ABSORPTION 155 



cork, and, ty a similar arrangement, attach to the other end a small scale 

 pan. Suspend the piece in front of a suitable measure and add weights 

 to the pan until the marks are brought to their original distance apart. Then 

 calculate the original diameter of the piece, and determine from these data 

 the power in atmospheres which must have been exerted by the osmotic 

 pressure. (Further particulars are given by DETMER, 154, or by DARWIN 

 and ACTON, 130.) The student should also note PFEFFER'S gypsum-cast 

 method described in the latter work, 131. 



Studies like the foregoing sooner or later bring up the ques- 

 tion whether all protoplasmic membranes are semi -permeable, 

 and whether the same membranes are constantly semi-permeable 

 under all conditions. The subject is rather difficult of experi- 

 ment, and the student had better inform himself upon it through 

 the literature, though there is one phase which he can readily 

 settle for himself as follows: 



SUGGESTED EXPERIMENT. Mount in water for the microscope a bit 

 of living tissue having a colored cell-sap, such as lower epidermis of Tra- 

 descantia discolor (or T. zebrina), or even a thin slice of very red Beet; place 

 the slide on the temperature stage, and, raising the temperature gradually, 

 note the degree at which the colored sap begins to come out, and interpret 

 this phenomenon. 



A crude demonstration of the same thing is given thus: take two simi- 

 lar small pieces (say cubes of i cm. side) of red Beet, boil one for a minute, 

 and place the two side by side in vessels of pure water; observe the differ- 

 ence between them after some hours. 



The student should now recall or look about for familiar 

 manifestations of osmotic phenomena, and then should interpret 

 these in terms of osmotic pressures, not simply in a general way, 

 but upon a molecular-pressure basis. Following is a list of some 

 of the more striking of these, several of them offering excellent 

 material for demonstration or experiment of a simple sort, and 

 the student should try to add others. 



FAMILIAR OSMOTIC PHENOMENA. These are: (a) the turgescence of 

 soft tissues as already demonstrated; (b) the power of soft tissues such as 

 young roots, or some kinds of Fungi, to lift heavy weights, break pavements, 

 force apart stones, etc.; (c) the crisping "of celery, cucumbers, etc., placed 

 in water; (d) the formation of a juicy syrup from fruits on which dry sugar 

 has been placed; (e) the bursting or collapsing, respectively, of berries 

 cooked with little or much sugar in preserving; (/) the plumpness of raisins 

 or currants when cooked, though collapsed when dry; (g) the powerful 

 swelling of soaking seeds (in part from osmosis, but in part from imbibition); 



