RESUMEN
Analyses of terrestrial sources of sodium and estimates of the sodium requirement of moose (Alces alces) on Isle Royale, Lake Superior, suggest that availability of the element controls the moose population. The terrestrial vegetation is very poor in the element, but, as elsewhere, submerged and floating leaved water-plants are relatively rich. Consumption of such plants in summer would provide an adequate source, if the animal can store the element. The fairly high sodium contents of freshwater vegetation have been little appreciated. In general, sodium concentration in water-plants, unlike that of potassium, is not correlated with chloride but the latter is ordinarily in excess of the sodium, so that uptake of the latter implies an equivalent supply of NaCl.