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Part IV, continued: 

  As indicated above, the chemical composition of root excretions, as well as that of the dying root hairs and cells of the root epidermis, varies in different plants. Consequently, root production will attract different soil microflora. Plants which excrete carbon compouonds attract to their roots a microflora which differs from the microflora attracted by plants which mainly excreted nitrogenous substances. With the presence of sugar in root excretions one kind of bacterial and fungal species will develop and, in the presence of organic acids, others will develop. These microorganisms which utilize the available nutrient substances more quickly and fully will predominate in the plant rhizospheres.

  The qualitative composition of root excretions and dying root residues determines the characteristics of the quantitative and species makeup of the microflora in the soil of the root region.

  The first to have observed the capacity of plants to affect the microflora of soils was the writer, S.T. Aksakov.

  In 1896 he wrote: "The mushroom is the child of the forest. . . . As is well known, if one sows, plants-- in a word, if one plants a forest in a bare field--the mushroom types characteristic of the varieties of the planted forest will undoubtedly start to grow there. However, the shadow (as many have thought) cast by the branches of trees is not the only secret force by which trees cause mushrooms to grow around them. It is true that the shadow is one of the primary reasons for this phenomenon. It protects the soil from the burning rays of the sun, and it create a moisture in the soil and even the dampness which is essential for the forest, as well as for the mushrooms. However, the main reason for the formation of the mushrooms, in my opinion, are the tree roots which, in their turn, having moistened the surrounding soil, give it the sap of the tree. This, in my opinion, is the secret of mushroom growth." (Notes and Observations on how to Collect Mushrooms, vol. V, 1896).

  Later, data on this subject were published In the scientific literature (Galakhov 1929; Danilov, 1943, 1949; Vasil'kov, 1953, and others). Detailed information on the interrelationship between edible mushrooms and higher plants in the forest phytocoenoses of Latvia is given in the dissertation presented by Mazelaitis (1952), and information on the distribution of mushrooms in the forests of the Moscow area in given in the book by Shiryamov "The Search and Collection of Mushrooms in the Forests of Moscow Area," (1948).

  An analysis of the obtained observation and studies shows that the distribution of edible mushrooms is closely connected with the composition of phytocoenoses. When the forest and plant species of an area are known, one can determine beforehand which species of mushroom will grow in the area and the distribution of the mushroom types of interest to us. For instance, the white mushroom is encountered in heather, mountain-cranberry, spruce-sorrel, and oak-bilberry forest. The ordinary brown mushroom is found in pine forests with birch and heather; Boletus lutens is found in young pine forests having a certain grassy vegetation, etc (Vasil'kov, 1953).

  We demonstrated experimentally the selective action of plants. Different plants were grown under sterile conditions, in sand or cotton, and soaked with Knop's nutrient solution. Bacteria were introduced into the medium, either separately, or in the form of mixture. The results are given in Table 76.

  As can be seen from the given data, some bacteria grow well, others grow only slightly, while others show intermediate growth on the same plant. Azotobacter, for instance, did not grow at all or grew very poorly in vessels in which wheat, corn, and flax were planted, showed intermediate growth on cotton, and abundant growth on clover, lucerne, and peas.

  The reaction of two strains, Nos. 1 and 2, of the same species of nonsporeforming bacteria. Ps. fluorescens, to the root excretions of plants differed. Strain No. 1 grew most abundantly in the rhizosphere of wheat, corn, and especially cotton, while strain No. 2 grew much better in the presence of the root excretions of clover and lucerne. The root-nodule bacteria reacted positively toward the roots of cotton, clover, lucerne, and peas, and less so to the action of the root system of sugar beets.

  We also grew a series of other sporeforming and nonsporeforming bacteria, mycobacteria and yeasts. The sporeforming bacteria Bac. mesentericus, Bac. subtilis, Bac. megatherium, as a rule, did not grow in a medium with the root excretions of the experimental plants, but did not die in it. The mycobacterium, Mycob. album, grew fairly well in the rhizosphere of corn and wheat and somewhat better in that of clover and peas. Brewer's yeast did not grow at all with corn, wheat, and beans, but wild yeast of the genus Torula, on the other hand, grew beautifully and multiplied.

  Similar data were obtained by Metz (1955) in experiments with sterile cultures, nonsporeforming bacteria and, on other plants, their growth stopped. According to his data, the growth of Azotobacter is strongly suppressed in the rhizosphere of celandine, buttercups (Ranunculus acer L., Ran. repens L), peonies (P.officinalis L.), and fumitory, (Fumeria officinalis L.), and is less suppressed in the rhizosphere of Viola tricolor Wittr., Allium schoenoprasum L., Rumex patientia L., and Epillobium montanum L.

  Plants such as Crepis virens K., Hieracium pilosella L., Armoracia rusticana Gaertn, and others, which strongly suppress the growth of sporeformong bacteria, do not have any deleterious effect on the growth of Azotobacter. On the contrary, many of them stimulate the growth of this microbe.

  A similar effect on the growth of Azotobacter by these or other plant species prevails under conditions of natural growth (Table 77).

  The effect of grass plants on the growth and accumulation of Azotobacter in the soil is especially well demonstrated in monocultures. The longer a plant is cultivated, the more bacteria and fungi accumulate in the soil. Such a long-term accumulation of microbes was observed by us in the soils of Central Asia, in the lucerne-cotton crop rotation, Usually, after lucerne is grown for three years whether in the pure form or in a grass mixture, cotton is cultivated for several years (six to nine). After this type of crop rotation, the microflora changes considerably in relation to Azotobacter, and to other species as well. (Figure 78). Under lucerne, the number of Azotobacter cells increases and, under cotton, it decreases. However, Azotobacter does not completely vanish under cotton.

Figure 78. Growth of Azotobacter in soils with a crop rotation of lucerne-cotton in the fields of the Vakhsh valley, Tadzhik SSR:

1-- cotton, 2--lucerne.

  'This regularity of change in microbial forms is observed in monocultures of plants, for example, under lucerne and wheat in the chestnut soils of the Trans-Volga region, or under clover-flax and oats in the podsol soils of the Moscow Oblast' (Figure 79).

Figure 79. Growth of root-nodule bacteria of clover on various plants:

1--clover; 2--wheat; 3--oats (monoculture) on the experimental fields of the Agricultural Academy im, Timiryazev.

  In the crop rotations of an ordinary farm with industrial crops, one observes thesame results, but with less markedly expressed numerical indices (Krasil'nikov, 1940a).

  Sheloutnova (1938) and Wenzl (1934) observed the growth of Azotobacter under a culture of tobacco and vineyards; Mashkovtsev (1934) and Uppal and coworkers (1939) observed its growth under rice.

  We have studied the growth of Azotobacter in forest plantations in the fields of Kirgizia, Moldavia, Ukraine, and the Moscow Oblast'. The arboretum of a seven- to ten-year-old plantation, in sectors previously under lucerne or other vegetation possessing a considerable number of Azotobacter cells was investigated. The growth of Azotobacter was quantitatively measured on strips sown with different tree varieties: maple, ash, poplar, acacia, spruce, pine, oak, etc (Table 78).

  As can be seen from the data given, afforestation, as a rule, removes Azotobacter from the soil. Only under certain species in it preserved in small numbers,. Under certain plants, the acacia and the ash tree, Azotobacter grows moderately well, although it does not reach the numbers found in plowed-up soils.

  Artificially planted birch, aspen, oak, and especially spruce and pine trees in podsol soils quickly remove Azotobacter. Azotobacter also perishes in Southern chernozem soils, if the latter are afforested with oak, pine, hornbearn, etc.

  Fruit trees, such am apple, pear. plum, cherry, and others, do not suppress the growth of Azotobacter and many of them are even favorable to its accumulation in the soil. The soils of the orchards investigated by us in Moldavia (chernozem). in the central belt of RSFSR (podsol), in Crimea, and in other regions of the USSR contain larger quantities of Azotobacter than the soils of fields which are intensively cultivated (Table 79).

  As can be mean from the above-mentioned, certain species of plants enhance the growth of Azotobacter in soil, others suppress it, and others neither enhance nor suppress its growth.

  This type of plant classification is only relative and only holds true for sterile cultures, where the microbes are subjected to a one-sided action by root excretions, with the exclusion of other external factors.

  Under conditions of natural growth in the field, the effect of root excretions is to a large extent annulled by many factors and, first of all, by microorganisms. Therefore, the summary effect in such cases will express itself in a weaker form and sometimes will not be observed at all. A comparison of the effectiveness of plants should be performed under the same soil-climatic conditions.

  The same plant in different soils and in differing climatic and geographical zones may act differently on Azotobacter. As was noted above, wheat acts negatively on the growth of Azotobacter under conditions of sterility, in open ground in the Trans-Volga region on chestnut soils, and in the central belt on podsol, but does not suppress the growth of Azotobacter under the conditions prevailing in Kirgizia on chernozem soil, on the chernozem soil of the Kuibyshev Oblast', and in other places. Even in the same region and in the same soil, the effect of wheat may differ, depending on the extent of the cultivation of the soil. Soils of the podsol zone, where cultivated, often contain a sufficient number of Azotobacter under wheat and other plants. Of great importance for the growth of Azotobacter is the agrotechnical cultivation of soil.

  The root excretions of young plants differ from those during the period of ripening and, therefore, the microflora during these periods of growth will also differ.

  Only by consideration of a multitude of factors affecting the growth of Azotobacter can the contradictory indices, obtained by different investigators, on the distribution of this microorganism be explained.

  These considerations not only pertain to Azotobacter but to all other microbial species in the rhizosphere of plants.

  The selective effect of plants is well demonstrated in the case of root-nodule bacteria. Korenyako (1942) tested three species of bacteria, Rh. trifoli. Rh. meliloti and Rh. leguminosarum, by introducing them into containers in which clover, lucerne, peas, wheat, corn, and cotton were grown on sand. Root-nodule bacteria of lucerne grew equally well under lucerne and under cotton; Rh. trifolii grew abundantly under clover, lucerne, peas, and wheat, less well under flax, and hardly at all under corn. Root-nodule bacteria of peas were found in large numbers under peas, clover, and wheat.

  These data were confirmed by experiments performed in open soil (podsol). Root-nodule bacteria of clover were introduced under clover, lucerne, peas, wheat, and corn. The growth of the bacteria in the rhizosphere was followed through the entire vegetative period, The results are given in Table 80.

  The intense growth of root-nodule bacteria under leguminous plants was also noted by other investigators (Wilson and Wagner, 1936, and Lewis, 1938).

  Rudin (1956) noted the activating action of corn sap, and especially that of pea sap, on root-nodule bacteria. According to his observations, the sap of inoculated peas having nodules on their roots is more active than the sap of noninoculated peas, The sap of corn roots enhances the virulence of the root-nodule bacteria of peas.

  According to our observations, the root-nodule bacteria of lucerne grow well under timothy grass, cotton, and rye grass. Their number in the serozem soil of Central Asia, reached hundreds of thousands in one gram of soil, i. e., almost the same as under lucerne. In the chestnut soils of the Trans-Volga region, these bacteria in the rhizosphere of timothy grass amount to tens of thousands in one gram of soil, and under orchard grass, oats, and millet, their number was considerably smaller.

  The number of root-nodule bacteria in the soil, after leguminous plants, decreases, if the subsequent culture in the crop rotation is not favorable to their growth this decrease in the number of bacteria in the soil takes place at different rates, depending on the soil and on the plant species. Under flax and wheat, the number of root-nodule bacteria of clover in the podsol of the Moscow Oblast, decreases comparatively rapidly.

  The experimental data obtained by Chailakhyan and Megrabyan (1955) on the specific action of the roots of leguminous plants on root-nodule bacteria are of interest. The authors found that the ground roots or the root sap of leguminous plants do not negatively affect the growth of root-nodule bacteria of their own species, but suppress the growth of bacteria of other foreign species (Table 81), The maximum expression of this selective action by the roots of leguminous plants is observed during budding and flowering stages, becoming less marked later.

  In our investigations we were unable to find such specificity in the selective antibacterial action of either the roots or the aerial parts of leguminous plants.

  Torne and Brown (1937) tested the bacterial effect of the sap of a great number of plants, leguminous and others. According to their data, the extracts of leaves of many plants inhibit the growth of root-nodule bacteria. These plants include clover, cabbage, carrots, turnips, and others. The authors did not observe any specificity in the inhibitory action of the sap. Lucerne sap inhibited the bacteria of its own species to the same extent as it inhibited that of clover, beans, and other plants. Root sap was less bactericidal than the sap of the aerial parts and, in many plants, the root extract had no toxicity for bacteria whatsoever.

  Among the other microbes which grow in the root zone of vegetating plants we also studied mycolytic bacteria. These bacteria are characterized by their ability to dissolve the mycelia, of fungi (Khudyakov, 1935; Novogrudskii, 1936). They differ in classification and comprise a mixed group of bacteria. They also possess a more or less defined specificity. Each species in this group of bacteria dissolves certain forms of fungi--saprophytes and phytopathogenic forms.

  According to Korenyako (1939). Raznitsina (1942), and Kuzina (1951, 1955), mycolytic bacteria grow abundantly and accumulate under certain plants. Under the conditions prevailing in Central Asia, in serozem, soils under lucerne, their number varies from 100,000 to one million in one gram. Under cotton, on the other hand, their number decreases. In soil which has been plowed for six to nine years, under this culture [cotton], the number of mycolytic bacteria is minimal, and under three-year-old lucerne it is maximal (Figure 80).

Figure 80. Accumulation of mycolytic bacteria in soil under lucerne and cotton (Central Asia)

1--under lucerne; 2--under cotton.

  Under the conditions prevailing in the central belt of the Soviet Union, many mycolytic bacteria grow well and accumulate in podsol soils under clover and under certain other leguminous plants. Some of these bacteria dissolve fungi of the genus Fusariam, and others dissolve fungi of the genus Helminthosporium. Mycolytic bacteria do not grow uinder wheat and especially under flax, and they are removed from the soil relatively quickly.

  Daraseliya (1949) found a large accumulation of mycolytic bacteria in the rhizosphere of the tea plant. According to the author, the limited distribution of phytopathogenic fungi of the genus Fusarium in the soils of tea plantations is caused by the abundant growth of these bacterial antagonists.

  Nitrifying bacteria also grow in the rhizosphere of plants, where, under certain species, for instance under leguminous and certain nonleguminous plants, their number is considerably higher than under cereals and certain vegetable cultures. In studying the species characteristics of denitrifying bacteria, which were isolated from the rhizosphere of lucerne, wheat, and millet at the Ershov Station (Saratov Oblast'), we successfully determined some of their characteristics. The strains which were isolated from the root zones of wheat were in the most cases more active than the strains growing under lucerne and millet. Nitrate reduction was completed by the former in three to five days, while the latter completed this reaction under the same conditions in five to ten days. In the former case, the process was accompanied by the violent evolution of gas (nitrogen), while in the latter, the formation of gas was not observed or was negligible.

  This example of the growth of denitrifying bacteria is probably not general and was observed only under the soil-climatic conditions of the given region. In the serozem soils of Central Asia and in the podsol soils of the Moscow Oblast' we have not observed such specificity. Certain differences in the properties of denitrifying bacteria, growing in the rhizosphere of lucerne and wheat, were observed in the chernozem soils of Moldavia.

  Among other species of nonsporulating bacteria growing in the rhizosphere of plants, specific forms also probably exist which have better adapted themselves to the conditions of life under certain species of plants. Unfortunately, with the methods available we are unable to detect specialized forms under these or other plants. Certain investigators have observed different concentrations of physiological groups of bacteria under different plant cultures.

  According to observations made by Starkey (1929, 1931), the process of the formation of nitrate from ammonium sulfate in the rhizosphere of certain plants, was more intensive than in others.

  According to our observations, the root system of peas activates the process of the decomposition of cellulose. In our experiments we used special growth containers, filled with turfy podsol soil. One wall of the container (made of glass) was covered with filter paper. The plant grew from the container, which had been placed in a sloping position, and the roots were established so that they grew on the surface of the paper. The consecutive stages of the process of the destruction of the paper could be followed through the glass. Wheat, corn, peas, vetch, and beans were planted. The experiments showed that the roots of cereals do not exert a noticeable effect on the process of cellulose decomposition. Among the leguminous plants, beans proved to be ineffective, vetch only slightly stimulatory, while the roots of peas strikingly enhanced the process of the decomposition of paper (Figure 81).

Figure 81. Decomposition of the cellulose in soil under the influence of the root system of plants

a) destruction of paper around the roots of peas; b) destruction of paper in control soil, outside the root area.

  Lockhead and his associates (1950, 1955) demonstrated that the composition of the microflora of the rhizosphere of various plants differs with respect to vitamin requirements. In the root area of some species of plants, bacteria prevail which require vitamin B₁ or B₂ and, in the rhizosphere of other plants, bacteria requiring biotin, vitamin B₁₂, cysteine, methionine, etc are prevalent.

  There are indications in the literature, that algae also differ in their growth in the root area of plants. According to data obtained by Shtina (1954a, 1955), rye, timothy grass, and potatoes mainly enhance the accumulation of diatom algae; in the rhizosphere clover and lupine enhance the growth of green algae and perennial grasses, and potatoes partially enhance the growth of blue-green algae (Table 82).

  There are many indications that if unfavorable conditions prevail during the growth of the plant, more or less great numbers of phytopathogenic organisms grow and accumulate in their rhizosphere.

  Timonin (1941) showed that under a culture of flax considerable numbers of the following fungi often develop: Fusarium lini, Alternaria, Cephalosporium, and certain others causing plant diseases. Sanford and Broadfoot (1951) observed the growth of the phytopathogenic fungi Helminthosporium sativum and Fusarium culmorum in soil under wheat and oat monocultures. The fungus Ophiobolus graminis, under the conditions which prevail in certain localities in Canada, is suppressed by oats and clover and, according to Winter (1940), it grows better in the rhizosphere of wheat, than in the soil outside the root zone. Martin (1950) found an accumulation of the fungi Fusarium solani, Pyrenochaeta sp., and other species in the soil under citrus plantations. The author is of the opinion that the weak growth of the seedlings and saplings of citrus plants in the soil of citrus groves is caused by the deleterious effect of the microflora. Cotton is favorable to the accumulation in the soil of the phytopathogenic fungi Verticillium dahliae and Fusarium vasinfectum, causing it to wither, At the same time, lucerne suppresses the growth of these fungi. The accumulation of phytopathogenic fungi in soil under the influence of vegetative cover was also noted by other investigators (Lockhead and others, 1940-1950; Weindling, 1946; Eaton and Rigler, 1946; Timonin, 1946; Grammer, 1955, and others).

  Under certain conditions, plants can accumulate microbial antagonists in the soil, which inhibit the growth of such useful species of microorganisms as Azotobacter, root-nodule bacteria, and mycorhizal fungi, which are the producers of various biotic substances: vitamins, auxins, amino acids, and other products of microbes.

  Plants may also favor the growth and accumulation in the soil of the microbial antagonists of phytopathogenic bacteria, fungi, actinomycetes, and even viruses.

  Hildenbrand and West (1941) observed a lower rate in the root-rot disease in strawberries when they were sown after soybean, and an increased incidence of the disease when they were sown after clover. According to data obtained by Cooper and Chilton (1950), in the root zone and on the roots of sugar cane, actinomycetes, antagonists of the fungus Pythium arrhenomonas, which is the causative agent of the root disease of this plant, grow abundantly. Their number in the rhizosphere reaches 77,000 and more, while outside the rhizosphere, away from the root, it does not exceed 2,000 in one gram of soil.

  Similar data wam given by Sanford (1946, 1948), Weindling (1948), Fallings (1954), and others.

  Plants have a beneficient effect on soil. not only in relation to phytopathogenic microbes, but also in relation to pathogenic microbes affecting men and animals.

  Bogopol'skii (1948, 1950) studied the effect of the root system of various plants on the viability of bacteria of the colon group in the soils of urban plantations, in the parks and squares of Kiev. According to his observations, certain grasses used for lawns considerably hastened the death of these bacteria. For instance, in a plantation of sweet clover, after 60 days, 25 organisms out of one and a half million originally introduced were found, 45 were found under clover, and 110 were found under oats, of the total number originally introduced in the soil. Under orchard grass the colon bacillus almost totally disappeared (10 organisms per gram), while In the control zone without plants, 180 bacteria per gram were observed.

  According to our observations, the colon bacillus and a pyogenic staphlococci die in the soil under some plants quicker than under others (Table 83).

  On the tenth day after the introduction of the staphylococci, it could not be found under a grass mixture of lucerne and rye grass, nor under clover with timothy grass. Under clover it disappeared after 20 days and in fallow soil, after 50 days. The colon bacillus disappeared more quickly under clover than under a grass mixture or even under fallow soil. The same results were obtained by Mishustin (1954) in vegetation experiments with orchard grass, rye grass, brome grams, clover, and fescue grass. Two cultures were introduced into the soil, Bact. coli and Bact. coli aerogenes. They died sooner under clover and fescue grass. Under other plants these bacteria died much later.

  Arkhipov (1951, 1954) studied the extent of the growth in soils of the bacterium which causes anthrax under different plants, under conditions of vegetation experiments, and under field conditions. Wheat, rye, clover, vetch, lucerne, barley, Euagropyrum, potatoes, buckwheat, millet, lupine, flax, garlic, onions, etc were grown. Experiments showed that certain plants (garlic, winter wheat, rye, onions, rhubarb, and vetch) completely remove the anthrax bacillus from the soil. Lucerne, spring wheat, hemp, and the castor-oil plant exert a weak inhibitory effect. Ornithopus sativus, carrots, radishes, rape, water cress, and others have no effect whatsoever. Finally, such plants as potatoes, Eusgropyrum, horseradish, radishes, and turnips stimulated the growth and accumulation in the soil of the above microbe. On the basis of the results of his own studies and data obtained from the literature, V. V. Arkhipov recommended the sowing of winter wheat, rye, vetch, clover, rhubarb, garlic, and onions for the more rapid removal of anthrax bacilli from the soil.

  The selective action of plants on the microflora is not only caused by the specificity of the nutrient substances excreted by the root system, but also by special antimicrobial compounds. In the chapter on toxicity of soils we shall give data showing that many plants form and excrete different toxic substances into the environment. Among them there are many compounds which strongly inhibit the growth of certain species of microbes.

  Earlier we noted (1934b, 1939) that the root excretions of wheat, corn, flax, and some other plants visibly supress the development of certain kinds of bacteria--Azotobacter, sporiferous bacteria, and other groups both under sterile experimental conditions and directly in the soil in their natural surroundings. It was shown that the root excretions of corn act differently to those of wheat. Under the influence of these excretions, the cells of the bacteria undergo a considerable deformation, degeneration, involution, and subsequently die. Sometimes this degeneration of the culture is accompanied by the birth of new forms and species.

  The presence of antimicrobial substances in the root excretions of plants was observed by Sidoranko (1940b), Meshkov (1953) and others. The soil studies carried out for many years in the Wareham forests (in England) by Rayner and Nelson-Jones (1949) showed that the obstacle standing in the way of afforestation of certain soil zones is not the lack or insufficiency of nutrient elements but the presence of special organic substances. These substances, according to these authors, retard the growth not only of young saplings of various wood varieties but also that of many microorganisms. Rayner has shown that by introducing organic fertilizer manure, or compost into these poisoned soils, the toxicity diminishes or even vanishes. According to her observations, this decrease in toxicity takes place due to the activation of the microflora in the presence of fresh organic substance.

  Extensive studies of the antimicrobial properties of toxins excreted by plants were made by Metz (1955). The root excretions and root sap of 100 species of plants were studied. It was shown that 10 species out of 100 (Armoracia rusticana Gaertn, Chelidonium majus L., Crepis viren K., Hieracium. pilosella L., Hypericum perforatum L.., Lampsana communis L., Ranunculus acer. L., Viola silvatica Schm., V. tricolor Wittr., Pulmonaria officinalis L.) greatly inhibited the growth of sporiferous bacilli or micrococci; 15 species--Brassica campestris L., B. napus L., Campanula rapunculoides L., Capsella bursa pastoris Mad., Fumaria officinalis L., Ranunculus repens L., Paeonia officinalis L., Sinapis alba L., Galium verum L., Allium schoenoprasum L., and others inhibited their growth moderately; 31 species: Achillea millefolium L., Aethusa cynapium L., Avena sativa L., Chrysanthemum lencanthemum L., Cichorium intybus L., Datura stramortium L., Hordeum sativum Gessen and others inhibited the growth of these bacteria only slightly. The remaining species of plants did not have any inhibitory effect.

  Stiven, (1952) found that in the soil under the plants Tragopogon plumosus L., and Pentanisia variabilis Harw., the processes of nitrification and the growth of Bac. subtilis, Bac. coli, and others are inhibited considerably. Pure substances obtained from the root excretions of these plants have the same effect.

  The action of root toxins is not specific, according to Metz. The roots inhibit the growth of bacteria, both when they were isolated from the rhizosphere of the given plant and when they were isolated from that of another species. For example, the roots of Chelidonium majus L. inhibit, to the same degree. the growth of bacteria isolated from their own root zone and those from the root zones of goutweed, violet, hawkweed, and others, and also bacteria from fallow soil. However, the author notes, the root-zone microflora is less sensitive to the action of the roots than organisms from outside this zone. The growth of bacteria and mycobacteria, isolated from fallow soil or from soil outside the root zone, in the majority of cases, is inhibited by the roots of many plants, while most of the bacteria of the rhizosphere are not inhibited at all.

  According to our observations, the roots of lucerne and peas, under conditions of growth in a sterile nutrient solution. excrete substances which inhibit the growth of Azotobacter chroococcum and Pseudomonas fluorescens, certain species of rootnodule bacteria, and others.

  The growth rate of the bacteria from two-month-old plants grown in a solution was determined (Table 84).

  The longer plants have been growing in a solution, the more strongly the solution affects the bacteria. After peas grew for three months in the solution, Azotobacter strain No 54 and strain A did not grow at all, nor did the root-nodule bacteria of clover and sweet clover.

  Antibacterial substances are also excreted by isolated roots if the latter are grown in an artificial nutrient medium. We cultivated the roots of peas. lucerne, lupine, and certain other plants in Bonner's nutrient solution for one to three months and, after various periods of time, we determined the presence of antibacterial substances in it. The bacteria were introduced into the solution and, by means of plating, their viability and degree of multiplication were established, Cultures of Azotobacter strain No 54, a halophilic strain and a garden strain, and, in addition, root-nodule bacteria of peas, vetch, lucerne, and others, were sown into the solutions.

  The results of the experiments showed that the excretions of roots grown in an isolated form act on the same species of bacteria as do the excretions of non-isolated roots. Only the degree of their inhibition was weaker. The antibacterial spectrum of root excretions was the same in both cases. This indicated the fact that isolated and nonisolated roots excrete the same antimicrobial substances.

  Thus, by this experiment, it was observed that certain antimicrobial substances excreted into the medium, are directly synthesized in the root system of the vegetating plant.

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