64 PLANT PHYSIOLOGY. 



supply stores) work equally well. When thoroughly soaked they are to 

 be tied over the lower end of the burette, but, having little surface, they 

 work far more slowly than the shell. Various simple forms of osmome- 

 ters are shown in Fig. 1 1. The advantage of the burette is that it allows 

 exact measurements. A very simple and effective form using parchment- 

 paper tubing is described by MacDougal in Journal of Applied Micros- 

 copy, 1,56. It is especially valuable for class demonstration. Diffusion- 

 shells of 40 mm. diameter are obtainable. 



The osmometer shown in Fig. 10 can easily be made autographic by 

 use of a frictionless float (see page 124) on the liquid, which is con- 

 nected by a thread with the wheel and recording cylinder later described 

 (page 103). 



Properly, to give a true measure of the osmosis in the preceding ex- 

 periment the two liquids should be kept at the same level by constantly 

 sinking the burette or raising the level outside, but practically this is 

 unnecessary, especially as the error is in the direction of a lesser and not 

 a greater result. The size of the outer vessel makes some difference in 

 the result, for if small the sugar passing exosmotically to it from the cup 

 rapidly raises its concentration towards that of the liquid in the cup, 

 hence diminishing both the rate and the amount of the rise in the latter. 

 But, as my experiments show, above a certain size (about that shown in 

 Fig. 10) the result is not appreciably affected by the size of the outer 

 vessel. 



Of course many crystal lizable substances besides sugar may be used, 

 such as potassium nitrate, etc. The advantage of the sugar is that it is 

 probably the chief substance effective in the endosmotic absorption of 

 water by the root-hairs, though it is possible that potassium nitrate 

 plays also a part in this process. A great advantage of the molasses is 

 its color. 



The rate of diffusion through a membrane, even the possi- 

 bility of its occurrence at all, varies with the membrane and 

 with the substances in solution. The parchment cup used in 

 Experiment 6 was readily permeable by the water, minerals, 

 and sugar. There are other membranes called semi-permeable, 

 which allow water and some dissolved substances to pass, but 

 not others. 



21. What effect has a semi-permeable membrane upon Osmosis? 

 Answer by Experiment 7. 



EXPERIMENT 7. An excellent semi-permeable membrane is formed 

 by a precipitation-film of copper ferrocyanide, which may be made 

 as follows: In a large upright test-tube, or its equivalent, place a 



