip, and draw the solution under the coversUp by 

 holding a fragment of blotting paper against the opposite edge. 

 Watch the effect on the cells — plasmolysis. Reverse the action 

 by running in a drop of water in the same way and watch the 

 recovery of turgor, 



