EFFECT OF ZINC COMPOUNDS
I. PRESENCE OF ZINC IN PLANTS.
The presence of zinc in the ash of certain plants has been recognised for
many years, especially in so far as the vegetation of soils containing
much zinc is concerned. Risse, before 1865, stated that most plants when
grown on such soils prove to contain greater or less quantities of zinc
oxide. He states that the soil at Altenberg, near Aachen, is very rich
in zinc, which rises as high as 20% in places. The flora of the soil
is very diversified and zinc has been determined qualitatively in most
and quantitatively in some of the plants. _Viola tricolor_ and _Thlaspi
alpestre_ are most characteristic under such circumstances, both showing
such constant habit changes that they resemble new species, while other
plants such as _Armeria vulgaris_ and _Silene inflata_ are peculiarly
luxuriant. Risse’s figures of the zinc content of these four plants may
prove of interest. The figures are based on the dry weights, air dried.
_Thlaspi alpestre_, var. _calaminaria_.
Root 6·28% ash, 0·167% ZnO, 1·66% ZnO in ash.
Stem 11·75% „ 0·385% „ 3·28% „ „
Leaves 11·45% „ 1·50% „ 13·12% „ „
Flowers 8·49% „ 0·275% „ 3·24% „ „
_Viola tricolor._
Root 5·59% ash, 0·085% ZnO, 1·52% ZnO in ash.
Stem 10·55% „ 0·065% „ 0·62% „ „
Leaves 9·42% „ 0·110% „ 1·16% „ „
Flowers 7·66% „ 0·075% „ 0·98% „ „
_Armeria vulgaris._
Root 4·74% ash, 0·17% ZnO, 3·58% ZnO in ash.
Stem 5·37% „ 0·02% „ 0·37% „ „
Leaves 9·36% „ 0·11% „ 1·17% „ „
Flowers 6·08% „ 0·07% „ 1·15% „ „
_Silene inflata._
Root 2·71% ash, 0·02% ZnO, 0·74% ZnO in ash.
Stem }
Leaves } 11·43% „ 0·22% „ 1·92% „ „
Flowers }
Freytag (1868) carried out various experiments on the influence of
zinc oxide and its compounds on vegetation, and found that all plants
are capable of absorbing zinc oxide by their roots when grown on soils
containing such oxide. Generally speaking the zinc is deposited chiefly in
the leaves and stems, very little being found in the seeds, such minute
traces occurring that he stated that the seeds must be harmless for men
and animals. The general content of ZnO in plants is given as ·5–1·0% of
ash, except in the abnormal case of plants growing on calamine.
Lechartier and Bellamy (1877) demonstrated the presence of zinc in such
food substances as wheat, American maize, barley and white haricots, but
they failed to find it in maize stems and beetroot, so they cautiously
concluded that if it does occur in the latter cases it must be far less
in quantity than in the former. Hattensaur (1891) analysed the ash of
_Molinia cærulea_ and discovered the presence of copper, manganese, zinc
and lead, zinc oxide forming ·265% of the total ash, (·006% of the air
dried plant).
Jensch (1894) observed that the flora on calamine soils was somewhat
scanty, the chief plants that came under his notice being _Taraxacum
officinale_, _Capsella Bursa-pastoris_, _Plantago lanceolata_, _Tussilago
Farfara_, and _Polygonum aviculare_, all of which showed certain
morphological peculiarities. Generally speaking the growth of these plants
on the calamine soils was weak and poor, the stems and leaves being very
brittle. Jensch found that the roots were deformed and showed a tendency
towards a plate-like superficial spread of root. The leaves of _Tussilago_
were uneven in shape and lacked the white hairs on the under side, the
flower stalks were twisted, while the flowers themselves were a deep
saturated yellow colour. The stems of _Polygonum aviculare_ were much
thickened at the nodes, the leaves weak and rolled in character, while
the flowers were long-stalked, the calyces being usually of a purple
red colour. The following figures are given for the quantities of zinc
carbonate (ZnCO₃) in the ash of these two plants:--
_Tussilago Farfara._
Root Leaf-stalk Leaf-blade
2·51%–3·26% 1·75%–1·63% 2·90%–2·83% ZnCO₃
= 1·629%–2·115% 1·136%–1·058% 1·882%–1·836% ZnO.
_Polygonum aviculare._
Root Stem Leaves
1·77%–1·93% 2·25%–2·86% 1·24%–1·49% ZnCO₃
= 1·148%–1·252% 1·46%–1·856% ·804%–·967% ZnO.
Other analyses of plants from zinc soils as against controls from normal
soils indicated the high water and high ash content of the zinc plants,
though the dry matter was low, and it is suggested that the increase of
the ash may be connected with a stimulation caused by the zinc salts,
unless it is due to phosphoric-acid hunger, since the calamine soils
concerned are very deficient in phosphorus.
Javillier (1908 c) corroborated the early statements of Risse as to the
presence of considerable quantities of zinc in certain species of _Viola_,
_Thlaspi_ and _Armeria_, and also he cited a list of other plants in
which zinc occurs in some quantity. Javillier, however, is of opinion
that zinc oxide, like the oxides of iron and manganese, is very common in
plant ash, being present in all plant organs. Zinc is specially abundant
in Coniferae, where it is probably characteristic, as is the presence
of manganese in the ash and manno-cellulose in the wood. The so-called
“calamine” plants show great powers of accommodation to large amounts of
zinc.
Klopsch (1908) analysed 17 species of plants grown on soil in the
vicinity of zinc works, and showed that the plants evidently absorb small
quantities of zinc from their surroundings. He also regarded zinc as a
normal constituent of certain plants.
II. EFFECT OF ZINC ON THE GROWTH OF HIGHER PLANTS.
1. _Toxic effect._
(_a_) _Toxic action of zinc salts alone in water cultures._
In comparison with copper little work has been done with regard to the
action of soluble zinc salts alone on higher plants when grown in water
cultures. Freytag (1868) stated that zinc salts must be very dilute if the
plants are not to be harmed, and that for zinc sulphate the concentrations
must not be more than 200 mg. per litre (= 1/5000). Baumann (1885) carried
out further experiments and concluded that zinc salts are far more toxic
than Freytag suspected, 44 mg. zinc sulphate per litre[5] killing plants
of 13 species belonging to 7 families (Coniferae excepted). The various
plants withstand the action of the zinc salts in different degrees, the
same concentration killing off the species in different times. With the
44 mg. zinc sulphate the following results were obtained:--
Trifolium pratense killed in 16 days
Spergula arvensis „ 21 „
Hordeum vulgare „ 30 „
Vicia sativa „ 31 „
Polygonum Fagopyrum „ 60 „
Beta vulgaris „ 76 „
Onobrychis sativa „ 194 „
[5] 44 mg. ZnSO₄.7H₂O = 10 mg. Zn = 1/22,727 ZnSO₄.7H₂O approx.
With still less poison, 22 mg. zinc sulphate per litre, all the species
mentioned were eventually killed with the exception of _Onobrychis
sativa_, while 4·4 mg. zinc sulphate seemed to be harmless for all the
plants tested except _Raphanus sativus_, which is evidently exceptionally
sensitive to this toxic substance.
Jensen (1907) again indicated the poisonous action of zinc salts and also
found that a relatively small reduction of toxicity was obtained by the
addition of finely divided quartz to the solutions.
(_b_) _Effect of soluble zinc salts in the presence of nutrients._
Krauch (1882) grew various plants in the presence of nutrient solutions
and quantities of zinc sulphate varying from ·1 to ·8 gm. per litre
(= 1/10,000 to 8/10,000). Barley proved to be very sensitive, even to
the weakest strength of the poison, as the plants soon showed reddish
flecks, while all were dead within six weeks, the control plants without
zinc remaining quite healthy. Certain grasses took longer to kill than
barley, those with ·4 gm. zinc sulphate per litre dying in about seven
weeks, while 13 weeks elapsed before the others were killed. Even after
this length of time the plants with ·1 gm. zinc sulphate per litre still
survived, although in a very sickly condition. With willow, again, even
·1 gm. zinc sulphate per litre made the plants very sickly after four
weeks, growth being weak, the leaves yellow, and the roots brownish. In
this case the solutions were renewed, but the plants treated with zinc
compounds were dead within eight weeks from the start, the controls being
very healthy.
The next year (1883) Storp repeated these experiments made by Krauch and
corroborated his results fully. Barley and grasses (timothy and others)
grown in solutions of zinc sulphate, both with and without nutrients, soon
lost their green colour and became covered with rusty brown flecks, the
barley dying within 14 days, and the grasses soon after. With willow, too,
the toxic action was again manifested.
True and Gies (1903) showed that the addition of calcium salts
in appropriate concentrations reduced the toxicity of zinc salts
considerably, a result similar to that which they obtained for copper.
Recent experiments at Rothamsted have shown that zinc sulphate is very
toxic to barley, though the plant is able to make some slight amount of
growth even in the presence of a solution of the anhydrous salt ZnSO₄
as strong as 1/5000, rapid improvement occurring as the concentration
decreases to 1/2,500,000 or less (Fig. 6). On the whole the higher
strengths of zinc sulphate are less poisonous to peas than they are
to barley. At a concentration of 1 in ¼ or 1 in ½ million in different
experiments the growth was nearly as good as with the control plants,
though it consistently lagged a little way behind until a dilution of
1/10,000,000 was reached (Figs. 7 and 8). Incidentally it is very striking
to see the desperate efforts that badly poisoned pea plants make to
reproduce themselves. Growth of the roots is nearly always checked in
advance of that of the shoots, probably on account of the contact of the
roots with the poison. In the greater strengths of such poisons as zinc
and copper sulphate root growth is checked from the outset, but usually
a very little shoot growth is made, and one frequently obtains ridiculous
little plants about an inch high bearing unhappy and diminutive flowers,
which are occasionally replaced by equally unhappy and miniature fruits.
The same thing has also been noticed when unsuccessful attempts have been
made to introduce spinach as a test plant for water cultures.
[Illustration: Fig. 6. Curve showing the mean value of the dry weights
of ten series of barley plants grown in the presence of anhydrous zinc
sulphate and nutrient salts. (March 2nd–May 8th, 1911.)]
[Illustration: Fig. 7. Photograph showing the action of anhydrous zinc
sulphate on pea plants in the presence of nutrient salts. (Sept. 30th–Dec.
20th, 1912.)
1. Control.
2. 1/5,000 zinc sulphate.
3. 1/10,000 „ „
4. 1/50,000 „ „
5. 1/100,000 „ „
6. 1/250,000 „ „
7. 1/500,000 „ „
8. 1/1,000,000 „ „
9. 1/2,500,000 „ „ ]
[Illustration: Fig. 8. Curve showing the mean values of the dry weights of
nine series of pea plants grown in the presence of anhydrous zinc sulphate
and nutrient salts. (May 18th–June 28th, 1910.)]
(_c_) _Effect of zinc compounds on plant growth when they are present in
soils._
As soon as the presence of zinc in members of the vegetable kingdom was
established the question arose as to its effect upon both the plant and
the soil.
Gorup-Besanez (1863) grew plants in soil with which 30 grams of metallic
poisons such as CuSO₄, ZnSO₄, HgO, were intimately mixed with 30·7 litres
(“cubik Decimeter”) of soil[6]. On analysing the ash of _Secale cereale_,
_Polygonum Fagopyrum_, and _Pisum sativum_ after six months growth he
failed to detect the presence of zinc in any one of the three. As the
results varied with different poisons on different plants he concluded
that the absorption capacity of the various kinds of soils for different
poisons varies, that basic salts are absorbed, while the acid salts may
pass completely through the soil in the drainage water.
[6] This is equivalent to about ·1% of poison.
Freytag (1868) stated that zinc is retained by the soil in the form of
oxide, which is derived from dilute zinc compounds as they filter through
the soil, by decomposition by the salts of the soil. For field earth the
limit of absorption of zinc oxide from zinc sulphate is between ·21%–·24%
of the earth.
F. C. Phillips (1882) corroborated Freytag’s statement as to the
absorption of small quantities of zinc by the roots of plants, but he
states as a fact that both lead and zinc may enter plant tissues without
causing any disturbance in the growth, nutrition or functions of the
plants, a conclusion that is obviously incorrect or at least incomplete
in view of later work on the subject. His choice of plants was certainly
unusual, including geraniums, coleas, ageratums and pansies, the poison
used being zinc carbonate.
Holdefleiss (1883) stated that in spite of a soil content of 2% zinc
the vegetation was not in any way harmed, clover fields and meadow lands
on zinc soil presenting a normal appearance. This observation was quite
inconclusive, as the author proceeds to say that of the plants that were
able to absorb zinc salts without disadvantage the most luxuriant were
the so-called zinc plants--the exceptions that prove the rule. Two years
later Baumann showed that such insoluble zinc salts as the carbonate
and sulphide in the soil cannot hurt plants. These salts are certainly
dissolved to some extent by water containing CO₂ but solution is hindered
by the constitution of the soil. He also found that the various kinds of
soil act differently upon zinc solutions, the absorptive power of pure
humus soils (“reinem Humusboden”) for zinc solutions being the strongest.
Clay and chalk soils also decompose such solutions energetically, while
poor sandy soils have only a weak power of absorption. This selectivity
of absorption may account for the difference in the toxicity of zinc salts
to plants in the various soils.
Storp (1883) experimented to determine the changes in the various
characters of the soil by the action of zinc salts on it, and he makes the
remarkable statement that in some soils the presence of zinc generates
free sulphuric acid, which is particularly injurious to plant life.
Grasses, young oaks and figs showed a decrease in dry weight, nitrogen
and fat, as the quantity of zinc compounds increased in the water added
to the soil. Both the quality and the quantity of the crop were adversely
affected. This decrease in the dry weight due to the presence of zinc was
confirmed by Jensch later on, and also by Nobbe, Baessler and Will (1884),
who state that both lead and zinc compounds work disadvantageously to
vegetation even when they are present in such small quantities that the
plants are outwardly sound, the harmful action appearing in the decrease
of dry weight. Contrary to Baumann’s opinion, zinc carbonate is said
to be one of the salts that exercises this insidious poisonous action.
Storp (1883) noticed that the direct poisonous action of zinc compounds
is largely destroyed by their admixture with soil, but he suggests that
a secondary cause of harm is introduced by the accumulation of insoluble
zinc salts, so that the fertility of the soil is impaired to the detriment
of the vegetation.
Ehrenberg (1908) throws out a suggestion that zinc is specially harmful
to plant life when it occurs in conjunction with ammonia, but no further
evidence has come to light.
(_d_) _Mode of action of zinc on plants._
The reason for the toxicity of zinc salts when present in soil forced
itself upon the attention of some of the early investigators in this
field. Freytag (1868) put forward the hypothesis that the zinc oxide is
partly or exclusively absorbed by the roots on account of the cell walls
of the root being corroded by the very thin layer of zinc salts lying in
contact with it--the same theory as has been held with regard to copper.
He stated also that the quantity of zinc oxide taken up by the plant
through its roots is strictly limited, not being proportional to the
quantity occurring in the soil, but varying between narrow limits. Krauch
(1882) found himself unable to accept another hypothesis which at one time
found favour, i.e. that the zinc salts kill the plants by coagulating the
protoplasm. If this were so, he argued, no plants at all could grow upon
soils containing zinc, and he was content to leave the cause as one yet to
be explained. Even at the present time, thirty years after, we know very
little more about the physiological cause of the toxicity of zinc.
2. _Effect of zinc compounds on germination._
In the course of his investigations on the influence of zinc on vegetation
Freytag just touched upon the question of seed germination. According to
his statement the presence of zinc oxide in the soil does not exercise
much influence upon germination and the growth processes of plants. Little
zinc is stored up in seeds and on this account seeds originating from
plants containing zinc germinate quite normally and do not seem to be
affected by the peculiar nutritive conditions of the parent plants.
In certain cases light seems to have something to do with the harm
zinc compounds work on plants. Storp found that when clover seeds were
germinated in the dark on filter paper moistened with water containing
·025 gm. ZnO per litre (added in the form of zinc sulphate) no deleterious
action was observed. Barley seeds were soaked for four days in (_a_)
distilled water, (_b_) water with ·9 gm. ZnO per litre, which was
frequently changed. These seeds were then placed in the dark on filter
papers soaked respectively with water and with the solution containing
ZnO. So long as no light was admitted, for a period of eleven days,
germination was uniform in both sets, but directly the covers were removed
the growth of the seeds with zinc ceased almost entirely, and they did not
assume the green colour taken on by the unpoisoned seedlings. With maize
the germination was retarded by zinc even in the dark, but the harmful
action of light on the plants with zinc was again established. These
results seem to indicate that the formation and activity of chlorophyll is
impaired by the toxic agent, and this hypothesis is borne out by the fact
that in many fungi and non-assimilating higher plants the toxic action of
zinc is not evident.
Micheels (1906) approached the matter from a totally different standpoint,
seeking to discover what influence the valency of a metal has upon the
toxicity of its salts. In each of a series of experiments 1000 c.c. of ⅝
decinormal solution of sodium chloride in pure distilled water were used,
with the addition of varying strengths of calcium sulphate. Grains of
wheat, which previously had been soaked in distilled water, were placed
in the solutions, and it was found that the stronger the calcium sulphate
solution (up to 1/64 normal--the limit of experiment), the better the
growth. The calcium sulphate was then replaced by salts of other bivalent
metals, as zinc, lead and barium, with analogous results, the quantity
necessary to obtain the maximum development varying with one and another;
with zinc, _n_/128 gave the maximum. In this case the toxic action of both
sodium chloride and zinc sulphate on germination were considerably reduced
by their mutual presence--a result which fits in perfectly with what is
known as to the masking effect of soluble substances upon toxic action.
The same fact obtains in the animal kingdom, where Loeb and others have
found that the toxicity of solutions of sodium chloride for marine animals
is reduced by the introduction of salts of the bivalent metals.
3. _Stimulation induced by zinc compounds._
While the toxic action of zinc on the higher plants is so obvious that
it forced itself upon the attention of investigators at an early date,
the question of possible stimulus is so much more subtle that it has only
come into prominence during the last twelve years, during which time an
extraordinary amount of experimental work has been done with regard to
it. One investigator, Gustavson, was somewhat in advance of his time,
for as long ago as 1881 he hinted at the possibility that zinc, aluminium
and other substances might act as stimulants or rather as accelerators.
He indicated that the rôle of certain mineral salts in the plant economy
is to enter into combination with the existing organic compounds, the
resulting product of the reaction aiding in the formation of yet other
purely organic compounds which ordinarily require for their formation
either a very high temperature or a long time--in other words, such a
mineral salt acts as a kind of accelerator.
This work was apparently not followed up immediately, but it evidently
contains the germ of the “catalytic” hypothesis of which so much has been
made during recent years.
The work dealing with zinc as a stimulant to plant growth has yielded such
various and apparently contradictory results that the question cannot yet
be regarded as settled--it is even still more or less uncertain whether
zinc compounds act as stimulants, or whether they are merely indifferent
at concentrations below the toxic doses.
(_a_) _Stimulation in water cultures._
True and Gies (1903) suspended seedlings of _Lupinus albus_ for 24–48
hours with their roots in solutions of zinc sulphate and calcium sulphate
(_m_/256)[7], and found that while zinc sulphate alone at _m_/8192
retarded growth, yet with _m_/2048 ZnSO₄ and _m_/256 calcium sulphate
growth was more than twice as rapid as in controls grown in water,
indicating a marked stimulation. The presence of the calcium exercised
a definite ameliorating influence, reducing the toxicity of zinc to
one-sixteenth at most. The hypothesis put forward is that interior
physiological modifications are responsible for the observed differences
in growth rate, the cell processes being so affected as to bring about
different results on cellular growth--i.e. that where mixtures of salts
are concerned growth rate represents the physiological sum of oppositely
acting stimuli or of antagonistic protoplasmic changes.
[7] _m_ probably = gram molecular weight.
Kanda (1904) found that peas were stimulated in dilute solutions of
zinc sulphate in the absence of nutrients, the optimum concentration
being between ·00000287% and ·000001435% (about 1 in 34,840,000 and 1 in
69,700,000), higher concentrations being poisonous when the solutions
were changed every four days. Jensen (1907) stated that he obtained no
stimulation at all with water cultures, even in a solution as dilute as
_n_/100,000 (about 1 in 1,239,000), but he suggested that it was quite
possible that in proper concentration the zinc sulphate might prove to be
a stimulant.
Javillier (1910) grew wheat in nutritive solutions with quantities of
zinc salts containing from 1/5,000,000–1/250,000 zinc, and found that the
dry weight of the plant was increased in so far as the stems and leaves
were concerned, though it remained uncertain whether a similar increase
occurred in the grain.
A consideration of the Rothamsted experiments shows that up to the
present time there is no conclusive evidence that zinc sulphate acts as a
stimulant to barley grown in water cultures. As a general rule the growth
of those plants with 1/5,000,000 ZnSO₄ approximates closely to that of
the controls. Beyond this the growth varies in different experiments.
In some cases lower concentrations from 1/5,000,000 to 1/50,000,000
seem to cause some slight improvement in comparison with the normal,
indicating a possible stimulus, but this improvement is not at all well
marked. In other cases these great dilutions are apparently indifferent,
neither a poisonous nor a stimulative action being exerted on the growth
of the plant (Fig. 6). With peas some increase has been obtained with
1/20,000,000, and although the rise is only slight, yet it is possible
that it may indicate the setting in of a stimulus which would make itself
more strongly felt with still weaker concentrations (Fig. 7).
(_b_) _Stimulation in sand cultures._
While Jensen denied stimulation in wheat grown in water cultures even
when the solutions were as dilute as _n_/100,000 zinc sulphate, yet
he found increase of growth with the same plant in artificial soil
(quartz flour) to which much stronger solutions of zinc sulphate, from
5_n_/10,000–_n_/10,000, had been added.
(_c_) _Increased growth in soil._
Nakamura (1904) dealt with a few plants of agricultural importance, adding
·01 gram anhydrous zinc sulphate to 2300 grams air-dried soil. The marked
individuality in the response of the various plants to the poison is
very striking. Allium showed signs of increased growth throughout; Pisum
was apparently improved in the early stages of growth, but when the dry
weights were taken at the end of the experiment no increase manifested
itself in the weights of the plants treated with zinc; with Hordeum the
same quantity of zinc exercised a consistently injurious action. These
results with peas and barley corroborate those obtained in the Rothamsted
experiments with water cultures in that zinc sulphate proved to be less
toxic to peas than to barley.
Kanda found that both peas and beans when grown in soil as pot cultures
were improved by larger quantities of zinc sulphate than when they were
treated as water cultures--a result in full accordance with current
knowledge.
Wheat is evidently peculiarly sensitive to the effects of zinc compounds
under differing conditions. Javillier (1908 c) pointed out that while
wheat is very susceptible to the toxic action of zinc, yet it can benefit
by the presence of sufficiently small quantities of the compounds of
the metal. Rice is another cereal that is said to respond to the action
of zinc sulphate, as Roxas, working in pot cultures with soil both with
and without the addition of nutritive salts, obtained an acceleration of
growth on the addition of _m_/1000 zinc sulphate, a quantity so remarkably
great that it might be expected to act as a toxic rather than as a
stimulant.
With phanerogams the zinc question is not only concerned with the effect
of the metal upon germination, but also with its effect upon the later
growth of the green plants, and on the physiological functions involving
the construction of substances at the expense of mineral elements and the
carbon dioxide of the air. Javillier holds that the indications are that
zinc would prove to be profitable if applied to crops as a “complementary”
manure.
4. _Direct action of zinc salts on leaves._
Dandeno (1900) applied zinc sulphate in drops to the leaves of Ampelopsis,
and found that the solution was not all absorbed by the leaf, but that a
slight dark ring of a yellow colour was produced, and he was induced to
think that some local stimulation was produced if the salt was presented
in sufficient dilution.
Klopsch (1908) discussed the effect on plant growth of zinc derived from
industries producing zinc fumes. Zinc oxide from the fumes is deposited on
the leaves, and Klopsch stated that the rain and dew containing dissolved
zinc compounds find entrance to the tissues by way of the stomates and
work injury to the plants. Against this, however, it must be remembered
that these same fumes also contain other substances which are admittedly
harmful to plant life, and so the deleterious effect may be partly or
even chiefly due to these substances rather than to the zinc. Yet it
is probable that at least some of the depreciation is due to the zinc.
Treboux (1903) tested the effect of zinc sulphate on shoots of _Elodea
canadensis_. If the shoots were placed in _n_/100,000 (= ·000016%) zinc
sulphate no reduction of assimilation (as observed by counting the number
of oxygen bubbles emitted per minute) took place, and replacement in water
apparently had no effect either way. When however the shoots were placed
in (1) water, (2) ·00008% zinc sulphate, (3) fresh ·00008% zinc sulphate,
(4) water again, it was found that while the first solution of zinc
sulphate had apparently no effect on assimilation, yet during the second
immersion a gradual reduction in assimilation set in, which reduction was
continued after the return to pure water, so that the toxic action of the
zinc sulphate upon the shoots was clearly demonstrated.
III. EFFECT OF ZINC ON CERTAIN OF THE LOWER PLANTS.
Among the fungi, one species stands out in special prominence on account
of the great amount of work that has been done on it with regard to
its reactions to zinc salts. _Aspergillus niger_ = _Sterigmatocystis
nigra_ van Tgh was used as a test plant by Raulin (1869), who evidently
considered that zinc was an essential primary constituent of the food
solutions of the fungi, ·07 parts zinc sulphate being added to each 1500
parts of water. In his experiments he tested (1) ordinary nutritive
solution, (2) nutritive solution with various salts added, as zinc
sulphate, (3) nutritive solution and salts (as 2) and also powdered
porcelain. (2) gave a crop of Aspergillus about 3·1–3·5 times better
than (1), while (3) was even better still. Sulphate of iron also proved
stimulating in its action, but Raulin stated that zinc cannot replace
iron, as both are essential.
Ono (1900) determined the relation between the weight of the mould crop
in grams and the quantity of sugar used up in the presence of varying
amounts of zinc sulphate. The amount of sugar used was always greater in
the crops with ·0037–·0297% zinc sulphate by weight than in the control
crops, indicating a stimulation caused by zinc.
Richter (1901) carried out rather similar experiments. When grown in
solutions without and with 1/700,000 gram molecule zinc sulphate the
dry weights of the mould were practically the same for the first two
days, then the dry weight of the zinc crop shot ahead for a day or two,
a depression setting in on the fifth day. Without zinc a less increase
took place, and a similar drop was noticeable about the sixth day. The
conclusion drawn is that the stimulation due to the zinc occurs chiefly in
the first few days and also that the rise in the sugar consumed is more
rapid at first with the moulds treated with zinc. Concentrations above
1/600 are harmful, but in weaker solutions zinc is a definite stimulant.
Coupin (1903) re-investigated some of Raulin’s work under more antiseptic
conditions in order to see what substances were really needed by the
mould and whether certain elements declared essential were really
so. He concluded that iron and zinc are of no use in the nutrition of
_Sterigmatocystis nigra_, but that the zinc retards the development of
mycelium when food is abundant, killing it if it is badly nourished. This
denial of stimulation was controverted by Javillier (1907) who re-tested
Raulin’s solution with extreme care, growing Sterigmatocystis in
(_a_) normal Raulin’s solution with zinc,
(_b_) Raulin’s solution without zinc.
The ratio of crops _a_/_b_ varied from 2·3–3·1 in four experiments,
vindicating the favourable action of zinc. With regard to the optimum
value for zinc the mould seemed to be perfectly indifferent to the
presence of medium quantities but very sensitive to extremes, the maximum
weights being reached in dilutions between 1/10,000,000 and 1/250,000,
while quantities above 1/25,000 were toxic in their action. At a dilution
of 1/50,000,000 stimulation was still evident, though in a less degree
than with the optimal concentrations.
Javillier maintains that zinc is fixed by the fungus, the whole of the
zinc present in dilute solutions being taken up, only part being utilised
in stronger solutions. The value of accordance between the quantity of
zinc fixed and the quantity supplied decreases rapidly with increase of
concentration. Sterigmatocystis is able to fix without harm a quantity
of zinc equal to more than 1/1100 of its weight. Zinc is regarded as a
catalytic element, as essential to the well-being of the plant as are the
more obvious nutrients, carbon, sulphur, phosphorus, &c., in spite of the
minute traces in which it occurs.
A few tests on yeasts made by Javillier showed that with vegetative yeasts
zinc has a specific action, a consistent increase occurring in the amount
of yeast formed and in the amount of sugar consumed as the quantity
of zinc increased from 0–1/10,000,000–1/10,000. With ferment yeast,
however, zinc exerted no appreciable action. These results lend force to
the conclusion of Richards (1897) who carried out experiments on fungi
with various nutritive media with the addition of certain salts of zinc,
nickel, manganese, iron, &c. He considered that his general results showed
that the fact of a chemical stimulation of certain metallic salts upon
the growth of fungi is established, although it must not be considered
without further investigations that all fungi react in the same degree to
the same reagent.
_Conclusion._
As matters stand at the present day, it appears that it is still
uncertain whether higher plants grown in water cultures are susceptible
to stimulation by zinc salts. If a stimulus does exist, it must be at
exceedingly great dilutions, but further evidence is needed. In soil
cultures, however, the fact of increased growth seems to be more firmly
established, certain species responding to zinc salts when used as manure,
though no increase has been obtained with other species. It must always
be remembered that the action may be an indirect one. The soil is very
complex in its constitution, and it is impossible to determine the exact
action of the added poison upon it, so that a stimulating effect need not
necessarily be due to a direct action of a substance upon the plant, but
it may be the result of more favourable conditions for life induced by
the action of the substance upon the soil.
Among the fungi the stimulation of _Aspergillus niger_ by minute traces
of zinc compounds seems to be well proved, though again it does not
necessarily follow that all fungi will react in the same way to zinc.