EFFECT OF BORON COMPOUNDS
I. PRESENCE OF BORON IN PLANTS.
The first claim to the discovery of boron in plants was put forward
in 1857 by Wittstein and Apoiger, who carried out investigations
on the Abyssinian Saoria (seeds of _Maasa_ or _Maessa picta_, N.O.
Primulaceae[12]). In the course of analyses a crystalline mass was
obtained which was found to contain chlorine, phosphoric acid, lime, and
boric acid. The discovery apparently attracted little attention and for
about another thirty years the matter was again allowed to sink into
oblivion. Then it came to the front again, and from 1888 onwards one
investigator after another demonstrated the presence of boron in various
plants.
[12] According to Engler’s classification this plant belongs to N.O.
_Myrsinaceae_.
In 1888 Baumert detected boron in French, German, and Spanish wines
without exception, while E. O. von Lippman (1888) demonstrated it in sugar
must and also in the leaves and root of the sugar beet. In the latter case
the reactions were so definite that the presence of more than a minimal
amount of boric acid was conjectured.
Crampton (1889) tested various fruits, but while he found boron in every
part of the watermelon, he could get no reaction with apples or with
certain samples of sugar cane. He predicted, however, that the occurrence
of boron would prove to be more general in the plant kingdom than had
previously been supposed. The next year (1890) Hotter extended the work
on fruits, testing for boron in the fruits, leaves, and twigs of certain
plants, and finding it in the apple, pear, cherry, raspberry, fig, and
others. His results indicated that fruits are relatively rich in boron.
Later on (1895) Hotter carried his experiments further, and he stated
that stone fruits are richer in boric acid than are berries and pomes.
The accumulation of boron is in the fruit itself, the other parts of the
plant containing little. The quantities of boric acid found in the ash of
the various fruits ranged from ·58% in the “Autumn Reinette” apple to ·06%
in figs. Bechi had previously (1891) detected boron in the ash of figs,
love-apple, and rubus fruits from Pitecio, but he attributed this to the
presence of boric acid or borates in the soil at the place.
Passerini (1891) found traces of boron in the stems of chickpea
plants, while in 1892 Brand determined boric acid in the ash of beer.
In consequence of this various samples of hops were ashed without the
addition of any alkali, and then the ash was distilled with sulphuric
acid and methyl alcohol. When tested all the hops showed relatively large
quantities of boric acid in comparison with beer, hence he argued that
the boric acid in beer is derived from the hops. Boron was discovered in
various parts of the hop plant--in the clusters, leaves, pedicels, and
stems.
Jay (1895) analysed many plants and plant products grown in various soils
and waters, and arrived at the conclusion that boron is of practically
universal occurrence in the plant world. Of all vegetable liquids wines
are the richest in this constituent, the amount varying from ·009 gram to
·33 gram per litre. He confirmed Hotter’s statement as to the richness of
fruits in this substance, finding from 1·50–6·40 grams in 1 kgm. of ash.
Chrysanthemums and onions, amongst other plants, are well off in this
respect, containing 2·10–4·60 grams per kgm. of ash. Jay also found that
the plants vary in their capacity for absorbing boric acid, those which do
so the least easily being Gramineae (as wheat, barley, rice), mushrooms
and watercress, the quantity in these plants never exceeding ·500 grams
per kgm. of ash.
Of all the workers upon boron, Agulhon has done the most to extend and
concentrate our knowledge of the subject. He used the most refined,
up-to-date methods for the detection and estimation of boric acid,
and so determined its presence in many plants, including angiosperms,
gymnosperms, ferns, algae, and fungi. Tobacco is so rich in boron that it
can be detected in the ash of one cigarette. Among the plants tested, the
highest percentages of boric acid were found in _Betula alba_ (1·175% of
ash) and _Laminaria saccharina_ (·682% of ash), the lowest in _Cannabis
sativa_ (·123% of ash). Generally speaking annual plants and parts of
plants seem to have the least boron in the composition of their ashes. In
one and the same plant the durable parts like bark and wood are richer
than the leaves, even in evergreen trees. He indicated that plants seem
to have a great affinity for boron, as even when plants are grown on soils
in which the boron is practically indetectable they always seem to extract
an appreciable quantity of the element.
From the foregoing results it is evident that boron is very widespread in
the vegetable kingdom, entering into the composition of many plants in all
the great classes. A general impression obtains that its distribution is
universal, and that it will ultimately prove to enter into the composition
of practically every plant, as the scope of the analyses is widened
and as methods of detection are improved. On the other hand, Agulhon is
inclined to think that boron may be a “particular element,” characteristic
of certain groups of individuals or of life under certain conditions.
The series of individuals differ among themselves as to their particular
needs of nutriment (in the widest sense) and doubtless each group has
special need of particular elements, a need that is possibly correlated
with morphological and chemical differences. It may well be that boron is
one of these elements, associated with certain vital functions in a way
as yet unexplained, though it may possibly be found to play some part in
the formation of vascular tissues, since it is most abundant in bark and
lignified parts.
II. EFFECT OF BORON ON THE GROWTH OF HIGHER PLANTS.
1. _Toxic effect._
(_a_) _Toxic action of boron compounds in water cultures._
Excessive quantities of boric acid are decidedly poisonous to plants, the
action being well marked in water cultures. Knop (1884) found that free
boric acid was poisonous in neutral food solutions when present at the
rate of ·5 gram per litre, but he was not able to detect boron in the
ash of the roots of the experimental plants. Archangeli (1885) placed
seedlings of maize, white lupins, _Vicia sativa_ and _Triticum vulgare_
in solutions of boric acid varying in concentration from 1–·05%, with
controls in spring water. In the latter case the development was normal,
with 1% boric acid the plants were killed, while it was found that the
weaker the solution (within the indicated limits) the stronger the root
and shoot growth.
Hotter (1890) stated that it was known that 1/20,000 boric acid by weight
was harmful to soy beans in nutritive solutions. He experimented with
peas and maize, placing the seedlings first in distilled water, later
in nutritive solutions. When the peas were nineteen days old they were
transferred to nutritive solutions containing 1/1000–1/100,000 boric acid
by weight per litre, and within three days the plants with 1/1000 showed
signs of injury. Two days later all the plants showed signs of poisoning
in that, even with the weakest strengths, the lower leaves were flecked
with brown, especially at the edges, while with the greater strengths
the lower leaves were dead and the flecking had extended to the upper
leaves. In eleven days from the start the plants with 1/1000 boric acid
were completely dead, while the other plants showed more or less signs of
poisoning. The dry matter and ash decreased steadily with the increase
in the boric acid, while the boric acid per 100,000 parts of dry matter
increased steadily from 8 to 557 parts. Similar experiments were carried
on with potassium borate and with borax; the results showed that, weight
for weight, borax is less toxic than potassium borate, which in turn is
less toxic than boric acid, while at a strength of 1/100,000 there is
little to choose between the three poisons. Similar results were obtained
with maize; plants treated with boric acid or potassium borate yielded
about 2300 parts boric acid in 100,000 parts dry matter. The general
conclusion arrived at by Hotter was that the effect is not so much that
of a general poisoning as of a bleaching of parts of the leaf, mere
traces of boron being harmless. The cause of injury is local inhibition
of assimilation and killing of roots in stronger concentrations. Increase
of the strength of boron raises the toxicity until 1/1000 practically
inhibits increase in dry substance. The boron was found to be fairly
evenly distributed through sound and affected organs.
Kahlenberg and True (1896) worked with seedlings of _Lupinus albus_ L.,
limiting their experiments to those of 15–24 hours in duration. Various
combinations of boron and other substances were tested. With boric acid
alone 2/25 gram molecule per litre killed the plants, with 1/25 they
were apparently just alive, while 1/100 and less had no injurious effect.
Boromannitic acid was possibly more poisonous than the boric acid, while
a combination of boric acid and cane sugar proved slightly less toxic.
The short duration of these experiments limited their scope considerably,
as with certain concentrations the toxic action would not become evident
within the prescribed limits of time.
Agulhon (1910 a) worked with sterile nutrient solutions, and found that
the higher strengths of boric acid hindered growth, 200 mg. boric acid
per litre rendering growth impossible. He supported Hotter’s idea that
the toxic action affects the roots and the formation of chlorophyll, and
he stated that the plants are less green as the dose of boron increases,
plants growing in doses of above 10 mg. per litre being yellowish. In
other experiments he found that at 100 mg. boric acid per litre life seems
impossible for the plant. The roots seem to be more adversely affected by
toxic doses than do the shoots. In control plants Agulhon determined the
stem/root ratio as 6, with a little boron as 7, while the ratio rose to
13 as the dose of the poison increased to 50–100 mg. boron per litre.
The Rothamsted experiments show that boric acid is definitely poisonous
to barley down to a strength of 1/250,000 (Fig. 15), the depressing
effect frequently being evident at much smaller concentrations, while
peas can withstand far more of the poison, the limit of toxicity being
about 1/25–1/50 thousand (Fig. 16). With the greater strengths of poison
the lower leaves of both barley and peas are badly damaged. In barley
the leaves turn yellow with big brown spots, giving the leaves a curious,
mottled appearance, while with peas the poisoning seems to begin at the
tip and edge of the leaves, spreading inwards, without, however, showing
the large spots as in barley. So far as chemical tests go at present, it
is very probable that boron is deposited in the leaves in the same way
as manganese, and that this is the cause of the degeneration. As with
manganese, the lower leaves are attacked first, and the trouble spreads
upwards, one leaf after another being involved. These observations fit
in very well with those of Hotter, and the hypothesis of direct boron
poisoning gains support from the fact that in dilutions which produce
stimulation of the shoot the leaves show hardly any sign of dying off,
even after prolonged growth in the solutions. With barley the effects of
boron can be seen in the leaves in concentrations as low as 1/2,500,000,
and it may be significant that this is the point at which the depressant
action of boric acid entirely ceases in many cases.
[Illustration: Fig. 15. Curve showing the mean value of the dry weights
of ten series of barley plants grown in the presence of boric acid and
nutrient salts. (May 1st–June 20th, 1911.)]
[Illustration: Fig. 16. Photograph showing the action of boric acid on pea
plants in the presence of nutrient salts. (Sept. 30th–Dec. 20th, 1912.)
1. Control.
2. 1/5,000 boric acid.
3. 1/10,000 „ „
4. 1/25,000 „ „
5. 1/50,000 „ „
6. 1/100,000 „ „
7. 1/250,000 „ „
8. 1/500,000 „ „
9. 1/1,000,000 „ „ ]
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 slime, probably fungal in nature.
In the presence of much boric acid the roots remain in a much healthier
condition, which suggests that the acid has in this case a strong
antiseptic action, and protects the roots. With high concentrations
the lower leaves of the plant 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 well 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 500 parts in 25 million.
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 unfair advantage due to the probable
antiseptic action of the boron. The effect of the boron poisoning is again
evident in the dying off of the lower leaves, which become flaccid and
drooping 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.
(_b_) _Toxic action of boron compounds in sand cultures._
Agulhon (1910 a) moistened 2 kgm. pure sand with 500 c.c. nutritive
solution for each pot, and boron was added at the rate of 0, 0·1, 1, 10,
and 50 mg. boric acid per litre of nutritive solution. Twenty wheat seeds
were sown in each pot, and after twelve days the healthy plants in the
first four pots were 6–8 cm. high, but those with the maximum amount of
boron showed yellowish leaves only 3 cm. long. After three months’ growth
the plants were harvested, when those with most boron were found to have
died after making about 10 cm. growth. The toxic doses in sand proved to
be weaker than those in water cultures, probably because evaporation from
the surface of the sand caused concentration of the poisonous liquid.
(_c_) _Toxic action of boron compounds in soil experiments._
Long before any experimental work was done with boron in water cultures,
the poisonous properties of the substance were recognised with regard to
plants growing in soil. Peligot (1876) grew haricots in porous earthenware
pots, the plants being watered by rain and by solutions, each containing
about 2 grams per litre of such substances as borax, borate of potassium,
and boric acid, other pots receiving various fertilisers, as potassium
nitrate, sodium nitrate, &c. This quantity of boron completely killed
off the plants receiving it, whether it was applied as free or combined
boric acid, while the fertilised plants completed their development well.
On this account the deleterious action was attributed to the boric acid
and not to the sodium or potassium base supplied. Peligot hinted at the
improbability of a substance like boron, which is so poisonous to plants,
being really innocuous to human beings when it is used as a preservative
for foods.
Nakamura (1903) also found that borax is harmful in pot cultures if
present in large quantities, 50 mg. borax per kgm. of soil exerting a
very injurious influence, while even 10 mg. per kgm. did some damage.
Agulhon (1910 c) found that the toxic doses of boric acid in soil cultures
approached those in nutritive solutions rather than in sand cultures, a
phenomenon that he attributed to the fact that the boric acid was fixed
by the soil, probably as insoluble borate of calcium, so that the surface
concentration obtained with sand cultures was avoided. He found that the
ash of plants grown with excess of boron contained more than the normal
amount of boron, while the weight of ash per 100 dry matter was also
increased. He concluded that the plant thus suffers an over-mineralisation
and in consequence an augmentation of its hold on water, so that the fresh
weight of the plant may indicate a more favourable action of the boric
acid than does the dry weight. Other investigators (Fliche and Grandeau
1874) had found the same increase in the proportion of ash in chestnut
trees grown on too calcareous soil, so Agulhon concluded that one is
here dealing with a general reaction of plants to an excess of a useful
element.
Other experiments were carried on in the open field, maize being grown
on control plots and on plots receiving 2 gm. boron per square metre.
At first the latter plants were behind, the dose being too strong.
Eventually, however, they pulled up level and the dry weights from the two
plots proved to be nearly the same, the fresh weights being identical.
Maize is evidently far less sensitive to boron poisoning than are peas
and oats, for with these one-half the original amount of boron (= 1 gm.
per sq. metre) proved toxic.
Interesting results were obtained (Agulhon 1910 a) by repeated experiments
with the same soil containing boron. It was found that sand or soil
containing a proportion of boron which is lethal or toxic to a first
culture will allow much better growth with a second and subsequent
crops. Repeated experiments on the same soil may show the change from a
lethal dose to a toxic one, thence to an indifferent and finally to an
optimum concentration. Furthermore (Agulhon 1910 b) the very plants may
accustom themselves to greater quantities of boron, the increased power
of resistance being transmitted. He concluded from his experiments that
the progeny of the second generation of maize were able to withstand
quantities of boron that were toxic to control plants[13]. Agulhon once
again emphasised the fact that for toxic doses of boron the first symptom
is the more or less marked disappearance of chlorophyll, though the aerial
parts are not affected so soon as the roots.
[13] “Il apparaît donc que les graines fournies par des plantes
ayant crû en présence d’une quantité de bore élevée présentent une
accoutumance vis-à-vis de cet élément; les plants auxquels elles
donnent naissance semblent non seulement faire un meilleur emploi
des petites doses de bore qui leur sont offertes, mais encore
supportent les doses toxiques plus facilement que les plants
témoins, issus de graines non accoutumées.”
2. _Effect of boron compounds on germination._
One of the first indications that boron compounds affect the germination
of seeds was given by Heckel (1875) who found that germination was
retarded for 1–3 days by weak solutions of borates (·25 gm. to 20 gm.
water), and was stopped altogether by stronger solutions (·60 gm. to 20
gm. water). Archangeli (1885) tested the germination of a variety of seeds
of Leguminosae, Gramineae, and of Cannabis, Iberis, Raphanus, Collinsia,
and Linum in the presence of boric acid. The seeds were placed in bowls
with solutions of ·25, ·5, and 1% boric acid at temperatures ranging from
16°–23° C. The bowls were covered with glass plates to prevent evaporation
and consequent increase of concentration, controls in spring water being
dealt with under similar conditions. 1% boric acid was found to check
germination altogether, and the weaker the concentration the less was
the process hindered. Morel soaked seeds of haricots and wheat in various
solutions of boric acid, and found that germination was generally hindered
or inhibited. The deleterious action diminishes as the strength of the
solution or the time of contact diminishes, but solutions of the same
concentration do not act equally on all seeds. Boric acid and borax proved
to be similar in their action qualitatively.
The deleterious effect of strong doses of boric acid on germination was
confirmed by Agulhon (1910 a), the higher quantities (above 10 mg. boric
acid per litre) retarding germination of wheat.
3. _Does boron stimulate higher plants?_
Of recent years a few investigators have thrown out hints as to the
stimulant action exerted by boron compounds on plants. Roxas indicated
that M/100,000 (M = molecular weight) of boric acid exercised a favourable
action on rice. Nakamura (1903) tested the point by means of pot cultures.
Peas and spinach plants were grown in soil which received 1 and 5 mg.
borax per kgm. With peas the 1 mg. exerted evident stimulant action,
as determined by the increase in height of the shoot over that of the
control, 5 mg. seeming to be slightly depressant in action. With spinach
a stimulation was observed both in weight and height with a dose of 5 mg.
borax per kgm.
Average weight Average length of leaves
5 mg. borax 10·35 38·2
Control 7·2 34·0
Agulhon (1910 c and d) took the matter up still more definitely and made
many tests of various kinds, in water, sand and pot cultures.
(_a_) _Water cultures._
His water cultures were made under sterile conditions, the seeds when
possible being sterilised with corrosive sublimate, the germinating
apparatus being also sterilised. With wheat a stimulant action was
evident, maximum growth being obtained with between 2·5 and 10 mg. boric
acid per litre, though the dry weight increase did not quite keep pace
with that of the fresh weight, a fact to which previous reference has been
made. The chief improvement is in the root, the stem/root ratio falling to
5, as against 6 in the control series. Visual observation indicated that
the roots of plants receiving 5–10 mg. boric acid per litre are longer
than the others, though they are less rich in adventitious roots. The
increased dry weight due to boron may amount to as much as 30%.
(_b_) _Sand cultures._
Agulhon again observed stimulation in this case. 2 kgm. of sand were
moistened with 500 c.c. nutritive solution, varying quantities of boric
acid being added in addition. ·1 mg. boric acid per litre of N.S. (·05
mg. per pot) gave an increase of 25% fresh weight, and 7·5% dry weight.
The stimulating doses seem to be weaker than in the experiments with
liquid media, probably because the evaporation from the sand increases
the concentration of the boric acid at the surface. It was also noticed
that the increase of weight varied in experiments made at different times.
With oats the stimulating influence is greater than with wheat, showing
that some plants are more sensitive than others to the influence of boron.
With radish 1 mg. boric acid per litre exercised a stimulating effect,
the enormous average increase of 61% in fresh weight occurring with this
strength, though this only represented an average increase of 9·6% dry
weight.
(_c_) _Soil cultures._
Here again the stimulating action was evident with higher concentrations
than in sand cultures, and Agulhon obtained good results with strengths
that are toxic in sand. The evaporation from earth is not so rapid as from
sand, so that the concentration is not increased, and also some of the
boric acid is withdrawn from the solution by interaction with the soil,
so that the stimulating concentration rises in the scale.
In field experiments Agulhon found that peas were more sensitive to the
toxic action of boric acid than is maize. A strength of boric acid (= 1
gm. B per sq. metre) that poisoned peas, gave an increase of 61% fresh
weight and 39% dry weight with maize; half the strength proved to be
indifferent for peas, the improvement with maize equalling 56% increase
fresh and 50% increase dry. Curiously enough, judging by appearances in
the first experiment, an unfavourable influence was at work, though in
reality a great stimulation was being caused. Colza gave a good increase
with similar strengths, but with turnips 1 gm. B per sq. metre only
favoured the aerial parts, while ·5 gm. B per sq. metre only increased
root development. Agulhon concluded that it is as yet impossible to
determine with any precision the exact part that boron plays in the plant
economy. He suggests that boron is a “particulier” element characteristic
of a certain group of individuals or of life under particular conditions.
In his summary he argues that each series of individuals adapted to
different environments has doubtless need of particular elements, and that
perhaps chemical causes and morphological differences are very closely
connected. Boron may be of this “particulier élément” type in the higher
plants of the vegetable kingdom, and it may be useful commercially as a
manurial agent, the “catalytic manure” of Bertrand and Agulhon.
While the higher concentrations of boric acid proved definitely toxic
to both peas and barley in the Rothamsted water cultures, some evidence
of stimulation was obtained with the lower strengths. With barley the
question of stimulation is still an open one, as below the toxic limit
growth seems fairly level in most of the experimental series. The lower
limit of toxicity varies from 40–4 parts boric acid per 10,000,000
according to circumstances. Below this critical concentration the boric
acid has apparently no action, either depressant or stimulant, unless the
stimulation should prove to begin at a dilution of 1/50,000,000, but the
evidence on this point is not sufficiently well marked or consistent to be
conclusive. This failure to detect stimulation was somewhat unexpected, as
when judged by the eye the plants treated with the lower concentrations
of boric acid seemed better than the controls, and also exhibited a
particularly healthy green colouration.
Peas on the other hand are definitely stimulated with traces of boric
acid, concentrations of 1/100,000 and less causing an improvement in
growth, while under some experimental conditions even higher amounts
of boric acid were beneficial. All the stimulated plants showed the
characteristic dark green colour which seems to be associated with
the presence of minute traces of boron in the nutritive solution. An
interesting morphological feature was the strong development of small side
shoots from the base of the plants in the presence of medium amounts of
boric acid, from 1 part in 100,000 downwards. This gave rise to a certain
bushiness of growth, which was less evident as the concentration of the
stimulant decreased. The general outcome of the tests seems to be that
boric acid needs to be supplied in relatively great strength to be fatal
to pea plants, and that the toxic action gives place to a stimulative one
high up in the scale of concentration. As far as experiments have already
gone it seems as though the stimulation is not a progressive one, as the
effect of 1/100,000 boric acid is as good as that of 1/20,000,000, a flat
curve connecting the two. This, however, needs confirmation.
Yellow lupins also give some evidence of stimulation with concentrations
of about 1/50,000 boric acid, the improvement being far more strongly
marked in some sets of experiments than in others.
III. EFFECT OF BORON COMPOUNDS ON CERTAIN OF THE LOWER PLANTS.
Our knowledge of the action of boron on the lower plants is less definite
and complete than with regard to the higher plants. Morel (1892) found
that boric acid acts as a strong poison to the lower fungi and similar
organisms, their development being completely arrested by very weak
solutions of the acid. He suggested, on this account, that boric acid
might be used in the same way as copper to attack such diseases as mildew,
anthracnose, &c., which attack useful plants.
On the other hand Loew (1892) stated that such algae as _Spirogyra_
and _Vaucheria_ showed no harmful influence for many weeks when the
culture water contained as much as ·2% (= 1/500) boric acid. This may
be supplemented by a recent observation at Rothamsted, in which certain
unicellular green algae (unidentified), were found growing at the bottom
of a stoppered bottle containing a stock solution of 1/100 boric acid.
Agulhon (1910 a) dealt chiefly with yeasts and certain ferments, and found
that yeasts grown in culture solutions are not influenced favourably or
unfavourably by relatively large quantities of boric acid up to 1 gram
per litre, while all development is checked with 10 grams per litre. The
presence of boron affects the action of yeast on glucose and galactose.
Galactose alone is not attacked even after 40 days in the presence of ·66%
boric acid. When glucose is mixed with the galactose the latter is said
to be at first left untouched, but later it disappears very slowly.
Boric acid exercises an antiseptic action on lactic ferments, 5 gm.
per litre checking their action sufficiently to enable milk to remain
uncoagulated. Lactic acid is still produced even with as much boric acid
as 10 gm. per litre. The microbe is not actually killed by the boric acid,
but its development is so arrested that reproduction cannot take place.
The same phenomenon was observed with yeast. With moulds again, while no
stimulation could be obtained with small quantities of boric acid, yet
the toxic action does not begin to set in until 5 gm. boric acid per litre
are present.
Thus it appears that such lower organisms as yeast, lactic ferment and
_Aspergillus niger_ are remarkably indifferent to the action of boric
acid, as is shown by the fact that the toxic dose is remarkably high,
while stimulation effects cannot be observed even in the presence of the
smallest quantities yet tried.
_Conclusion._
Boric acid is less harmful to the growth of higher plants than are the
compounds of copper, zinc, and arsenic. Evidence exists that below a
certain limit of concentration boron exercises a favourable influence upon
plant growth, encouraging the formation of stronger roots and shoots. This
stimulation is more strongly marked with some species than with others,
peas responding more readily than barley to the action of boric acid.
Fungi are very indifferent to boron, whether it is present in large or
small quantities, and there is evidence to show that certain of the green
algae can also withstand large quantities of it.