CELL-CONTENTS AND CELL-WALLS. 35 



glass of the jar. Note that the ice is colourless, showing 

 that only water, not the coloured sap, has been frozen out 

 of the cells. 



(j ) Freeze another Beetroot slice, and suspend it in a 

 beaker of water at the ordinary temperature ; arrange a 

 slice of unfrozen root in a similar beaker for comparison ; 

 the frozen slice yields its coloured sap to the water, the 

 other does not. 



(k) Mount filaments of Spirogyra in water on a slide, 

 place the preparation in the freezing apparatus, and note 

 that the cells are strongly plasmolysed and shrunken, but 

 on thawing the cell- wall is seen to be intact. Freezing 

 does not cause rupture of the cell-wall. 



(I) To illustrate the fact that freezing causes a mole- 

 cular change in the protoplasm a rearrangement of the 

 molecules make some starch paste in a beaker or test- 

 tube. Freeze the paste ; when it thaws it is no longer a 

 homogeneous liquid, but has become spongy, the "pores " 

 being filled with fluid. 



(m) Our experiments with frozen Beetroot slices and 

 Spirogyra threads suggest that on freezing the formation 

 of ice takes place not inside the cells themselves, but on 

 the outside in the intercellular spaces in the case of a 

 mass of tissue. To demonstrate that this is usually the 

 case (though under some conditions ice is formed within 

 the cell), cut off the upper part of a Beetroot, scoop out 

 a cavity in the lower part, and fix the upper part on again, 

 like a lid, with thread. Freeze to about 8C., and on 

 removing the lid-like upper part note that ice has accumu- 

 lated in the cavity. 



(n) That ice usually forms, at any rate at first, in the 

 intercellular spaces may be directly observed. Freeze a 

 Potato or a Carrot, and with a very cold razor (chilled by 

 being put in the freezing apparatus or in ice-water) cut 

 sections and mount them on a chilled slide. Observe 

 quickly, and note that the ice crystals have been formed 

 between the cells. As thawing proceeds (check its rate by 

 placing a bit of ice at the side of the cover-glass), note 

 that the intercellular spaces have expanded as the ice 



