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No. 45. 



VARIATIONS IN THE PERMEABILITY OP LEAF-CELLS. 



By henry H. DIXON, Sc.D., F.R.S., 

 Professor of Botany in the University of Dublin. 



(Read Jdnb 24. Printed July 1.5, 1924.) 



From many points of view changes in the permeability of cells and the factors 

 controlling them are of interest. Variations in the permeability of the 

 protoplasmic membranes lining the leaf -cells have special significance with regard 

 to the problems of the export of dissolved substances from leaves to the rest 

 of the plant and of their import from the stem to the leaf (6 and 7). 



Certain conditions, however, render the study of the permeability of leaf- 

 cells difficult. The plasmolytic method is unsatisfactory in their case, owing 

 to the difficulty of obtaining good contact between the surfaces of these cells 

 and the watery solutions used. The injury to the cells of leaf-tissues necessary 

 to render them visible also introduces disturbances unlmown in nature and 

 magnitude. 



Osterhout's important and pioneer researches (9, 10, 11, 12) have introduced 

 the electrical method for the determination of changes in permeability in 

 homogeneous tissues like those of the great kelps; he and others have shown 

 that the electrical resistance of a tissue is a measure of the permeability of 

 the protoplasmic membranes of its component cells. The methods of making 

 electrical contact with the tissue under experiment, which he found suitable 

 with Laminaria, are, however, not available for leaf-tissues, which are more 

 or less spongy, and are coated with an unwettable cuticle. Consequently it 

 was necessary, if the electrical resistance method was to be used, to find 

 some other means of effecting this end. 



After a considerable amount of experiment the following method was found 

 satisfactory, and adopted : — A square centimetre is cut with a template from 

 the leaf to be investigated. It is laid across two electrodes {E, fig. 1) formed 

 of two square pieces of platinum foil 0-5 cm. on the side. Each electrode is 

 given a rectangular fold so as to appear Ij-shaped in profile. The two horizontal 

 parts face each other and support the leaf-square. To the vertical part of 

 each electrode is soldered a piece of platinum wire (D) which passes through 

 the sealed end of a glass tube. The end of the latter supports the electrode 

 rigidly. The glass tubes are filled with mercury (ff) and are fixed in a small 

 frame made of three discs of cork {A, B, C) connected by a glass rod (L). 

 The ends of flexible leads dipping into the mercury connect the electrodes 

 with one of the anns of a Kohlrausch bridge. In order to secure good contact 

 between the electrodes and the cut edge of the leaf-square a little sap, pressed 

 from leaves of the experimental plant, is placed upon each electrode. The 

 electrodes themselves are coated with platinum-black to obviate polarization 

 effects. Before cutting out the leaf-square from the leaf, it is advisable to 

 smear the upper surface of the latter with a thin film of vaseline. This prevents 

 the sap on the electrodes from creeping across and short-circuiting the tissue. 



SCIENT. PKOC. K.D.S., VOL. XVII, NO. 4.5. 3 X 



