ROOTS 53 



Put some living threads of pond-scum (Spirogyra) (Chapter XX) 

 into a 4 per cent solution of glycerine in water, a 16 per cent solution 

 of cane sugar, or (for quick results) a 2 per cent solution of common 

 salt. 1 Any one of these solutions is much denser than the cell-sap 

 inside the cells of the pond-scum, and therefore the watery part of 

 the cell-contents will be drawn out of the interior of the cell and 

 the protoplasmic lining will collapse, receding from the cell-wall. 

 The cell-contents are then said to be plasmolyzed. Sketch several 

 cells in this condition. Remove the filaments of Spirogyra and 

 place them in fresh water. How do they now behave ? Explain. 

 Repeat the plasmolyzing operation with another set of cells which 

 have first been killed by soaking them for five minutes in a mixture 

 of equal quantities of acetic acid, three parts to 1000 of water, and 

 chromic acid, seven parts to 1000 of water. The pond-scum threads 

 before being transferred from the killing solution into the plas- 

 molyzing solution should be rinsed with a little clear water. Note 

 how the cells now behave. How is it shown that they have lost 

 their power of causing a liquid to be transferred mainly or wholly 

 in one direction? Why do frozen or boiled slices of a red beet 

 color water in which they are placed, while fresh slices do not? 



65. Osmosis in Root-Hairs. The soil-water (practically 

 identical with ordinary spring or well water) is separated 

 from the more or less sugary or mucilaginous sap inside 

 of the root-hairs only by their delicate cell-walls, lined 

 with a thin layer of protoplasm. This soil- water will pass 

 rapidly into the plant, while very little of the sap will 

 come out. The selective action, which causes the flow of 

 liquid through the root-hairs to be almost wholly inward, 

 is due to the living layer of protoplasm (Chapter XII), 

 which covers the inner surface of the cell-wall of the root- 

 hair. When the student has learned how active a sub- 

 stance protoplasm often shows itself to be, he will not be 

 astonished to find it behaving almost as though it were 



1 This should be done as a demonstration by the teacher. 



