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Part IV, continued: 

  Increased accumulation of microbes in the root soil was first observed by Hiltner in 1904. He proposed the term "rhizosphere." In investigating the root system of various plants, Hiltner came to the conclusion that the accumulation of microbes in this area was not accidental and that it was caused by the biological activity of the roots.

  It should be noted that some investigators before Hiltner observed the accumulation of certain species of bacteria in the root region, but they looked upon this phenomenon from a narrowly specialized point of view.

  Epstein (1902) observed the development of special bacteria on the roots of beets, which differ from those of the soil. Welich (1903), Grüber (1909), and Maassen (1905) found large numbers of Clostridium gelatinosum cells in the rhizosphere of the sugar beet.

  After Hiltner, the phenomenon of the accumulation of microorganisms around roots was observed by Layon (1918). He pointed out that the root systems of plants strongly change the natural microbial associations of the soil. The ratios between species in the rhizosphere microflora, according to this author's observations, differ from the ratios prevailing in the microbial biocoenoses of nonrhizosphere soil. Hoffman (1914). Maaasen and Behn (1923), Stoklasa (1926) and Richter and Werner (1931) observed the increased development and accumulation of fungi in the rhizosphere.

  A more detailed and systematic study of root-region microflora began in the 19301's.

  Detailed studies made by Starkey (1929, 1934) showed that the roots of plants have a considerable influence on the accumulation of microorganisms in the soil. According to his observation, the number of microorganisms in the rhizosphere is several times higher than that in ordinary soil outside the root area. For example, on the rhizosphere of beets, 427 million bacteria per gram were found, while in the control soil only 8.2 million per gram were found; in the rhizosphere of clover, he found 11,320 million per g and in the control soil only 6.6 million per g; in the rhizosphere of wheat, 653.4 million per g were found; and in the control soil, only 22.8 million bacteria per g were found. Pochenrider (1930) studied the microflora of the soil in the root region of cruciferous plants and also observed the intensified development of certain bacterial species (Azotobacter and others).

  The most numerous and detailed investigations were performed by the Soviet specialists, Isakova (1934-1940), Krasil'nikov and others, (1934, 1945), Sidorenko (1940), Obraztsova (1936), Berezova (1941, 1945), Korenyako (1942), and many others. Berezova published data on her study of the microflora of the rhizosphere of flax; Isakova on that of rubber plants, tangerines, tung trees, and the tea plant, and Obraztsova, on the tea plant.

  Large accumulations of microbes in the region of the roots of forest plants were found by Samtsevich and his associates (1949) and by Kozlova (1953).

  Romeiko (1954) observed the development and accumulation of microflora in the rhizosphere of hops. Noting the concentration of microbes in the root region the author emphasized the connection between this phenomenon and the developmental phase of the plant.

  Popova (1954) obtained data on the intensified accumulation of microbes on the root systems of vines which grow in the serozem soils of Uzbekistan.

  Our investigations (1934-1945) of the soil of the root area have shown that plants in any soil under different climatic conditions, regardless of the zone, possess the capacity of concentrating soil microorganisms in the vicinity of their roots, We studied the microflora of cereals (wheat, rye, oats, barley) grasses (orchard grass, rye grass, etc), leguminous plants (beans, peas, vetch, clover, lucerne, etc), commerical crops (cotton, tobacco, flax, hemp, etc), and several varieties of fruit trees and decorative plants (acacia, poplar, lime tree, oak, apple, plum, pear, etc). Studies were performed in various parts of the Soviet Union: in the South--in Crimea, in the Caucasus, Armenia, and Georgia, on the coast of the Black Sea, in Central Asia, and on the fields of the Tadzhik, Uzbek, and Kirgiz Republics, in Kazakhstan, in the lowlands of the Volga area and in the Astrakhan steppes; in the Ukraine, Moldavia, Siberia, on the Sakhalin; in the northern regions of Yakutiya, Igarka, and the Kola Peninsula. The microflora of plants of the moderate region was also studied: Leningrad, Moscow, Yaroslavl', Kuibyshev, and other regions, and the plants of Belorussia, Latvia, Estonia, and other places,

  According to our observations, the total number of microorganisms in the zone of the plant roots exceeds the number of microbes in ordinary soil by tens, hundreds, and thousands of times. The differences in the quantitative relationships depend on the species of plant and the soil-climatic conditions (Table 73).

  In recent years, the microflora of the rhizosphere has gained more of the attention of many foreign specialists. In addition to the above-mentioned studies, made by Starkey, others have been made by Lockhead and his associates, and by Jensen, Katznelson, Timonin, and others, on the bacteria of the rhizosphere.

  All of these investigators obtained data on the considerable accumulation of microorganisms in the root region. Katznelson (1946) found 100-1,000 times more microbes in the rhizosphere of beets than were found outside the root zone. Thom and Humfield (1932) obtained the following data: in the rhizosphere of corn, there are 136 million bacteria per one gram of soil, while outside the rhizosphere (control) only five million.

  According to Starkey ( 1955), in the rhizosphere of beans, there are 200 million bacteria per gram of soil; in beets, 427 million per gram; and in grain crops, 653 million per gram. The control samples of soil contained one to five million bacteria per gram of soil.

  The poorer the soil in organic substances, the less fertile it is, and the weaker is the influence of the root system on the quantitative composition of the microflora of the rhizosphere. The quantitative relationship of the microflora of the rhizosphere (M.R.) to that of the control (M.C.) is most strongly expressed in poor soils. It is more marked in the primary, slightly fertile podsol soils of the Kola Peninsula than in the chernozem soils of Moldavia or Kuban,.

  In the primitive noncultivated or slightly cultivated soils of the Kola Peninsula the total number of bacteria reaches from 50,000 to 300,000 cells per gram. In the rhizosphere of clover in its first year of life, there are 150-400 million per gram. The M.R. to M.C. ratio is 1000:1. In the well-cultivated chernozem soils of the Khar'kov Oblast, the number of bacteria in the rhizosphere of Lucerne is from 2,500 to 5,000 million per one gram, and outside the rhizosphere, from 150 to 300 million per gram. The M.R. to M.C. ratio is 17:1.

  Central Asia's slightly cultivated serozem soils contain comparatively small numbers of bacteria, within the range of five to ten million per gram. In the root region of lucerne in its first year of life, we counted up to 1,000 million bacteria per gram. This same soil, after five to eight years of cultivation and the introduction of the corresponding fertilizers, contained 7,000 million bacteria in the rhizosphere of one-year-old lucerne, and, outside the roots, 60 to 100 million organisms per gram. The M.R. : M.C. ratio was 100 to 200) in the first case, and 70 to 100 in the second.

  Such a shift in the M.R. : M.C. ratio was observed by us in chernozems, podsols, serozems, chestnuts, and other soils, regardless of the geographical region, Only the quantitative expression differed.

  The quantitative ratios between the number of microbes in the rhizosphere and outside it increase, with the depth of the penetration of the root system. In the lower layers, the numerical indices are expressed more strongly (Table 74).

  At a depth of 90-110 cm in the rhizosphere of clover, growing in podsol soil, the number of bacteria is 2,000 times higher than outside the rhizosphere, and in the rhizosphere of lucerne in chernozem soil at the same depth, it os 270 times higher. Approximately the same ratios are observed in the case of wheat.

  In the deep layers of soil there are usually very few bacteria, while in the rhizosphere of plants, even at the depth of two to three meters, they grow abundantly.

  Under conditions of deep growth, the root systems of plants create favorable conditions for the growth of microorganisms, not only by their excretion of nutrient substances, but also evidently due to such factors as the improvement of their conditions of respiration and metabolism.

  The number of microbes growing in the root area varies with the ago of the plant. As should be expected, the maximal number of bacteria is observed during the period of the most active growth of the plant. The more intense are the life processes, the more organic substances are excreted by the root, and the more intense the multiplication of microbes in the rhizosphere. Observations show that an abundant growth of microorganisms takes place in the early stages of the plant's growth; however, the most vigorous growth of microbes ensues during the period of flowering and in the period directly preceding it. (Figure 70). Sometimes one also observes three elevations in the growth curve of microbes: the first small elevation is seen in the early stage; a second great elevation ensues before and during flowering; the third elevation occurs before ripening. The last is usually barely noticeable.

Figure 70. Quantitative composition of the microflora of the root area during different phases of the plants' growth

  Under conditions of irrigation, small rises in the growth curve of microbes are observed after each irrigation (Krasil'nikov, Kriss, and Litvinov, 1936b). Under these conditions, they are caused by the increase in soil moisture. The question of whether the moisture was the direct cause of the enhanced growth of the microbes or whether this growth was strengthened as a result of an increased amount of nutrient substances excreted by the roots due to the higher intensity of metabolism in the plant may probably be answered as follows: both mechanisms are operative.

  When soils in the rhizosphere are analyzed after harvest, one can observe that the activity of microorganisms does not cease. In the presence of moisture and higher soil temperatures, a sharp increase in the number of microbes in the root zone is observed. In this case, the dead roots are subjected to intense decomposition. The number of microbes during this period may quite often exceed that of the rhizosphere of living plants during rapid growth.

  In cases when there is little moisture in the soil (in arid regions), the root residues after harvest decompose slowly and there is no noticeable increase in the number of microbes. The roots remain in the soil undergoing no major changes for long periods.

  Differences in the quantitative composition of microbes in the rhizosphere of different plants are slight. In our investigations we could not detect regular and significant differences in the numbers of bacteria in the rhizospheres of different plants of the cereal and leguminous families. In the same field sector, under the same agrotechnical and soil-climatic conditions, the total number of bacteria in the rhizospheres of wheat, oats, and barley are approximately the same. When comparing the microflora of the rhizospheres of cereal and of leguminous plants, one notices a slightly higher number of microbes in the leguminous type. However, this difference is not always noted.

  As to the nature of microbe distribution in the rhizosphere of vegetating plants, we noticed regular diffuse growth of microbes, as well as the existence of more or less isolated colonies.

  In the literature, one finds indications that microbial cells are present not only on the surface of roots but also inside them, having penetrated the tissue of the epidermis, the intercellular substance, and also the protoplasm of the cells themselves (Berezova, 1953; Rempe, 1951; Hennig and Villforth, 1940). According to the data obtained by Schanderl (1939, 1940), the cells and tissues of plants are not sterile and always contain bacteria. In his last paper, the author claims that plant cells grow in symbiosis with bacteria. The latter are present in the protoplasm in greater or smaller amounts.

  Our investigations did not confirm Schanderl's data. Neither in the tissues nor in the cells of healthy plants could we detect bacteria, fungi, or actinomycetes. Upon microbiological analysis, the plant tissues always remained sterile. Even the roots of leguminous plants did not have bacteria in their tissues outside the nodules. Other scientists also, (Burcik, 1940; Schaede, 1940) did not detect bacteria in tissues of healthy plants. Detailed studies were recently made by Stolp (1952). This author attempted to verify Schanderl's data. He investigated various plants, leguminous, cereals, and others. in not one case did he find microbial cells in plant tissues. Bacteria can penetrate into the dying tissues and cells when the re sistance of the latter is weakened. For example, the root hairs, upon dying, are completely filled with bacterial cells. Obviously, Schanderl and others studied such dying cells and tissues, or they accepted as bacteria the intracellular structures.

  In order to elucidate the nature of the distribution of microorganisms on and in the vicinity of roots, we employed the method of growing vegetation on glass plates mounted on special vegetative frames, set up in such a way that the roots of the growing plant spread on the glass, leaving their imprints on them. For these frames we used flat boxes with glass walls, 5-6 cm wide, 20-25 cm high and 50 cm long.

  The frames were filled with soil or sand and the plants were grown in them. In order to make sure that the roots would stick to the glass, the frames were arranged at a slope of 40-50° (Figure 71). The roots, due to geotropism, grew in a downward direction and touched the glass, sticking tightly to its surface.

Figure 71. Frames for growing plants:

A--frame in a sloping position, for obtaining imprins of roots and microflora on glass; B--nature of the growth of roots on the lateral side of the frame (glass).

 

  For the sake of the convenience of microscopic examinations, we placed microscopic slides on the inner side of the glass surface of the vegetation frame. After certain time intervals, the glass with the roots growing over it was taken out and examined microscopically after having been previously stained or left unstained,

  When the growth of microorganisms was abundant, one could see with the naked eye a zone of film, measuring three to five mm in radius, around the root, This zone was especially noticeable on the glasses of overgrowth which had been immersed in sand (Figure 72). The sand was easily removed from the surface of the glass while the microbial cells remained. One found by simple microscopy that the microorganisms grow diffusely in the zone of the roots in the form of isolated colonies or small foci. The colonies are located between the hairs on the surface of the roots, or in their vicinity (Figure 73). In cases where there was a great deal of moisture covering the roots and the root hairs, the bacterial cells spread. often occupying considerable segments along the roots (Figure 74).

Figure 72. Imprints of root branches on glass with surrounding microflora:

A--natural size of the slide; imprints of roots in the form of hairs (a) are seen; B--the same preparation magnified 1:100; branch of the root overgrown by hyphae of fungi and actinomycetes.

Figure 73. Colonies of bacteria and actinomycetes on the surface of roots:

a--colonies of actinomycetes (marked by +), b--bacterial colonies, c--colonies of mycobacteria (our own observations); d and e--bacterial colonies (after Linfold, 1942).

Figure 74. Diffuse distribution of bacteria on root surfaces:

a--nonsporiferous bacteria; b--mycobacteria; c--nonsporiferous bacteria and mycobacteria.

  Isolated colonies, as wen as profuse areas of microflora, mainly consist of nonsporeforming bacteria and mycobacteria. One also finds colonies of actinomycetes and mycelia of fungi. Very seldom does one see small colonies or single cells of sporeforming bacteria.

  Nonsporeforming bacteria and mycobacteria form well-defined, compact colonies, located most often on the surface of the roots, between the root hairs, and also at a certain distance from them (Figure 75). Colonies of actinomycetes often consist of proactinomycete elements, (Figure 75), or of entangled mycelia. Fungi also grow around roots in the form of single hyphae and mycelia.

Figure 75. Colonies of actinomycetes around roots. The zone of massive growth of bacteria, at a certain distance from the root, is clearly seen

  If one prepares imprints of the root area of microflora growing in sand, and not in soil, one does not encounter this picture. Upon the microscopic examination of these imprints, one can observe only mycelial threads of fungi and actinomycetes, occasionally with sporangiophores on branches. One seldom observes single colonies of actinomycetes, Around the root branches and hairs bacterial cells are also encountered, but in limited numbers and, as a rule, they appear as single cells or in pairs and very seldom in colonies. When such a preparation is being grown, by covering it with a thin layer of an agar medium, a considerably larger number of bacteria and mycobacteria appear than can be observed under direct microscopy. The great majority of cells are not detected by microscopy. This in due to the fact that the cells of bacteria and mycobacteria, and possibly certain actinomycetes, exist in a fragmented state in the form of small granules hardly discernible from soil particles. These granular elements, stained with erythrosin, are encountered in the soil of the root zone in great numbers. They probably form the major part of the mass of the rhizosphere microflora. These elements, when transferred to a nutrient medium, grow out, giving rise to cells of normal size, which are detectable under laboratory conditions.

  The methodof overgrowing the root zone ofplants on glass was employed by Linfold (1942). He grew seeds of corn, lettuce, pineapple, and the cowpea (Vigna sinensis) in soil in a special chamber. The imprints obtained on glass plates were microscopically examined. The author observed an abundant growth of bacteria in the form of large colonies around the roots and root hairs; colonies were often located at the hair tips (Figure 76). At a certain distance from the roots, according to the author, amoebae, Infusoria, and nematodes grew.

Figure 76. Bacterial colonies on the tips of root hairs (according to Linfold, 1942)

  Starkey (1938) employed the method developed by Rossi-Kholodnii for the study of root-area microflora. He found that the bacteria grew on the roots in clusters, and fungi and actinomycetes, in the form of threads. The author buried the slides in the soil. under the root system. Of course, he could not see the greater part of the bacteria on the overgrown glass. As in our experiments, the bacteria were probably in a fragmented state and could not be detected among the numerous soil particles without being cultured.

  Stille (1938), using this method, found an abundant growth of bacteria around the root hairs.

  These data show that, in the root zone, the microflora probably grows in the same way as it generally does in the soil, in colonies or in aggregates. Only when there is a high moisture content in the substrate do bacterial cells grow in extensive areas.

  Group composition of the microflora of the root area. Studies have shown that around and on the roots of vegetating plants one finds various representatives of microorganisms--bacteria, actinomycetes, fungi, algae, yeasts, protozoa, phages, and other living organisms.

  However, the prevailing group in the rhizosphere, regardless of the conditions of growth and the age of the plants, are the nonsporiferous bacteria,

  The second place (largest group) among rhizosphere microflora is occupied by mycobacteria.

  The remaining forms of microbes, actinomycetes, fungi, sporiferous bacteria, etc are encountered in much smaller numbers. The quantitative ratios of these microorganisms can be found in any plating of rhizosphere soil on agar media containing protein or on synthetic media. In Table 75 data are given on the of different groups of microbes growing in the rhizospheres of various plants immeditately before flowering. They were obtained by plating one drop (0, 05 ml) of a 1:1,000 dilution.

  Approximately the same group-composition ratio of rhizosphere microflora is also found in other plants. Nonsporeforming bacteria are the prevailing group among the rhizosphere microflora of wheat, oats, clover, lucerne, cotton, and other plants either growing in the southern regions on chernozem, chestnut, and serozem soils, or in the northern regions in podsol or other soils. In some cases, deviations in the number of bacteria of the different groups may exist, the percentage of this or that microbial group may change, but the general nature of the ratio between the different groups of microbial biocoenoses in preserved.

  The group relation of the microflora in the root area varies considerably with the age of plants.

  It was noted above, that upon the ripening of plants, the total number of microorganisms in the rhizosphere decreases. During this period the quantitative ratio between the different representatives and groups changes; the number of sporeforming bacteria, fungi, and actinomycetes increases, and new organisms appear. At the same time, the total amount of nonsporeforming bacteria decrease, some species disappearing altogether, etc.

  On the diagram in Figure 77, data are given on the analysis of the rhizosphere microflora of oats during different periods of its growth, obtained in studies of the plants of the Trans- Volga region (Krasil'nikov, Rybalkina, Gabrielyan, Kondratleva, 1934).

Figure 77. Quantitative relationship between microorganisms at different stages of the plant's growth (oats):

A--flowering period; B--period of ripening.

  In later works we noted the same changes in the ratios of the representatives of the rhizosphere microflora of many other plants, growing in different areas on differing soils (Krasil'nikov, Kriss, and Litvinov, 1936).

  The change in the quantitative composition of rhizosphere microflora with the age of the plant was also noted by Starkey (1938), Isakova (1939), and some other investigators.

  Nonsporeforming bacteria comprise the main, and the most numerous and versitile group of soil microflora, in general. This group embraces representatives of various families, genera, and species; these include such organisms as Azotobacter, rhizobia, thiobacteria, photobacteria, Azotomonas, Sulfomonas, nitrifying bacteria, denitrifying bacteria, and others.

  All these representatives are encountered in the rhizosphere of plants. Most frequently, organisms of the genera Bacterium and Pseudomonas reach the largest numbers, These organisms, comprise the main microflora of the root area,

  The species composition of these organisms is scarcely known. We have no adequate methods for the diagnosis and differentiation of these bacteria and we are not in a position to distinguish between the nonsporeforming bacteria in the rhizosphere of one genus of plants and those of another. Only a few genera can be accounted for: Azotobacter, Rhizobium, and some others.

  Kostychev and others (1926) were of the opinion, that Azotobacter adapted itself to the rhizosphere of certain plants: tobacco, rice. and others and is their essential companion.

  Later, many investigators found Azotobacter in the rhizosphere of plants. Sidorenko (1940b, c) studied its growth in the root region of wheat, barley, beets, lucerne, sudan grass, soybean and other plants grown on the chernozem soils of the Khar'kov Oblast'. Popova (1954) found Azotobacter in the rhizosphere of grape vines: Obraztsova (1936) and Isakova (1935) found it in the root zone of subtropical plants: lemon, tangerine, and tung tree; Daraseliya (1950) found it in the root zone of the tea plant. Raznitsyna (1947) and Tanatin (1949, 1953) observed the distribution of Azotobacter in the soils of Central Asia under cotton, lucerne, wheat, and various vegetable crops. Pavlovich (1953) studied the distribution of Azotobacter in the rhizosphere of various plants in the Latvian Republic; Banak (1956), in Moldavia; Gerbart (1952), in the western Ukraine districts, Petrosyan and his associates (1949) and Afrikyan (1953) in Armenia.

  According to our observations, the growth of Azotobacter takes place in the root area of various plants under differing climatic conditions and in different geographical regions, from the northern regions to the southernmost points, on mountain tops and in valleys. The growth of Azotobacter is observed in the rhizosphere of forest trees (Samtsevich and others, 1952); Krasil'nikov, 1945), fruit trees, bushes, and in other plantations (Kanivets, 1951; Kulikovskaya, 1955, and others).

  The number of Azotobacter organisms in the rhizosphere of plants may reach considerable dimensions. We counted from 0 to 200 million Azotobacter cells in 1 g of soil in the root area. Tanatin (1953) counted tens of thousands and hundreds of thousands in a gram of rhizosphere soil. Approximately the same numbers were found by other investigators (Pavlovich, 1953; Babak, 1956; Raznitsyna, 1947).

  No less frequent are the rhizobia. Their identification in the rhizosphere is not only of diagnostic. value but also of practical significance, Depending on the abundance of their growth in the root area of leguminous plants, their effectiveness will differ under the same conditions of activity.

  Root-nodule bacteria, as is well known, grow abundantly on the roots of those plants on which they can form nodules. However, they can also grow on roots of other plants. For instance, the nodule bacteria of lucerne grow luxuriously on the roots of lucerne and also in the root zone of cotton; the nodule bacteria of clover also grow well in the rhizosphere of lucerne, peas, and certain other plants. The nodule bacteria of peas grow well in the root zone of peas, clover, wheat, etc. The total number of nodule bacteria in the rhizosphere of plants may be considerable. Korenyako (1942) counted them in hundreds of thousands and in millions per gram of soil. Similar data is given by Raznitsyna (1947), Petrosyan and his associates (1949), Petrosyan (1956), and others.

  Ammonia-producing bacteria, denitrifying bacteria, and certain other groups of nonsporeforming bacteria grow abundantly in the rhizosphere. Of the first two groups many millions and billions are present per gram of soil in the rhizosphere (Krasil'nikov, Rybalkina, and others, 1934; Krasil'nikov, Kriss and Litvinov, 1936, Raznitsyna, 1947; Starkey, 1928; and others).

  Nitrifying bacteria, cellulose bacteria, nitrogen-fixing anaerobic bacteria of the genus Clostridium, and certain other groups do not grow well, comparatively speaking, in the rhizosphere (Rempe, 1252; Krasil'nikov, 1934, 1936).

  The second place, with respect to their quantitative growth in the rhizosphere, is occupied by mycobacteria. Their number in the root area reaches hundreds of thousands and millions. Their species composition was not studied. Most often one encounters the nonpigmented forms of Mycob. album, M. mucosum, and others.

  One finds sporeforming bacteria in the rhizosphere of plants much less frequently and in smaller numbers. In general, they comprise only a fragment of one percent of the microflora. They are especially scarce during the period of vigorous vegetative growth of plants. Usually these bacteria begin growing abundantly at the end of vegetation, especially on dead, decomposed roots.

  Actinomycetes also occupy a small place among root-area microflora during the early stages of the development of plants. This group of organisms is very widespread and versatile in its species make-up. In the rhizosphere, there are various representatives of actinomycetes. Toward the end of vegetative development their number increases considerably. They multiply with special intensity on semidecayed dead roots. One often sees rootlets covered completely with the mycelia of these organisms in the form of a fluffy or a mealy-white coating.

  Fungi are detected in the rhizosphere by the conventional analyses of small quantities.

  It is known that certain plants have a well-developed fungal coating on their roots, coalescing with the root tissue. This: fungal coating is called mycorhiza. Depending on the nature of its relation to the roots one can distinguish between endotrophic and ectatrophic mycorhiza. Mycorhiza fungi are widespread in the root systems of many plant species, both woody types and grasses. Some investigators are of the opinion that all plants have mycorhiza. The fungi participating in the mycorhiza belong, according to their systematic positions, to different classes and orders, families and genera. It is supposed that these organisms are of great importance to the plants. However, this problem has been only slightly studied (Lobanov, 1953; Reiner and Nelson-Jones, 1949; Kelly, 1952).

  Many plants do not grow well without mycorhiza fungi, and some do not grow at all, However, one seldom encounters roots with an abundant growth of these organisms in grassy field crops.

  It should be noted that upon an ordinary microbiological analysis of the rootlets, one observes, as a rule, single mycelial hyphae. Fungi are detected by special studies with the use of special analytical methods.

  The question of algal growth in the root area of vegetating plants has been only slightly studied. The research done by Katznelson (1946) and Shtina (1953, 1954 b) showed that various algae live in the rhizosphere of plants in considerable quantities., Their total number reaches tens and hundreds of thousands in one gram of soil.

  Shtina studied the growth of algae in the rhizosphere of rye, timothy grams, clover, lupine, potatoes, barley, and oats. Among some of these plants, the number of algae in the root area was two to three times higher than outside it (rye, timothy grass, clover, lupine). For example, in the root zone of clover, 149,000 cells were found in one gram of soil, and in the control area (outside the rhizosphere), only 99,000 cells per gram were detected.

  Qualitatively, the algae composition within the rhizosphere is approximately the same as outside it. It consist mainly of diatoms, green and blue-green algae (Shtina 1954 a and b). They probably also have a certain importance in the life of root-area biocoenoses.

  In the rhizosphere of plants, one observes invertebrate animals: protozoa, nematodes, insect larvae, etc. The number of these organisms in the root area of healthy plants is small and does not exceed a few thousands per 1 m² under various plants. Shilova (1950) studied the soils under grasses, rye, and lupine, in long fallow, and fallow soils. According to her calculations, in 1 m of the surface horizon (0-10 cm) of soil, there are from 7,000 to 187,000 invertebrates, and the number of certain members of Apterygota in this area, under grasses, reached 760,000.

  The composition of this fauna is extremely diverse. One finds in the rhizosphere members of Acarina (mainly under forest cultures), and representatives of Apterygota. Enchytraeidae, and others (Shilova, 1950; Gilyarov, 1949, 1953). Katznelson (1946). Linfold (1942), and Brodskii (1935) have described the distribution of the protozoa, amoebae, ciliates, flagellates, and others in the soil of the root area. These organisms are frequently encountered when studying the soil of the root area in ordinary laboratory studies.

  Nikolyuk (1949) has established that there are two to three times more protozoa in the root zone of lucerne than outside it.

  The greatest number of these organisms are accumulated in the rhizosphere of cotton during its first year of cultivation (up to 100,000 in 1 g soil). The author ascribes this accumulation of protozoa to the abundant growth of bacteria in the rhizosphere, which serves as nutrient material for the protozoans.

  Ressel (1955), Brodskii (1945). Nikolyuk (1949), and others ascribe great importance to this group of organisms, as a factor affecting the composition of microbial biocoenoses in the soil.

  Quite often one encounters worms and nematodes in the root zone of plants. Certain nematodes, as is well known, grow well on roots and, in penetrating into plant tissues, cause diseases. Arkhipov (1954) noticed the death of nematodes under certain plants.

  Pathogenic microbial forms--bacteria, actinomycetes and fungi--often grow and accumulate in the root area. Under certain conditions, these organisms penetrate the tissues of plants. in agricultural practice, cases of mass infections of plants, as a result of the growth of pathogenic fungi and bacteria in the root zone are not infrequently encountered. The development of microbial antagonists inhib iting phytopathogenic organisms in the rhizosphere is also possible.

  In general, many different forms of organisms may grow in the rhizosphere of plants, both useful and harmful; those which facilitate the nourishment and develop ment of plants, and, an the contrary, those which inhibit and poison them. The prevalence of these other organisms depends on soil-climatic conditions, on the manner in which the farm is handled, and on the whole agrobiological complex.

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