Inorganic Plant Poisons and Stimulants

CHAPTER V
EFFECT OF ARSENIC COMPOUNDS

I. Presence of Arsenic in Plants.

The occurrence of arsenic as an occasional constituent of plants has been recognised for many years. Chatin (1845) found that if a plant were supplied with arsenical compounds at the roots arsenic was absorbed, but that it was distributed unequally to the various tissues. The greatest accumulation of the element was in the floral receptacle and the leaves, while it was scarce in the fruits, seeds, stems, roots and petals. E. Davy (1859) commented on the presence of arsenic in plants cultivated for food. He grew peas in pots and watered them for a short time with a saturated aqueous solution of arsenious acid, the application being then discontinued. The plants, apparently uninjured by the treatment, flowered and formed seeds. On analysis arsenic was readily detected in all parts of the plant, including the seeds. Other analyses revealed the presence of the element in cabbage plants (from pots) and turnips (from field), both of which had been manured with superphosphate containing some amount of arsenic. This absorption of arsenic by the roots of plants was further established by Phillips (1882).

Various physiological workers have pointed out that this element is frequently or usually present in animal tissues. Cerný (1901) reached the general conclusion that minimal traces of arsenic can occur in animal organisms, but that these play no part in the organism and indeed are not constant in their occurrence. Bertrand (1902) established its presence in minute quantities in the thyroid glands of the ox and pig, hair and nails of the dog, and the feathers of the goose. Gautier and Clausmann (1904) realised the constant presence of arsenic in human tissues and recognised that it must inevitably be introduced into the body with the food. This led them to estimate the arsenic present in various animal and vegetable foods, some of their results being given in the following table.

Arsenic was also found in wine and beer and in considerable quantities in sea water and various kinds of salt. Since it cannot be found in some things even in the least traces, the authors conclude that it is incorrect to say that the element is always present or that it is essential to all living cells.

S. H. Collins (1902) found that barley is able to absorb relatively large quantities of arsenic. The plants were grown in pots on soil which originally contained a certain amount of the substance, and various combinations of arsenic acid, arsenious acid and superphosphate were added. Particulars and details are not given by the author, except that arsenic was detected by Reinsch’s test in the grains from all the experimental pots, and in one case (not specified) in the upper and lower halves of the straw and in the threshed ears. The analyses of the soil at the close of the experiments showed the presence of 7–22 parts arsenious acid per million.

Wehmer (1911) quotes references to the occurrence of arsenic in Vitis vinifera. The element was detected in the ash of the must and its presence was attributed to treatment of the plants with arsenical compounds. In this connection it is interesting to note the observation of Swain and Harkins (1908), who, while acknowledging the absorption of arsenic from the soil by many plants, yet indicate that in the case of those plants which are exposed to smelter smoke the arsenic is deposited on the vegetation, and is not absorbed by the latter from the soil.

II. Effect of Arsenic on the Growth of Higher Plants.

1. Toxic effect.

(a) Toxic action of arsenic compounds in water cultures in the presence of nutrients.

The poisonous action of arsenic on plants has long been recognised. Chatin (1845) gave accounts of tissues poisoned by strong arsenical solutions. Nobbe, Baessler and Will (1884) carried on water culture experiments with buckwheat, oats, maize and alder, and found that arsenic was a particularly strong poison for these plants. When small quantities of arsenious acid (As₂O₃) were added to the food solutions, growth was measurably hindered by a concentration of 1/1,000,000 As (reckoned as As). The element only appears in plants in very small quantity and can never be detected in notable quantities. The aerial organs show the effect of arsenical poisoning by intense withering, interrupted by periods of recovery, but eventually followed by death. It was also found that if plant roots were exposed to the action of arsenical solutions for a short period, say ten minutes, and then were transferred to normal food solutions, the action of the poison was delayed, but eventually hindering of growth or death occurred, according to the strength of the poison used in the first solution.

At the same time that Nobbe, Baessler and Will were establishing the great toxicity of the lower oxide of arsenic, Knop (1884) was carrying the matter a step further by comparing the action of arsenious and arsenic acid and their derivatives on plant growth. He established the fact that while arsenious acid is a strong poison for maize plants, arsenic acid in small quantities is not toxic to the roots and that the plants can produce flowers and fruit in its presence. Arsenic acid applied as potassium arsenate proved to be harmful to young maize seedlings if the solutions contained ·05–·1 gm. arsenic acid per litre (= 1/–2/20,000 arsenic acid). If however the plants were allowed to form 10–15 leaves in a pure food solution and then when strongly rooted were transferred to a solution of ·05 gm. arsenic acid per litre, they were found to grow strongly and develope big healthy leaves. Careful measurements indicated that the development is unchecked by the addition of the poison, though arsenic was determined in the ash of the treated plants.

Stoklasa (1896, 1898) tested the effect of arsenic compounds on plant growth with special attention to their comparative relation to phosphoric acid. He corroborated Knop’s statement as to the greater toxicity of arsenious acid and arsenites in comparison with arsenic acid and arsenates, stating that 1/100,000 mol. wt. arsenious acid per litre causes definite trouble in plants, while with arsenic acid 1/1000 mol. wt. per litre first shows a noticeable toxicity. Water culture experiments were made with and without phosphoric acid, in each case with and without the addition of arsenic and arsenious acid. It was found that the arsenic acid was unable to replace the phosphoric acid, the plants decaying in the flower in the absence of the latter. In the complete absence of phosphoric acid, arsenic acid causes a strong production of organic substances up to the flowering time. The following figures were obtained with maize:—

Comparative experiments with the two arsenical oxides showed that varying times were required to kill different plants. Young seedlings were brought into solutions containing 1/10,000 mol. wt. arsenious acid (= ·019 gm. As₂O₃ per litre) and the plants died in a very short time.

With ten times the strength of arsenic acid (1/1000 mol. wt. = ·23 gm. per litre) the plants took much longer to kill.

Various experiments have been carried on at Rothamsted with peas and barley. With arsenious acid on barley a depressing influence is manifest even at a concentration of 1/10,000,000, while no growth at all is possible with 1/10,000 and upwards. Apparently the toxic action on the root ceases at a higher strength than on the shoot, as with 1/1,000,000 and less the dry weight of the root remains practically constant. At this same strength the shoots look better than the controls, but this is not apparent in the dry weights (Figs. 9 and 10). With peas the depression is again evident to 1/10,000,000, but the plants are more sensitive to the higher concentrations, as no growth can take place in the presence of 1/250,000 arsenious acid (Fig. 11). A striking difference is observed with arsenic acid on barley, as apparently this does not act as a toxic even with such comparatively great concentrations as 1/100,000, though possibly the shoot is slightly depressed by this strength (Fig. 12).

With sodium arsenite the dilutions were carried further, to 1/250,000,000, but this still depressed barley to some extent (Fig. 13). With peas the results vary somewhat in the different tests, the depression with 1/2,500,000 and less being usually slight, though occasionally it is much more strongly marked (Fig. 14). In a single series with sodium arsenate barley was apparently unaffected by a concentration of 1/1,000,000, but from this point down to 1/250,000,000 a constant depression showed itself, which was paralleled by a similar depression in the sodium arsenite series from 1/25,000,000 to 1/250,000,000, the curves grading downwards instead of up towards the normal. With peas sodium arsenate has little or no action, though it is just possible that the rather irregular curves indicate a very slight depression below the normal throughout.

(b) Toxic effect of arsenic compounds in sand cultures.

Comparatively few tests seem to have been made as to the action of arsenical solutions in sand cultures. Stoklasa (1898) repeated his water culture work, using sand as a medium, and found analogous results by the two methods, i.e. that arsenites are far more toxic than arsenates, and also that the degree of toxicity of a salt varies with the plant to which it is applied, as was shown by the fact that different plants lived for varying times when treated with similar strengths of solution.

(c) Toxic effect of arsenic when applied to soil cultures.

Daubeny (1862) watered barley plants with a solution of arsenious acid, 1 ounce in 10 gallons, five times in succession, and found that the crop arrived at maturity about a fortnight earlier than the untreated part of the crop, though the amount harvested was rather less. With turnips four waterings had no effect upon the time of maturity, but again the crop was slightly decreased. The analyses made indicated that no arsenic was taken into the tissues, but that it merely adhered to the external surfaces.

Gorup-Besanez (1863) mixed 30 grams arsenious acid with 30·7 litres[9] soil, growing two plants on this quantity of earth. Most of his experimental plants (Polygonum Fagopyrum, Pisum sativum, and Secale cereale) developed normally, but Panicum italicum died soon after the plants appeared above the surface, the leaves being very badly coloured. Analyses by Marsh’s test showed no trace of arsenic in 20 grams dry matter from Secale cereale, but in 148 grams Polygonum Fagopyrum the presence of arsenic was evident, though the mirror formed was weak. With such a large proportion of arsenious acid in the soil it seems hardly conceivable that the plants were not injured to some extent, and also it is probable that with more careful analyses arsenic would have been detected in those instances in which its presence was denied. Yet it must be remembered that Davy (1859) had treated pea plants in pots with a saturated solution of arsenious acid for a short time and had stated that the plants were uninjured. Thus both Gorup-Besanez and Davy concur in the opinion that Pisum sativum is indifferent to relatively large quantities of arsenious acid when presented in the soil, whereas the Rothamsted experiments show that in water cultures the plant is extremely sensitive even to minute traces of the substance. It is possible that the arsenic in the solution added to the soil enters into combination with other substances, forming insoluble compounds, thus being removed from the sphere of action and rendered unable to affect plant life. If this be so, the apparent immunity of certain plants to arsenious acid is explained. F. C. Phillips (1882), in his experiments on various flowering plants, such as geraniums, coleas and pansies, found that compounds of arsenic in the soil exercised a distinct poisoning influence, tending, when present in large amount, to check the formation of roots, so that the vitality of the plant was so far reduced as to interfere with nutrition and growth, or even to kill it outright. He also stated that traces of arsenic were found in all the plants grown upon the poisoned soil.

In this connection it is interesting to note that a certain proportion of arsenic is frequently present in the superphosphate used as manure. In view of the known toxicity of arsenical compounds to plant life the question arose as to whether superphosphate manuring would exercise a detrimental influence on account of its arsenic content. Experiments carried out by Stoklasa (1898), however, indicate that there is not sufficient arsenic in maximum doses of superphosphate to exercise a toxic action in the field.

(d) Physiological considerations.

The physiological action of arsenic compounds on plant life early attracted the attention of investigators. Chatin (1845) put forward some rather curious and unexpected considerations with regard to this action. He stated that the effect of arsenic on plant growth is determined more by the constitution and temperament of individual plants than by their age, and that apparently difference in the sex of plants is of no significance. The chief determining agent, however, is the species, and Chatin found that as a general rule Cryptogams are more sensitive than Phanerogams, and Monocotyledons than Dicotyledons, as is shown by the fact that under treatment the former perish first. Some extreme exceptions exist, though, as Mucor mucedo and Penicillium glaucum will grow on moist arsenious acid, whereas leguminous plants are killed by an arsenical solution in a few hours. Chatin held the view that elimination of the poison succeeded its absorption, and that this elimination is complete if the plant lives long enough. Here again the species exerts a great influence on the excretory functions of the plants. Lupins and Phaseolus are presumably able to eliminate in six weeks all the arsenious acid they can absorb without dying. Most Dicotyledons need 3–5 months, while Monocotyledons retain traces of poison for six months after its absorption. Lichens are said to eliminate it more slowly still. Again, woody species are longer in freeing themselves than herbaceous, and young plants carry out the elimination more easily than old plants. The excretory function is influenced by other physiological factors such as dryness and season. The toxic effects and elimination are supposed to act inversely and parallel, the absorbed arsenious acid combining with alkaline bases, making a very soluble salt which is excreted by the roots. Calcium chloride is given as the antidote to arsenious acid, all soluble acid being “neutralised” by it. This view of the elimination of arsenic apparently did not gain much support, as no further references to the matter have so far come to light. In view of the work of some modern investigators (Wilfarth, Römer and Wimmer) on the excretion of salts by plant roots, the idea may prove of fresh interest. Chatin also found that moving or still air influenced the working of the poison, indicating that the external physical conditions affect the toxic action considerably. Nearly forty years later Nobbe, Baessler and Will found that, if transpiration were hindered by placing plants in a dark or moist room, it was possible to keep the plants turgescent in arsenic solutions for a long time without thereby increasing the toxic effect later on. The poisonous action proceeds from the roots, of which the protoplasm is disorganised and the osmotic action hindered. Finally, in the presence of sufficient of the poison, the root dies without growth.

Stoklasa (1896, 1898) again found that phanerogamic plants can withstand arsenic poisoning for some time in the dark or in CO₂-free air, provided that glucose is given in the food solution. The arsenic poisoning is at its maximum during carbon assimilation by means of chlorophyll. The toxic action of arsenious and arsenic acids, especially in phanerogams, is due to injury to the chlorophyll activity. The destruction of the living molecule is far more rapid in the chlorophyll apparatus than in the protoplasm of the plant cell.

Thus it seems that the physiological cause of the toxicity of arsenic is partly a direct action on the root protoplasm, whereby its osmotic action is hindered, and partly a detrimental action upon those functions which are directly concerned with the elaboration processes of nutrition.

2. Effect of arsenic compounds on germination.

In view of the great toxicity of arsenic to plants in their various stages of development, one would naturally expect to find a similar action with regard to the germination of the seeds. Davy (1859) casually mentioned cases in which watering with arsenical solutions or dipping seeds in arseniated water prevented germination. Heckel (1875) found that arsenious acid checks germination and kills the embryo at relatively feeble doses, ·25 gm. to 90 gm. water.[10] Guthrie and Helms (1903–4–5) carried out a systematic series of experiments to test the effect of arsenic compounds upon different farm crops. Various amounts of arsenious acid were added to soil in pot experiments, and the seeds of the several crops were then sown. With barley, wheat and rye 0·10% arsenious acid had little or no effect on germination, while an increase in the poison exercised a retarding action. Maize could withstand 0·40% arsenious acid without retardation being perceptible. The aftergrowth with the different crops varied considerably. The wheat plants with 0·10% arsenious acid grew all right at first, but later on they developed weakly. The toxic action increased rapidly as the strength of the poison rose in the different pots. Barley proved even more sensitive than wheat, for even 0·05% arsenious acid affected the growth adversely. After a time the plants with 0·05–0·06% recovered and grew strongly, though not so well as the controls, but those with 0·10% practically died off. Rye behaved in the reverse way from wheat. The plants with 0·10% were slightly checked at first but later recovered and made growth quite equal to the check plants. Growth was stunted with 0·20% arsenious acid, and the plants were killed with 0·30%, so that rye is far less sensitive than barley. With maize the growth was slightly affected with 0·05% As₂O₃, and increasingly so with greater quantities. It was also found that the action of 0·8% As₂O₃ was strongly adverse to the germination of all plants, and that above this strength germination was altogether prevented.

The results show very clearly how impossible it is to draw any general conclusions with regard to the action of arsenic compounds on plants, as they emphasise the strong individuality of the species in their reaction.

3. Do arsenic compounds stimulate higher plants?

The question of stimulation due to arsenic does not seem to have engaged the attention of investigators to any extent. Water culture experiments at Rothamsted have so far yielded negative results, and no stimulation has yet been obtained with any plant, with the possible exception of white lupin with sodium arsenite. In a single series a stimulus was suggested, beginning to make itself felt at 1/500,000, rising to an optimum at 1/10,000,000. No stress can be laid on this result, as it is never safe to draw any certain conclusions without several repetitions of the same experiment. With arsenic acid on barley a possible stimulus is sometimes indicated to the eye, the plants being fine and of a particularly healthy dark colour, but this is not corroborated by the dry weights. Additional tests were made with peas and barley, treated with sodium arsenite and arsenate, the dilutions being carried down to 1/250,000,000, but no evidence of stimulus was obtained, so that it hardly seems possible that arsenic can act as a stimulative agent for these two plants when grown in water cultures. It had been thought that the failure to find a stimulation point hitherto might be due to the too great concentration of the toxic substance rather than to the actual inability of the poison to stimulate, but this hypothesis must now be dismissed so far as these plants are concerned.

III. Effect of Arsenic Compounds on Certain of the Lower Plants.

1. Algae.

Loew (1883) was sceptical concerning the specific toxicity of arsenic for plant protoplasm. He was convinced that arsenic and arsenious acid were poisonous to algae, not because of their specific character as arsenical compounds, but because of their acid nature, algae being peculiarly sensitive to any acid, and he maintained that these substances were not more poisonous than vinegar or citric acid. He placed various species of Spirogyra in solutions of ·2 gm. potassium arsenate per litre water (1/5000), and found that the algae grew well without making any abnormal growth in a fortnight, showing hardly one dead thread. Some of this alga was then transferred to a 1/1000 solution of potassium arsenate. This suited it excellently and it increased and the appearance under the microscope was very fresh and strong, which was attributed more to the potash than to the arsenic acid. Loew maintained that for the lower animals and for many of the lower plants arsenic in the form of neutral salts is not a poison. When the differentiation of the protoplasm into certain organs reaches a specific degree in the higher plants, then the poisonous action of the arsenic compounds comes into play.

Knop (1884) found that certain unicellular green algae grew luxuriantly in a neutral solution supplied with potassium arsenate. Bouilhac (1894) concerned himself chiefly with the possibility of the replacement of phosphates by arsenates. He recognised that the influence of arsenic is not the same on all species of plants, so he confined his attention to certain of the algae. Stichococcus bacillaris Naegeli was found to live and reproduce itself in a mineral solution containing arsenic acid. Even in the presence of phosphoric acid the arsenic acid favours growth, the best dose being about 1/1000. The arsenic acid is capable of partly replacing phosphoric acid. Other species of algae, Protococcus infusionum, Ulothrix tenerrima, and Phormidium Valderianum invaded the original culture of Stichococcus from the atmosphere, but with no arsenic or phosphoric acid their development was poor. The jars with arsenic compounds were invaded by still more species which grew strongly. Under these conditions it is evident that these algae are capable of assimilating arsenic, and the addition of arsenic acid to a solution free from phosphoric acid is sufficient to enable these algae to live satisfactorily, the arsenates in this case replacing the phosphates. Ono (1900) found that algae are favourably influenced by small doses of poisons, the optimal quantity for algae being lower than that for fungi. Protococcus showed a possible stimulus when grown in concentrations of potassium arsenate varying from ·00002–·0005%. This possible stimulus is interesting in view of the failure to observe stimulation in higher plants by minute traces of arsenic.

2. Fungi.

The effect of arsenic on fungi is of special interest in that it has a direct bearing upon hygienic and commercial interests. Gosio (1892, 1897, 1901) found that certain of the fungi, Mucor mucedo and Aspergillus glaucum, will grow on various arsenic compounds and exercise a reducing influence on them. These moulds attack all oxygen compounds of arsenic including copper arsenite, and develope arsenical gases. Sulphur compounds of arsenic are not influenced by these fungi. The same moulds would, if cultivated in soil containing arsenic, develope hydrogen arsenide. Penicillium glaucum has such a strong and definite action on arsenic compounds that he states that there is no doubt of the possibility of poisoning by arsenical gas in a room hung with paper containing arsenic. The compounds are so extraordinarily potent that if a mouse is placed in a vessel in which the mould is strongly developed in the presence of arsenic, it dies in a few seconds. Penicillium brevicaule uses the element in its development as a food substance. If material containing arsenic is placed in contact with dead fungi no reaction occurs. The life activity of the mould is evidently necessary for the reaction by which the arsenic-containing gases are liberated. Csapodi (1894) put forward the earlier results of Gosio and noted that the so-called arsenical fungicides do not only fail to kill the mould fungi but actually favour their development. This action explains why wallpaper containing arsenic is so disadvantageous in a room. Abba (1898) severely tested Gosio’s method of detecting arsenic by means of growths of Penicillium brevicaule, whereby arsenic gases are liberated, vindicating the method completely, and establishing the test as an exceptionally delicate one. Segale (1904) applied the same method to the detection of the presence of arsenic in animal tissues.

Ono (1900) grew Penicillium cultures with solutions of potassium arsenate and found no important differences either of depression or stimulation. Orlowski (1902–3) stated that small doses of arsenic (1/1000–1/100% Sodium arsen—[11]) stimulate the growth of Aspergillus niger, larger doses up to ¹⁄₈% retard growth, while ¹⁄₆% kills. Spores of the fungus taken from soil containing arsenic are said to possess an immunity against arsenic, in that they germinate in the presence of an arsenic content which rapidly kills control fungi. This immunity is not specific for arsenic, but extends also to other poisons. The chemical composition and water content are not altered.

Conclusion.

The toxic effect of arsenic upon higher plants is much more marked with arsenious acid and its compounds than with arsenic acid and its derivatives. No definite evidence of stimulation has yet been obtained with any arsenic compound, however great the dilution at which it is applied. With certain algae a stimulus may occur, and it is possible that arsenic acid is capable of replacing phosphoric acid to some extent under certain conditions. With fungi the toxic effect of great concentrations is marked with certain species, but there are others which are capable of living happily on arsenical compounds and of liberating highly poisonous arsenic gas.

CHAPTER VI
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.

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.