348 Hoyt: CULTURES OF SPIROGYRA 
solutions thus made were all more toxic than those from nontoxic 
water. 
A solution similar to the one without calcium, but with mag- 
nesium also absent, was prepared with equal parts by weight of 
KNO;, KzgHPO,, and KCl. This was more toxic than the calcium- 
free solution with magnesium present; both 0.4 and 0.1 per cent 
concentrations were fatal to the plant within two days (V, 6). 
Animal charcoal produced no improvement in the stronger solu- 
tion (V, 7). The addition of calcium carbonate in considerable 
excess of its solubility enabled the plant to grow for several days 
(V, 8). Asolution containing 0.1 per cent of the three potassium 
salts and 0.1 per cent of CaCl, was fatal within three days (V, 
9), while the addition of 0.1 per cent of MgSO, to the 0.3 per 
cent solution of potassium salts prolonged the life of the organism 
for six days, but did not completely counteract the toxicity (V, 10). 
The last is a repetition of the first experiment of this section, where 
the organism lived 28 days (V, 1). 
A simple solution of magnesium sulphate was fatal to the plant 
in concentrations ranging from 0.4 to 0.01 per cent (V, II). 
Animal charcoal produced improvement only in the weakest of 
these concentrations (V, 12). Wood charcoal (with some calcium 
present) slightly improved both the 0.1 and the 0.4 per cent. 
solutions (V, 13). While the simple solution of MgSO, killed the 
organism in two or three days, a 0.4 or 0.1 per cent solution of 
MgSO, plus 0.1 per cent of CaCl, (V, 14), as well as 0.1 per cent 
solution of MgSO, plus 0.1 per cent of KCI (V, 15), allowed the 
alga to live fourteen days. In these experiments the addition of 
potassium was just as efficient in counteracting the toxicity of 
magnesium as was the addition of calcium. 
To determine whether the effect of CaCO; in counteracting the 
toxicity of MgSO, is to be considered as due solely to the addition 
of calcium to the solution or partially to other causes, the following 
experiments were carried out. A saturated solution of CaCO; was 
prepared and to this was added 0.4 per cent of MgSQ,. At the 
same time, to a 0.4 per cent solution of MgSO, was added an 
excess of finely divided CaCO . In the former solution, the alga 
was markedly injured within nineteen hours, and was totally 
dead at the end of fourteen days (V, 16). In the solution contain- 
