the Broad Bean and certain other Plants. 
667 
increase in the percentage of nitrogen, though the actual nitrogen absorbed 
was either unaffected or considerably depressed. Cook and Wilson (17 ( b )) 
in field experiments found a higher percentage of N both in the straw and 
grain of wheat treated with boron than from the control plots ; in fact it is 
a common experience that the presence of some injurious factor is accom¬ 
panied by a rise in the percentage of nitrogen found in the plant. However, 
since the actual nitrogen absorbed is not increased by the presence of toxic 
agents, the rise in the percentage of nitrogen must be attributed mainly to 
a reduction in the output of dry matter of which the chief constituents are 
the carbohydrates, and not solely to the direct influence of the injurious 
factor on the intake of nitrogen. In the case of barley grown in water 
culture most of the concentrations of H 3 B 0 3 were toxic and the nitrogen 
percentage gradually rose as the quantity of boron was increased (Text- 
fig. 4). With the broad bean the majority of the concentrations were 
beneficial and the percentage of nitrogen remained approximately constant, 
irrespective of the strength of boric acid supplied, but where the dose 
became toxic, i. e. 1 : 5,000, a rise was obtained as in the case of barley 
(Text-fig. 2). 
However, the unhealthy condition due to a deficiency of boron, which 
was seen only with the broad bean in water culture, apparently had no effect 
on the percentage of nitrogen, although the actual amount was consider¬ 
ably lower than in the well-grown plants, in some cases falling even below 
that of the poisoned sets. 
The right interpretation of the term ‘ stimulation ’ is still an open 
question, for it is equally difficult to define ‘ normal’ growth. Peas in ordinary 
nutrient solution, for instance, apparently develop quite normally, but when 
the addition of a small quantity of boron causes a still better growth, the 
question arises as to whether the so-called ‘ normal ’ development was not 
really abnormal owing to the omission of some necessary element. And 
also, it is equally uncertain whether the plant might not be further improved 
by the addition of some other substance as yet undetected. 
With broad beans the case is perhaps simpler, as it is clear that 
growth in the so-called c complete ’ nutrient solution is abnormal; but here, 
again, it has not yet been shown whether other substances might not be 
able either to supply the same need or to still further benefit the plant. 
The possibility of boric acid acting merely as an antiseptic agent has 
been shown by Agulhon (1 (a)) to be quite unlikely, and in support of this, 
in the present experiments, moulds have occasionally developed on the 
roots of plants grown with toxic concentration of H 3 B 0 3 . The acidifying 
action he also considered negligible, and this is confirmed by the fact that 
1 :5,000 boric acid is less acid than the normal nutrient solution used in 
this water-culture work, as determined by pH values. After analysing 
a large number of plants and plant organs for the presence of boron, 
