the Broad Bean and certain other Plants. 
655 
Since small quantities of boron compounds are common constituents 
of the soil, and traces have been detected in the soils used in this pot-culture 
work, it is probable that the plant is able to absorb sufficient of the element 
from the untreated soil to make normal development: if this were not so, 
it would be difficult to account for the success with which broad beans are 
usually cultivated in garden soil, 
though it is still an open question \ 
whether growth might not be im- $ 
proved on occasions by the addition 
of further small quantities of boron 1 
compounds. 1 
(b) Boric acid applied as a top- 1 
dressing. Applied as a top-dressing, i 
boric acid was more toxic than } 
when the same quantity was mixed 
throughout the soil, and this is 
borne out by the green and dry 
weight figures (Table VI). The 
application was made twenty-four 
days after sowing. All the plants 
died when treated with 4 grm. or 
2 grm. H3BO3 per pot, and even 
1 grm. proved very injurious, only 
a few plants surviving for a short 
time. 
The exceptional drought con¬ 
ditions of 1921, in spite of regular 
watering of the pots, may have 
caused this result to be unduly 
accentuated, but it is to be expected 
that the boric acid would have -Boric Acid gms 
a more marked effect on the plant Text-fig. 5. Broad benns grown in pot culture 
with various quantities of boric acid mixed throughout 
when concentrated in the upper soil the soil. Mar. 4-May 31,1921 (average of five pots): 
layers where the young roots were 7 Weight; percent. Nitrogen;. 
developing. 0-5 grm. H.,BO a per 
pot was also decidedly injurious, the plants showing the typical leaf 
poisoning; but o-i grm. or 0-02 grm. caused no toxic effect, the plants in 
the former set being even slightly taller than the controls, while in both 
cases a small increase over the untreated plants was obtained in the green 
and dry weights (Table VI). 
/ 3 . Comparison between the Effect of Boric Acid and Borax in the Soil. 
In this series Na 2 B 4 0 7 + o-io H 2 0 was tested simultaneously with 
X x 
