on Plant Growth in Nutrient Solutions , cfc. 361 
ferric phosphate and ferrous sulphate, on the plants grown in the two types 
of solutions, varying amounts of iron in the two forms were used in an 
experiment comprising two series of twenty cultures each and two controls. 
Throughout each series the same solution was used for all the cultures, the 
cultures of each series differing only in the amounts of iron added. To 
represent one type Tottingham’s solution TjRjCg, as given in Table I, was 
chosen, and, as a representative of the other type, this solution was modified 
by substituting ammonium sulphate for the potassium nitrate as previously 
explained (solution T 1 R 1 C 5 of the ammonium sulphate series, Table I). The 
series in which the Tottingham solution was used will be designated 
series C, and that in which the modified Tottingham solution was used will 
be designated series D. Ferric phosphate was supplied to half the cultures 
in each series in amounts varying from o-oi mg. to 5*° rag- of iron per 
litre of nutrient solution, and to the other half of the cultures in each series 
iron was supplied in corresponding amounts in the form of ferrous sulphate, 
but no iron was added to the controls. The culture methods pursued 
throughout were precisely the same as those previously described. The 
cultures were conducted during a growth period of thirty-five days. 
In the cultures of the Tottingham series C containing iron in the form 
of ferric phosphate a chlorotic condition appeared within five days after the 
experiment was started. In this series at harvest time those cultures 
receiving one milligram of iron or more had recovered. The cultures of the 
ammonium-sulphate series D receiving the same form of iron (ferric phos¬ 
phate) were all green and healthy except the one culture receiving the 
smallest amount of iron (o-oi mg.), which was slightly chlorotic. 
In the cultures supplied with ferrous sulphate, specking of the leaves 
became apparent within ten days, appearing in the Tottingham series C in 
cultures receiving 0*75 milligram of iron or more, and in the ammonium- 
sulphate series D in cultures receiving o-io milligram of iron or more. No 
specking was observed in cultures grown in solutions supplied with ferric 
phosphate. At the time of harvest the plants in the Tottingham solutions 
supplied with ferrous sulphate were healthy and in a vigorous condition, but 
in the solutions containing ammonium sulphate supplied with ferrous 
sulphate as the source of iron, the plants presented a very poor appearance, 
with yellow, dying, or dead leaves, and this sickly appearance increased 
in intensity with increase of iron above 0-25 milligram per litre of solution. 
At the end of the growth period of thirty-five days the dry weights of 
the tops and roots were obtained in the usual way. The yields of the 
cultures of the two series, together with the averages of the hydrogen-ion 
concentrations in terms of pH values of the solutions obtained at the end 
of the various growth intervals, are given in Table II. The yield values 
of tops and roots from the cultures supplied with ferric phosphate as the 
source of iron are represented graphically in Fig. 1, the upper set of graphs 
