ISO PLANT PHYSIOLOGY 



are difficult to obtain, is a Pastetjr-Chamberland bourie filter, obtainable 

 of all bacteriological supply companies at a cost of about 60 cents. I bave 

 also found that the gauge may be brought into communication therewith 

 by use of a plumber's ground brass joint (thinly greased), one part of which 

 is cemented by hard sealing-wax to the bougie and the other to the glass 

 gauge, the various joints being made tight by glass sleeves and sealing-wax. 

 Molasses, however, is not to be used in such a gauge, since its strength soon 

 bursts the cup; and besides it is far preferable to work with the dilute solu- 

 tions found in the plant. 



The student should now inform himself as to our knowledge 

 of the amounts of osmotic pressure developed by different sub- 

 stances, both outside the plant and within, the relations which 

 exist between osmotic and gas pressures, the relations of the pres- 

 sures of different substances to the numbers of molecules involved 

 and to the ionization of the substances, and the theories now held 

 to account for the pressure. And he should make sure that he 

 understands the invaluable De Vries-Pfeffer table of osmotic 

 data (Pfeffer, i, 146), excepting for the isosmotic coefficients, 

 which are later to be studied. 



So much for the physical and general aspects of the subject; 

 we turn next to the direct observation of the osmotic phenomena 

 of single living cells. Obviously these do not admit of direct 

 osmometric measurements, but advantage may be taken of 

 the fact that exosmotic is as easy as endosmotic movement, and 

 an exosmotic movement may be induced by the use of strong 

 solutions outside the cell. We may now bring the subject defi- 

 nitely before us in the form of a problem thus: 



What phenomena are exhibited on the application to living 

 cells of strongly osmotic solutions? 



Experiment. Mount for the microscope, on one slide but under 

 separate covers, streaming cells of Nitella, and apply to both, by draw- 

 ing under the cover with filter-paper, a normal solution of cane-sugar, 

 keeping one cell under observation the while. As soon as a marked 

 effect (plasmolysis) is produced, replace the sugar in one by water 

 but leave the other for an hour; keep the preparations from drying 

 out, and both observe and interpret the results. 



Experiment. To other specimens apply, respectively, .5, .25, .24, 

 .23, .22, .21, .20. normal cane-sugar, and observe the effect, leaving 

 them for one hour. Which strength just produces a visible plasmo- 

 lytic result? 



