Zinc , Arsenic , < 2 ;^ Boron on the Growth of Plants . 297 
observations. 1/100,000 boric acid seems to be a definitely critical strength 
for peas, generally indicating the point of cessation of the toxic influence and 
the setting in of the stimulative action, although rarely it introduces an 
indifferent range of concentrations below the stimulating strength. 
The general outcome of the tests seems to be that boric acid needs 
to be supplied in relatively great quantities to be fatal to pea plants, and 
that the toxic action gives place to a stimulative influence high up in the scale 
of concentration. As far as experiments have already gone, it seems that the 
stimulation is not a progressive one, as the effect of 3/100,000 boric acid 
is as good as that of 1/20,000,000 boric acid, a flat curve connecting the two. 
Lupins. The tests with white lupins gave no conclusive results, as for 
some reason it proved very difficult to get satisfactory plants in water 
cultures. When they are grown under such conditions the roots always 
tend to get more or less diseased and covered with a thick hanging slime, 
probably fungal in nature. In the presence of boric acid the roots remain 
in a much healthier condition, the slime being conspicuous by its absence, 
which suggests that the acid has in this case a strong antiseptic action which 
protects the roots. With high concentrations the lower leaves of the plants 
are badly affected, just as with peas and barley, turning brown and 
withering at an early date. 
Various experiments have been made with yellow lupins, but these 
again are very difficult to grow in water cultures, as they are apt to drop 
their leaves for no apparent reason. Generally speaking, the evidence goes 
to prove that boric acid is toxic down to a concentration of about 1/50,000, 
and that below that strength stimulative action sets in. It is difficult to get 
a true control with which to make comparisons, as the plants without boric 
acid are encumbered with the slime on their roots, which naturally interferes 
with normal growth, while the plants in the presence of boric acid have the 
advantage due to the antiseptic action of the boron. The effect of the 
boron poisoning is again evident in the dying off of the lower leaves, which 
become drooping and flaccid, and finally drop off. The lupins grown with 
boron are very active in the putting forth of lateral roots, so much so that 
the cortex of the roots is split along the line of emergence of the laterals, 
which are very numerous and crowded. 
Discussion of Results. 
It is quite usual to find that in different series of plants grown under 
similar conditions, the degree of toxicity of a particular strength of poison 
varies within certain limits. This may probably be accounted for by 
difference in the variety of plant grown, and also by difference in season. 
There is no ground for assuming that each variety of a given species will 
react in precisely the same way to an external factor, and consequently such 
variation may reasonably be expected. Seasonal variation is of two kinds : 
