46 Papers from the Marine Biological Laboratory at Tortugas. 



their behavior is one of degree rather than of kind. In other words, the 

 motor centers are merely a more sensitive part of the nervous system than 

 is the nerve-net of the subumbrella. 



For example, a medusa of Cassiopea xamachana (record, fig. 8) with 

 motor centers intact, and pulsating at the rate of 33 per minute in natural 

 sea-water, pulsated very irregularly at the rate of about 8 per minute after 

 having been for 10.5 hours in a solution of 100 volumes of sea-water -f 700 

 volumes of 0.6 molecular NaCl. Here we see the decline in pulsation-rate 

 is greater than in the nervous network without motor centers. If, however, 

 the sea-water had been diluted with 0.9 dextrose, the medusa would have 

 ceased to pulsate when the sea-water had been mixed with about an equal 

 volume of dextrose ; hence the effect of the lowering of concentration of Mg, 

 Ca, and K and proportionately increasing the concentration of the sodium 

 ion is to stimulate and maintain pulsation. 



The initial effect of an increased concentration of NaCl upon the normal 

 medusa is to increase its rate of pulsation. Thus the medusa which pul- 

 sated at the rate of 33 per minute in sea-water, immediately pulsated at the 

 rate of about 116 per minute when placed in i volume of sea- water plus 7 

 volumes of 0.6 molecular NaCl, although after half an hour its rate was 

 about normal and after 10.5 hours it was only about 8 per minute. Thus 

 the initial efifect of the relatively increased concentration of sodium was to 

 stimulate, but later it produced depression in a manner resembling the 

 effect of its close chemical ally, potassium. It is interesting to see that 

 Magowan (1908) and Osterhout (1909) find that sodium and potassium 

 are closely similar both in their toxic and protective action upon plants.^ 



THE INERT NATURE OF THE MAGNESIUM ION. 



In order to determine the effects of the magnesium ion, we may add 

 magnesium to sea-water in the form of a 0.057 molecular solution of MgCla 

 dissolved in 0.78 molecular dextrose. This decreases the concentration of 

 the sodium, calcium, and potassium cations, but maintains the normal con- 

 centration of the magnesium ion and also the normal osmotic pressure of 

 the sea-water. The effect must, therefore, be due to the decline in con- 

 centration of the sodium, calcium, and potassium. 



Table 10 and figure 9 (dotted curve with open circles) show the results 

 of this experiment upon 7 rings of subumbrella tissue of Cassiopea, and it 

 indicates that the magnesium ion, in the concentration found in sea- 

 water, is not in any sense a stimulant; for the rate of nerve-conduction 

 declines in practically the same manner is it does when the sea-water is 

 diluted with distilled water or with 0.9 molecular dextrose. 



The non-toxic nature of magnesium is clearly shown by the good and 

 rapid recovery of the rate of pulsation when the rings are replaced in natural 

 sea-water. 



Figure 9 shows the effects of magnesium on the rate of nerve-conduction. 

 The full-line curve shows the effect of adding 0.4 molecular MgCl2 to sea- 



> Botanical Gazette, vol. 4S, P- 4S; vol. 48, p. 98. 



