on Plant Qrowth in Nutrient Solutions , &c. 359 
means of the colorimetric method, using the indicators recommended by 
Clark and Lubs ( 2 ), the double-tube colour standards of Gillespie (10), and 
the apparatus devised by Van Alstine ( 42 ), and these concentrations weie 
recorded in terms of pH values. 
The plants were grown in the culture solutions during a period of five 
or six weeks. At the end of the growth periods of the different experiments 
the dry weights of the tops and roots were separately obtained by the usual 
method. 
The non-solution environment was made uniform for all the cultures by 
employing the rotating table ( 36 ). Daily records were kept, in so far as this 
was possible, of the measurements characterizing the aerial conditions. 
Experimental Results. 
The detailed results of the first two experiments are purposely omitted, 
as their inclusion would only be indicative of the more conclusive data that 
are here presented. The first of these experiments consisted ingrowing the 
plants for a five-week period in the various solutions of Table I, with iron 
supplied in the form of an aqueous suspension of ferric phosphate in the 
amount of 0-83 milligram of iron per litre of nutrient solution. In general 
appearance the plants of many of the cultures of the Tottingham series 
(series A) soon showed the chlorotic condition which is characteristic of 
plants suffering from lack of iron. The plants in the series containing 
ammonium sulphate (series B) did not present a chlorotic appearance. 
However, a yellow mottling was present later in both series, obscuring, 
if present, the chlorotic symptoms of lack of iron. The dry weights of the 
series containing ammonium sulphate were slightly superior to those of the 
Tottingham series, and the fact that the solutions containing ammonium 
sulphate grew more acid in contact with plant roots, while the Tottingham 
solutions grew less acid, suggests that this slight advantage of the series 
containing ammonium sulphate is due to the greater availability of the iron 
from the insoluble ferric phosphate in these solutions which have a tendency 
to become more acid. 
In the second experiment the same amount of iron was supplied in the 
form of ferrous sulphate in a fresh aqueous solution. There was no chlorosis 
apparent in either series. About two weeks after the experiment was 
started many of the plants in both series exhibited a peculiar brown speck¬ 
ing, especially on the unfolding new leaves. Later on this was succeeded by 
a general mottling of the leaves. That this was due to the ferrous sulphate 
was experimentally determined by setting up a supplementary experiment 
in which both ferrous sulphate and ferric phosphate were used as sources of 
iron. The specking and mottling persisted in the ferrous sulphate cultures, 
but were absent in those cultures receiving iron in the form of ferric 
phosphate. 
