514 Reed . — The Value of Certain Ntitritive Elements 
algae, moss protonema, and fern prothalli. Ammonium nitrate appears to 
be a very suitable form in which to supply a source of nitrogen. The plant 
probably derives benefit sooner or later from the reduced form of nitrogen 
as ammonium, as well as from the oxidized form as nitrate. There are 
undoubtedly different processes of metabolism for which each is better 
suited. This solution does not tend to become more unsuited for algae, 
the longer they grow in it. The presence of potassium dihydrogen 
phosphate gives the solution a faintly acid reaction. If the potassium 
phosphate has passed entirely into solution before the calcium chloride 
is added, there is practically no precipitation of calcium phosphate until 
heat is applied in the process of sterilization. This precipitate did not 
prove troublesome, however, because the solutions were measured out 
before being sterilized, and errors in measurement due to the presence 
of a solid substance were avoided. The calcium is furnished as calcium 
chloride in a much smaller quantity than in the solution of Knop. This 
smaller amount must be regarded as more rational if we regard the needs 
of the plant. As the calcium radical is used more rapidly than that of 
chlorine, the solution tends to become slightly more acid, and the conditions 
are correspondingly better for algae. 
The nutrient solution which was used in the cultivation of phanerogams 
was one given by Pfeffer (Physiology of Plants, English trans., vol. i, 
p. 420):— 
Calcium nitrate . . . . . . . 4 g. 
Potassium nitrate . . . . . . 1 g. 
Magnesium sulphate . . . . . . ig. 
Potassium dihydrogen phosphate . . . ig. 
Potassium chloride 0-5 g. 
Ferric chloride trace. 
Distilled water . . . . . . 3 or 7 litres. 
This solution gave good results with phanerogams, which are apparently 
favoured when the reaction of the solution is slightly alkaline. It is some- 
what more concentrated than necessary, especially when only three liters of 
water are used. At that dilution the solution would have a concentration 
of 2,500 parts per million (0*25 per cent.) ; when seven liters of water are 
used the solution has a concentration of 1,070 parts per million (0-107 per 
cent.). This solution is somewhat more concentrated than necessary for 
the growth of higher plants. Indeed, the work of Breazeale (’ 05 ) indicates 
that the optimum concentration of salts for wheat plants in water culture 
is approximately 300 parts per million. 
The volume of the solution employed in cultures was always made 
large. The advantages of a large volume are numerous ; the concentration 
of the solution is altered more slowly as the plants remove the nutrients ; 
