70 PLANT PHYSIOLOGY. 



26. Can the turgidity resulting from osmotic pressure afford 

 mechanical rigidity? 



Answer by Experiments I I and 12. 



EXPERIMENT 1 1. To test this an apparatus should be constructed 

 which is flaccid to begin with, but capable of absorbing water osmot- 

 ically. This can be done thus : Take a piece of parchment-paper 

 tubing 40 mm. diameter and 1 5 cm. long ; soak it well ; gather one 

 end into close folds and tie tightly with waxed thread ; fill nearly full 

 with molasses, gather the open end into folds, squeeze until the 

 molasses begins to run out (to expel air) and tie tightly with thread. 

 (Or, the ends may be tied tightly to large corks, preferably rubber, 

 fitted into them.) The tube will now be so limp as to bend in the 

 hand, lacking all rigidity. Place in a basin of \vater for a few hours 

 and examine. Holding it in the air, prick the membrane with a 

 pin. (Rubber bands joining the threads at the ends show instruc- 

 tive results.) 



EXPERIMENT 12. A correlative experiment to the above, to test 

 the effect of removing the pressure from a turgid and rigid structure, 

 is needed. It may be done thus : Take a young internode of a twin- 

 ing plant or its equivalent (thus, roots of Lupinus albus or beans 

 grown in sphagnum) and place in a io salt solution (which causes 

 an exosmotic movement) for a short time, and observe effect. It is 

 well to measure the length of the internode before and after this 

 treatment, and also to determine by use of weights the amount of 

 force required after the treatment to make the piece its former 

 length. Or a delicate leaf may be placed in the salt solution, with 

 an instructive result. 



Here the student should make some microscopical observations upon 

 plasmolysis, using Tradescantiaor Nitella, with salt or sugar solutions of 

 different strengths, from 2% upwards. Very exact quantitative determin- 

 ations of the turgor-pressure in cells may be made by use of equimolecular 

 solutions of potassic nitrate, directions for which may be found in Det- 

 mer-Moor, 151. As material, rings from dandelion scapes or pieces of 

 Ricinus hypocotyl are recommended. 



It is doubtful if the measurement of the force necessary to pull a plas- 

 molyzed tissue back to its length before plasmolysis has much meaning 

 as an index of the turgor-pressure in the cells. 



Since the absorbing systems of roots consist of closed 

 membranes, it is obvious that minerals cannot pass into them 

 in the solid form. The only alternative possibility is that they 

 may be absorbed dissolved in the water. 



