TOXIC CONCENTRATION OF MAGNESIUM NITRATE FOR THE SQUASH 409 
the light, a diffused daylight intensified by a tungsten lamp was used. 
Heat radiation from the lamp was minimized by a double glass window 
in the constant temperature room. In every plant of this series there 
was a deep green coloration equal to that of plants grown in daylight. 
As is shown by the graph, a concentration of i^on X 10-6 Mg(N03)2 
is not toxic to the squash under the above conditions of illumination. 
In the dark, however, this concentration is toxic. The toxicity here 
is plainly shown both by the leach of electrolytes with an increase in 
concentration as shown by the graph, and in the lack of root growth as 
shown by the photograph. The growth of tops in the etiolated seed- 
lings is of course greater than those exposed to light. The concentra- 
tion 125W X 10-6 seems to be just at the border line of toxicity for 
etiolated squashes. It, therefore, appears that such light exposure 
and chlorophyll formation is accompanied by a rise in the minimal 
toxic concentration of the solution. 
Either of two conditions can produce this rise in the minimal toxic 
concentration. Either the resistance of the protoplasm to the toxic 
effect of magnesium may be increased, or the concentration of mag- 
nesium within the cell may be reduced by light exposure. In regard 
to increased resistance of the protoplasm no evidence is offered here. 
It seems probable, however, that the decrease in concentration brought 
about by light exposure may be sufficient to account for the change. 
To become toxic the magnesium must reach a certain concentration 
within the cell. From the etiolated series this minimum toxic con- 
centration is seen to be a little below the equilibrium concentration 
established within the cells in i2on X 10-6 Mg(N03)2. Since a con- 
centration of 150W X 10-6 Mg(N03)2 is not toxic to plants exposed 
to light, the concentration of Mg(N03)2 actually in condition to 
produce toxic effects within the cell is probably less than the minimum 
toxic concentration found for etiolated seedlings. This decrease in 
concentration may be brought about by the removal of magnesium to 
form non-toxic compounds. One such group of compounds com- 
paratively rich in magnesium whose formation in the squash depends 
upon light is the leaf-green compounds found by Willstatter (i) to 
consist of two parts, chlorophyll a and chlorophyll b. 
From the work of Willstatter and others (2), it has been shown 
that magnesium forms an important part of the chlorophyll molecule. 
Mameli (3, 4), has shown that the presence of magnesium favors 
chlorophyll formation. 
