the Broad Bean and certain other Plants. 669 
they are found to act rather as catalysts in certain normal processes of 
metabolism ’ ( 24 ). 
But it must here be emphasized that no suggestion is made that boron 
is in any way analogous to a vitamine, but rather that their respective effects 
afford a striking parallel. 
Summary. 
1. In water culture a continual supply of boric acid appears to be 
essential to the healthy growth of the broad bean plant, concentrations of 
1 :12,500,000-1 : 25,000 H3BO3 being beneficial. 
In its absence, death occurs in a characteristic manner, and the apex of 
the shoot becomes withered and blackened, though the addition of boric 
acid after these symptoms have set in results in a renewal of growth by 
means of new lateral shoots and roots. This type of dying never occurs 
in broad bean plants grown in pot culture, and it is concluded that suffi¬ 
cient boron is present, as a trace has been detected in the soils used. 
2. The absence of boron does not cause death in barley, growth being 
healthy in ordinary culture solution. 
3. Excess of boric acid is poisonous to the broad bean, injury being 
apparent with 1 : 5 )°°° H 3 B 0 3 in water culture and with 0-5 grm. or 
over per 22-J lb. of soil in pot culture, according to the method of appli¬ 
cation. Smaller quantities added to the soil are either without effect or 
cause an increase in the green weight only. 
4. Boric acid is more poisonous to barley than to the broad bean ; in 
water culture a concentration of 1 : 2,500,000 H 3 BO s and in pot culture 
i-o grm. or 0-5 grm. per 22^ lb of soil is injurious, according to the method 
of application. Smaller quantities are either ineffective or slightly favourable, 
though the benefit is usually evident to the eye only and not shown in the 
dry weight. 
5. Injury is marked by ( 1 ) retardation of germination ; ( 2 ) first chlorosis 
and later brown markings of the leaves; the barley leaf becomes spotted, 
but that of the broad bean shows a band of brown along the margins ; ( 3 ) 
retardation in maturing in the case of barley in soil culture. 
6. Preliminary experiments show that several other plants, and 
especiall y Phascolus nmltijlomis and Trifolium incarnatum, appear to derive 
benefit from the addition of small quantities of boric acid to the nutrient 
solution, though rye behaves similarly to barley, and is apparently indifferent 
to low concentrations. 
7. Boron is found to be present in considerable quantity in the dried 
shoots of broad bean plants grown in a nutrient solution containing no 
boron, and also in the seed. In garden-grown plants h larger proportion of 
boron was present in the pods than in either the stems or leaves. A trace 
