MEASURING ELECTRICAL CONDUCTIVITY 25 



(arranged in a circle as shown in the figure) each of 



which receives the end of a glass rod about 9 inches long, 



the other end of each rod being fastened in a similar 



support. The circle (dotted line) just inside the seven 



small circles represents a disk of tissue inserted between 



the glass rods with its surface at right angles to them. 



The smaller circle, H, in the centre 



represents an opening in the electrode 



holder through which the current 



passes from the platinum electrode to 



the disks of Laminaria. The arrange- 



ment is shown in Fig. 3, where H 



represents the opening and L repre- 



sents the disks. Before reaching the 



disks the current passes through K, 



(Fig. 3), a hard rubber disk (with an 



opening in the centre) which provides 



mechanical support for the tissue. 



The disks are cut from the fronds 

 by means of a cork borer and have 



D 



about the diameter and thickness of a whichare B eta3 S rods 



., , mi i -11 (seen in section, as a series 



silver quarter. They are packed to- O f circles) which hold 



i-i TI n f " / i A. -inn in place th , e d ^ sk8 , of 



gether like a roll OI COinS (abOUt 100 tissue (een in 1 section] asla 



dotted line). 



in all). They are firmly held in 

 place by the glass rods which surround them and by 

 the electrode holders which press against them at either 

 end. At the same time the spaces between the glass 

 rods allow free circulation of liquid. 



Each disk is placed in sea water as soon as it is cut 5 ; 



"It was at first thought that cutting might injure the tissues at the 

 edge of the disk sufficiently to interfere with the results, but experiments 

 proved that this is not the case. Not only do the cells adjoining the cut 

 surface live as long as those in the centre of the disk, but it is found that 

 experiments (made by another method) on intact fronds give the same 

 results as experiments on the cut disks, 



