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  The diversity and abundance of the living population of the soil was discussed in the preceding chapter. The living mass of microbes: bacteria, fungi, actinomycetes, and algae, by itself comprises more than 10 tons in the plow layer of 1 hectare of a well-cultivated, fertile soil. Besides, a great number of protozoa, insects, worms, and other representatives of the living world exist in the soil.

  All this soil population performs an immense work, processing considerable amounts of different mineral and organic substances, decomposing plant and animal residues, participating in the transformation of their decomposition products.

  Microbes synthesize and excrete different metabolic products, which, upon entering the soil, confer upon it fertility.

  Thanks to the microbial population the dead, loose, geological formation assumes life and becomes a productive body. Conditions necessary for the nutrition and growth of higher plants are thereby created. Microorganisms constitute the most important and indispensable link in the nutrition of plants.

  Microorganisms of the soil, not only create the conditions necessary to the growth of higher plants, but also have a direct effect on them through their metabolic products. This chapter deals with the effect of microorganisms as agents of mineralization of organic compounds of plant and animal residues and as a biological factor necessary for the normal nutrition and growth of plants.

  To obtain high yield, fertilizers have been used in agriculture from ancient times. Men used fertilizers long before science established and solved the main problems of soil science, agriculture, agrochemistry, and plant physiology. The rules of fertilization were worked out empirically but, as Pryanishnikov pointed out, many of these rules attained high accuracy.

  The Romans, for example, knew of the valuable fertilizer properties of animal excrements and of some mineral substances such as ash, gypsum, calcium and marl. Moreover, they knew that the fertilizing value of excrements of different animals varied. Of highest esteem was the excrement of birds.

  The Romans also knew about green fertilizers. Thus, they plowed in green manure on the slopes of Vesuvius in order to increase the fertility of the soil.

  The theory of plant nutrition had not yet been elaborated. Vague assumptions on the presence of some "fat" of the earth (terrae adeps) were made, This "fat" was responsible for the fertility of the soil. In order to -increase the amount of this "fat", animal excrements were necessary.

  Those assumptions contained the nucleus of the humus theory, which subsequently became widespread. This theory assumes that the organic compounds are of utmost importance in the nutrition of plants. "Echoes of these ideas can be heard in the language of different nations. Let us recall that our word 'TUK' (fertilizer) once had two meanings, one of them being 'fat'." (Pryanishnikov, D. N., Plant Nutrition, Coll. Papers, Vol 1, 1952, p 58).

  The humus theory of plant nutrition stated that plants feed on organic substances present in the excrements of animals and in general in humus. This theory, widespread in the 18th century, was favored by the plant breeders and agrotechnicians of that time, since it was well confirmed by agricultural practice.

  The explanations of the favorable effect which organic substances had on the growth and fertility of plants differed. The most widespread conception was the assumption that the plant derives (from the organic substances) carbon which it subsequently incorporates into its body (Davy, 1813).

  Other authors thought that the excrements contained certain special substances. Thus, for example, Prof. Vallerius in 1766 assumed that only the organic substances or "fat" substances of humus of soil play any role in plant nutrition, other components of the soil serve merely as "fat" solvents.

  Some scientists of the 16th--18th centuries and in particular Olivier de Serres, (1600), assumed that the cause of the action of fertilizer is in its heat.

  Bernard Palissy in his paper (1563) put forward the idea that the salt content of the fertilizer plays a role in the growth of plants.

  First experimental attempts to elucidate the problem of plant nutrition were made by Van-Helmont. In his paper (1629) he presented results of five years of experiments on growing ivy branches in soil which was given only rain water. In 5 years the branch grew and attained a final weight which was 33 times higher than the initial weight. The soil thereby did not lose weight. Since the composition of the air was not known in those times, Helmont concluded that the plant utilized only water and this was sufficient for the plant to build its body.

  Analogous conclusions were reached by the French investigator Duhamel. He grew plants in Seine water. He did not have distilled water at his disposal for control experiments, and positive results obtained by him are therefore not trustworthy, since the water from the Seine contained various organic and inorganic substances.

  Woodword showed that the point of view of Van-Helmont was erroneous. In his experiments he demonstrated that mint grows better in river water than in rain water and it grows even better and gives greater increase if soil is added to the water. He presents the following data: the plant-weight gain in grains* during 77 days, was as follows: in rain water, 17; in the sewage water of Hyde Park, 139; the same with addition of soil, 284. [*"Grains" is a unit of weight.]

  Woodword concluded that not only water but also something else which is present in the soil is necessary for the growth of plants.

  The agricultural practice of those times did not possess any scientific basis for the elucidation of the observed link between the growth of plants, its yield and the soil. Foundations of sound scientific knowledge were layed down when first M. V. Lomonosov and then Lavoisier discovered the law of the conservation of matter.

  Lavoisier discovered the composition of air and the essence of the processes of oxidation, burning and respiration. Not long before his death (1794) he wrote, concerning the nutrition and growth of plants, that plants get materials necessary for their organization from the surrounding air, from water, and generally from the mineral kingdom (according to Pryanishnikov, 1952).

  New methods of chemical investigations elaborated by Lavoisier were employed by Priestley, Senebe and Sossur, in order to study metabolism in plants. These scientists showed that plants obtain carbon from air, and that apart from carbon plants require salts.

  While noting the importance of atmospheric carbon dioxide as a source of carbon, Sossur also thought that the humus of the soil is of great importance. Humus contains a certain substance indispensable for plants; the daily experience of farmers pointed to a close link between the fertility of the soil and the presence of organic substances in it.

  The English scientist Davy (1813) drew attention to various sugary, glueing, oily, slimy, extractable substances, and to the carbon dioxide of the soil. These elements, according to him, contain all the elements needed for the life of plants.

  The humus theory became very popular after the well-known works of Teer (1752-1828), the founder of the first agricultural school and the propagator of crop rotation instead of the three-field system. He wrote that fertility of the soil depends entirely on humus. since apart from water, humus is the only substance of the soil which can serve as a nutrient for the plants (according to Pryanishnikov,1952).

  These ideas were the basis of a theory of the exhaustion and regeneration of the soil's fertility. It was assumed that the more the plants absorb humic substances, the better they grow, and the greater the crop. Different plants absorb different amounts of organic substances. For example, wheat requires more humus than rye does.

  Teer and his adherents considered humus to be an important product of plants and a substance of the utmost importance for the life of plants. Humus is formed at the expense of plants; some plants excrete more humus than they absorb. In other words, some plants enrich the soil in humus and others exhaust its reserves. To the former category belong clover and other leguminous plants.

  The propounders of the humus theory did not take into account mineral substances. The latter, according to them, enhance the decomposition of humus and transform it into a form which can be assimilated by plants.

  The humus theory was very popular even in the thirties of the last century in different countries such as Germany, England, France and Russia.

  Of the Russian investigators of the late 18th and the beginning of 19th centuries, who drew attention to the importance of organic fertilizers, A. Bolotov, I. Komov and A. Poshman should be mentioned. The works of these scientists showed that fertilizers have an immense influence on the crops, Komov assumed that organic substances are necessary for plants to the same extent as they are necessary for animals, and he recommended to use them in practice. According to him, the organic fertilizers cannot be replaced by salts.

  At the beginning of the 19th century the role of nitrogenous compounds in the nutrition of plants began to be better understood.

  Liebig assumed that plants obtain nitrogen at the expense of atmospheric ammonia and that its presence in the atmosphere is sufficient for them. However, experiments disproved this. It was found, on the contrary, that this substance is most insufficient for plants.

  The well-known studies of Bussengo revealed the sources of the nitrogenous nutrition of plants, In 1837-38 he developed his theory on nitrogenous fertilizers and recommended the use of fertilizers rich in nitrogen. He connected soil exhaustion with the depletion of sources of nitrogenous nutrition. In this process, he ascribed varying importance to different plants. Some plants absorb more nitrogen from the soil, others less. He ascribed an active role in the enhancement of soil fertility to certain plants, e. g., clover. "One should think", said he "that cultures which improve the soil do not limit themselves to its enrichment with C, H, and 0₂ only, but also enrich it with nitrogen" (according to Pryanishnikov, 1945).

  Hellriegel (1889), discovered the reason for the peculiar action of leguminous plants on the fertility of the soil. By his experiments he found that leguminous plants assimilate nitrogen from the air. Voronin (1886) studied the root nodules of leguminous plants and found microorganisms in them, which in his words are: "The culprits of the formation of the nodules." Later, Hellriegel by thorough experimentation showed that these symbiotic organisms are the cause of the nitrogen fixation by leguminous plants.

  Thus, the conclusions of the old investigators, for example Teer, have been confirmed by the later experiments of Bussengo, Hellriegel, and others, who found that plants not only exhaust but also enrich the soil with nutrient organic substances.

  Thus, the humus theory of plant nutrition, of the 18th and first half of the 19th centuries, was very popular. Fertilization with organic substances was considered to be an essential measure, not only for the increase of yields, but on the whole for the increase of soil fertility.

  The attitude toward the theory of humus or organic nutrition of plants, and in general toward fertilizers changed abruptly after Liebig put forward his theory of the mineral nutrition of plants. He severely criticized the humus theory of nutrition and considered it basically wrong. He considered all the studies, performed before him by physiologists and agronomists, to be inconsistent and meaningless for the solution of the problem of plant nutrition. He criticized the humus theory of nutrition: "Taking it", in Pryanishnikov's words,"in its extreme expression, and bringing it ad absurdum."

  Liebig completely rejected even the possibility of assimilation of the organic substances by plants. In his opinion only inorganic compounds can serve as sources of nutrition for plants. He considered humus as a source of CO₂ which enhances the process of the erosion of silicates and prepares the mineral food for the plant.

  Liebig did not recognize the role of plants in the enrichment of soil. He severely negated and criticized the notion of "enrichment of soil in sources of nutrition." Plants, according to him, deplete soil, carrying off elements of mineral nutrition with the crops. But the depletion of the soil is carried out by various plants at different rates and in different directions. Some of them take out from the soil mainly calcium (for instance, peas), others--potassium and silicon (wheat grains). Therefore, the crop rotation, recommended by previous investigators, only slows down the depletion of the soil.

  In accordance with his theory, Liebig recommended the introduction into the soil of mineral fertilizers. The amount of these fertilizers applied should take into account the utilization by plants.

  Owing to the authority of its author--Liebig, the theory of mineral nutrition of plants was accepted by his contemporaries with hardly any criticism at all. The authority of Liebig's chemical school supressed all the previous ideas and theories of organic nutrition of plants. Liebig, says Ressel (1933), gave the final blow to the theory of humus. Only the most daring would still venture to maintain that plants take the necessary carbon not from CO₂ but from another source. Although. one should admit that we have no proof that plants obtain all the necessary CO₂ in this way.

  Liebig's theory was developed and subjected to changes corresponding to the newly acquired factual data. In our country the theory of Liebig was subjected to a thorough revision by the Academician Pryanishnikov and his followers. Pryanishnikov originated his own direction in agrochemistry; he and his students elaborated a series of valuable postulates laying a basis for practical measures in agriculture. To him goes the honour of solving the problem of basic improvements of the nitrogen balance in the agriculture of the USSR. In counterbalance to the theory of Liebig, he ascribed a major importance to the biological processes of the soil and especially of nitrogen accumulation. Pryanishnikov did not deny the possibility of the assimilation of organic substances by plants and he himself showed it experimentally.

  Notwithstanding this, the theory of Liebig is still reflected in the studies of many specialists. In the theory of plant nutrition one observes an obvious underestimation of the role of the organic substances of the soil, and its importance of the nutrition of plants is often denied altogether. As a rule, the significance of the organic substances of the soil is basically formulated in two postulates:

  1. Humus substances are reserves of plant-nutrient elements which become available only after they are mineralized.

  2. Humus improves the physicochemical properties of the soil, increases its absorptive capacity, and therefore, promotes also the accumulation of nutrient substances--it strengthens the structure of soil particles and with it improves many other soil properties. Organic fertilizers--manure, compost, etc, prepare the soil for the acceptance of mineral fertilizers, increase its buffer capacity, etc.

  All this is quite true and is confirmed by age-old practice and by many experiments. However, to reduce the importance of these fertilizers only to the given postulates will, in our opinion, be inadequate.

  The fact of positive action of humus and organic fertilizers on the growth of plants cannot be explained by the action of the mineral elements of nutrition present in them. Russel (1933), summarizing the experimental data of 60 years work of the Rothamstead Station, said that, although plants can grow satisfactorily and reach full development on inorganic nutritious substances only, under natural conditions, however, their nutrition takes place in the presence of organic substances. The question whether these substances play any active role in this process has been very much discussed. The experimental data are not very conclusive. In the Rothamstead field experiments none of the combinations of the artificial fertilizers is as effective as manure in maintaining crops from year to year.

  The extensive field experiments of the German investigators Gerlach, Hansen, Schultz, Wagner, Schneidewind, and others, lead us to the conclusion that whatever the amount of the mineral fertilizer may be, if using mineral fertilizers only, one cannot reach as high yields as by the simultaneous introduction of manure (Schneidewind, 1933). Summarizing experiments of many years in the Lauchstadt Station, Schneidewind gives us as an example the following data from sugar-beet yields in centners per hectare: with only mineral fertilization--roots, 487.6; sugar content, 77.66; tops, 291.7; under mixed fertilization mineral and organic (manure)--roots, 533.6; sugar content, 88.11 and tops, 366.6.

  Similar data are given by Schneidewind for potato crops. By using artificial fertilizers only he could not get more than 240 centners of potatoes per hectare, while by the introduction of mineral fertilizers and manure the yield rose to 306-312 centners per hectare. This effect of manure repeated itself year after year and was independent of the amount of precipitation whether it was a dry, wet, or average year.

  Russel (1933) and his co-workers have shown that the organic substances of the soil stimulate the growth of plants and increase crops. He concludes that no mixture of artificial fertilizers can be as effective as manure in maintaining steady high crops year after year.

  Academician V. I. Palladin (1924), concerning the problem of plant nutrition, wrote that green plants can nourish themselves on ready-made organic compounds. Such nutrition may go on parallel to the assimilation of carbon from the atmosphere. According to the author's observations, green isolated leaves of plants grown with artificial, sugar-containing nourishment always accumulate more organic substances in their tissues and have a higher turgor than plants nourished with mineral elements. The carbohydrate increment on sugar nutrition reaches 5 grams per 1 M² of leaf area.

  Lebedyantsev (1936) in an annotation to his translation of Liebig's book (Chemistry as applied to Agriculture and Physiology) mentioning the work of Sossur on nutrition of plants, has written: "Sossur considered the CO₂ of the air to be the main source of carbon nutrition, not denying, however, the possibility of utilizing carbon of organic compounds of the soil, since he was not in possession of facts enabling him to deny this. We shall note that today we do not possess such facts and the question of the assimilation of organic compounds from the soil still remains to a large extent an open one, although, undoubtedly, the main source of carbon for green plants is after all CO₂." (page 396)

  As can be seen from the above, the question of plant nutrition, notwithstanding the numerous studies which have been carried out, remains unresolved in may respects.

  It is hard to agree with the concept according to which during all the history of their evolution, plants, although having been in contact with organic substances of the soil, did not acquire the ability to assimilate them in one form or another.

  There is no basis for denying the well-known facts and experimental data showing that plants utilize mineral compounds for their nutrition. Numerous observations speak in favor of this method of plant nutrition being the most important one under natural conditions. However, is such a nutrition sufficient to obtain high yields and fully viable seeds year after year? This question seems to us not to have been sufficiently solved.

  Not so long ago, the assimilation by roots of CO₂ from the medium was said not to take place, but now this may be considered an established fact. Plants absorb CO₂ not only from the atmosphere but also from the soil (Samokhvalov, 1952; Kursanov, 1953, 1954).

  It should be assumed that plants that experience lack of CO₂ in the atmosphere gladly assimilate it from the soil. Under various unfavorable conditions the photosynthetic activity of plants may decrease considerably. Thus, for example, during drought the stomata are closed, and the influx of CO₂ stops or weakens. Respiration of plants, however, under these conditions does not cease and may even increase. Starving ensues, to a larger or a smaller degree. Weakening of photosynthesis may also be caused by other factors. In all such cases, evidently, plants can switch to a heterotrophic nutrition, assimilating organic compounds from the soil supplementing their nutrition.

  The ability to assimilate ready-formed organic compounds and use them as nourishment is observed in many representatives of the plant kingdom; from the lower forms such as blue-green and green algae, to the higher flowering plants.

  It is known that many (if not all) algae, may under certain conditions, grow and develop on synthetic media with mineral sources of nutrition, as well as on organic media containing different nitrogenous and carbonaceous complex compounds. It was shown by special experiments that these organisms, and especially the green unicellular organisms, which usually grow on pure mineral nutrious media, grow much better upon the addition of organic substances to the solution. They can assimilate carbonaceous and nitrogenous substances (Gollerbach and Polyanskii, 1951).

  Blue-green algae of the Cyanophyceae--Oscillaria, Nostoc Anabaena, Lyngbya, and others, also green algae of the groups Chroococcum, Spirogyra, etc, cannot be cultivated indefinitely on artificial organic media without a noticeable weakening of their viability. In our laboratory we have been maintaining a pure culture of the green alga Chlorella for already more than 25 years. We do not observe any essential lowering of life functions. This alga assimilates nitrogen from peptone and amino acids and grows quite well on must agar and even on meat-peptone agar (MPA) and gelatin.

  According to the observations of many investigators, green algae Chroococcum and Spirogyra, blue-green algae Phormidium, Nostoc, Anabaena, Gloeocapsa, and others, readily assimilated sugars, amino acids, alcohols, organic acids, urea, casein, and other organic substances. Studies with pure cultures of algae performed first by Pringsheim (1913) and later by many others (Harder and Humfeld, 1917; Cataldi, 1941; Gerloff, 1950) have shown that these organisms grow on organic media in the dark as well as in the light. Chlorella assimilates sodium and potassium oxalates in the light, and malic, tartaric and some other acids in the dark.

  Allen (1952) studied 26 cultures of blue-green and some green algae belonging to 11 genera--Anabaena, Nostoc, Oscillatoria, Lyngbya. Phormidium. Gloeocapsa. Aphanocapsa, Plectonema Cladothrix, Chroococcum. and Synechococcus. These cultures were grown in mineral media with different sources of organic nutrients, Yeast extract, various sugars (glucose, saccharose, lactose, maltose, etc) mannitol, glycerol, ethanol, organic acids, amino acids, casein, urea, casein hydrolysate, etc were added to the media. In the presence of these substances many algae grew better than in media with mineral sources of nutrition only. Assimilation of organic substances takes place in many species at the same intensity in the dark as in light. Upon prolonged cultivation on organic media some algae lose their pigment and continue to live as typical saprophytes (Fogg and Wolff, 1955).

  Authors who studied algae concluded that they can live and grow as heterotrophs, utilizing organic compounds, such as nitrogenous and carbonaceous substances, In the same way as the ordinary saphrophytes. This is understandable because the two kinds of organisms live in a milieu rich in organic substances--in soils and water reservoirs, where animal and plant residues serve as sources of organic nutrients after their decomposition.

  Green moss of the genera Splachnum and Getrapladon, settle and grow on excrements of animals, utilizing organic substances for their nutrition.

  Among higher flowering plants a group is known as the so-called "humus plants"--various representatives of which can be observed to be in different stages of degradation of the chlorophyll apparatus. The humus plants are so called because they grow on substrates rich in humus and decomposing organic animal and plant residues.

  The character of nutrition and the conditions of the existence of these organisms have left a certain imprint on their structure and appearance. Some of them lose the green coloration, the leaves are reduced (but only in the lower part of the stem) and lose the capacity to assimilate carbon dioxide from the atmosphere. To such plants belong the species of the following genera: Monotropa Dum., Zymodorum, Neottia Sw., Corallorhiza Rale, Epipogon S. G. Gmel., Voyria Aubl, Fletia, Pogonia Less, Voyriella Miq., and others (see below).

  In other representatives of humus plants the green leaves are preserved and they remain autotrophs by assimilating carbon dioxide of the atmosphere but, nevertheless, they can utilize ready organic substances. To such plants species belong the species: Dentaria L., Pyrola L., Goodyera R. Br., Cephalanthera Rich., Epipactis Adans,Platanthera Rich., and others. All these plants, to a greater or lesser degree, live and nourish like heterotrophs, assimilating organic compounds together with mineral sources of nutrition.

  The group of insectivorous plants is well known. In our latitudes such flowering plants are encountered an Drosera rotundifolia L., and Utricularia vulgaris L. To this group belong such southern plants as Cephalotus follicularis Labill, many species of Nepenthe L., Dionaea muscipula Ellis, Drosophillum Link, Aldrovanda Monti, Sarracenia L., and others.

  About 500 species of such insectivorous plants are described in the literature. These plants have a complex apparatus for catching insects and special glands for their digestion.

  Small animals falling into the trap of these plants are decomposed by enzymes to the soluble forms of organic compounds containing nitrogen and nonnitrogenous compounds which are then absorbed by the plants and utilized by them for nutrition.

  It is characteristic that those insectivorous clearly heterotrophic plants do not lose their capacity to assimilate atmospheric CO₂. Many species which obtain ready-made organic nourishment from the living body of other plant species also belong to the heterotrophic plants. These are the parasitic plants. They are divided into facultative and obligatory parasites.

  Among the obligatory parasite plants, the climbing plants of the genus Cassytha L. (fam. Lauraceae L.) and Cuscuta L. (fam. Convolvulaceae L.) are the well-known species.

  The biology of Cuscuta L. is known in great detail (up to 50 species are recognized). They are parasites of the cereals, bushes and trees. They penetrate the tissues of the host with their haustoria and such nutrient substances out of the host. Leaves of this plant are weakly developed and can hardly be seen, the stems lack chlorophyll.

  Orobanche L. are also etiolated plants with weakly developed leaves (in the form of scales) lacking chlorophyll. They are parasitic on the roots of sunflower, flax, cabbage, and others. About 180 parasitic plants of this genus are known. The shoots of Orobanche are so coalesced with the roots of the plant host that it becomes impossible to distinguish between the cells of the host and the parasite.

  Lathraea squaniaria also belongs to the group of parasitic plants. In contrast to the aforesaid this plant does not climb; it consists of a thick, colorless, watery stem. The leaves of this plant are weakly developed and are in the form of colorless scales. The root system of this plant is close to the root of the host, from which the parasitic plant sucks the sap of the host with the aid of haustoria. The plant at first grows under the earth's surface, then its shoots pierce through the surface. The aerial parts are of a violet-red color.

  The Balanophoraceae are tropical plants--parasites living on roots of various trees. Forty species of these parasites are known (14 genera). These plants, like the Orobanche grow in such intimacy with the roots of the host that no visible border between them is discernible.

  Parasitic plants related to Balanophoraceae are the Rafflesiaceae Dum., which parasitize trees of tropic and subtropic regions. These plants penetrate the bark of the tree host, embrace its stem and roots and suck the, sap of the latter.

  Parasitic plants from the family Loranthaceae D. Dow include more than 300 species. They live exclusively on stems and branches of different trees. A typical representative of this group of plants is our ordinary Viscum album L., which lives on deciduous and coniferous trees. This plant penetrates the branches of the host with its roots and haustoria. The stem of Viscum album which has the form of a dichotomically branched bush carries elongated leathery leaves of a yellowish-green color.

  The group of plants of the mistletoe family is characterized by the fact that its representatives--arboreal plants--did not completely lose the capacity of photosynthesis and preserved the normal appearance in their stems. They are the intermediate group placed between the nonchlorophyll, flowering, obligatory parasites, which nourish on ready-made organic food, and the facultative parasites which preserve their normal structure in all their parts, as well as the capacity of assimilating atmospheric CO₂.

  To the facultative parasites belong about 100 species of the family Santalaceae R. Br., and more than 500 species of the family Scrophulariaceae R. Br. In our flora, species of the genus Thesium L. (family Santalaceae) and species of the genus Euphrasia L., Alectorolophus Boehm., Melampyrum L., Pedicularis Boehm., Odontites Pers and others (family Scrophulariaceae) are encountered.

  They are all grasses growing in meadows or in forests. They possess green leaves and a well-defined photosynthetic capacity. In the initial stage they develop as typical autotrophs without the slightest inclination to become parasites. However, when their roots reach a definite size (1- 2 cm) haustoria appear, with the aid of which they become attached to the root of another plant host, encountered during their growth. From this moment on the semiparasite plants begin to supplement their nourishment at the expense of the plant host. They can, however, grow without the host in the event of their not having encountered it in the course of growth. They can also parasitize each other.

  There are in nature a great many epiphytic plants developing and growing on plants as on substrate. It is assumed that they nourish independently, at the expense of mineral substances present on the plant barks. However, the possibility of their using organic substances such as dead tissues, or their decomposition products, is not excluded. This problem is still inadequately studied.

  Many plants assimilate organic substances, metabolic products of microbes which live on and inside the roots. The best known plants are those in the tissues of which there are bacterial microbe-symbionts, actinomycetes, and fungi.

  The best known symbiosis is that of plants with mycorhiza fungi. According to the data in the literature, there are mycorhiza fungi on the roots of almost all plants which are to a greater or lesser degree symbionts. The symbiosis is such, that some plant species cannot grow without the fungi. Such plants are apparently obligatory parasites, heterotrophs whose nutrition depends on the metabolic products of the fungus. Some orchids serve as an example of such parasitism. They require fungi for their normal growth and development. They grow badly without them, and certain species such as Neottia nidusavis Rich., encountered in our oak and pine forests cannot grow at all without the symbiotic fungi. This orchid grows and develops under the soil surface and appears above the surface only for flowering and fruit bearing. It has no green color and its pale, thick stems carry weakly developed, slightly yellowish sprouts with scaly leaves. Neottia nidusavis Rich. Has completely lost the capacity of independent nourishment on mineral compounds. It is almost completely devoid of chlorophyll. Its nutrition is at the expense of decomposition products of fungi which grow inside its stem and roots.

  There is a theory that orchids of this species can assimilate organic compounds directly from the soil, and that the fungi serve merely for the processing of these compounds into the form in which they are more easily assimilated. The cytological picture of the growth of fungi inside the orchid tissues shows that the mycelium grows to a certain size followed by coiling and lysis. The products of lysis are assimilated by the plant.

  Other species of orchids which have lost their green color and are entirely dependent on fungi can be encountered in our forests. To such orchids belong: Epipogon aphyllus Sw. and Corallorhiza trifida Chat., which grow in forests of the moderate belt. Such orchid parasites are also encountered in tropical forests (Burgeff, 1932). In the literature, orchids are described in which parasitism becomes manifest only at a certain growth phase. The species of the genus Myrmechis (Myrm. glabra Bl., Myrm. gracilis Bl. ) when young grow in the form of a root obtaining their nourishment entirely at the expense of fungi; afterward they form aerial organs, flowering sprouts with green leaves and start to nourish themselves. When the sprouts fade away they become parasites again.

  According to the data of Bernard, the seeds of orchids cannot germinate without the mycorhiza fungi. Bernard has shown that the orchids cannot germinate without fungi and he elaborated methods for the artificial infection of these plants with the mycorhiza.

  Parasitic plants which have lost their green coloration are encountered among the other representatives of green plants. Such for example is Monotropa hypopithys L., of the family Pirolaceae Drude. They are devoid of green coloration and their leaves have been transformed into brownish scales. This and other plants of the genus Monotropa L., receive their nourishment entirely at the expense of fungi growing inside their tissues. Gordyagin (1922) described a fern Ophioglossum symplex the nonsexual sporiferous generation of which is nourished at the expense of fungi, absorbing organic products of their metabolism and decomposition.

  According to the observations of that author, ferns and other green plants are encountered in the forests of the Tatar Autonomous Republic, which assimilate carbon from the atmospheric CO₂ but cannot exist without symbiosis with fungi. Such plants are widespread in nature. Higher plants of such a type preserve the green coloration and the capacity of assimilation of atmospheric CO₂, but cannot normally develop and reach the stage of flowering and fruit bearing in the absence of the fungus.

  The majority of plants have fungi on their roots. They are useful though not indispensable. Fungi, in such cases, promote their growth and nutrition. The plants, as experience shows, grow better and adapt themselves to new places and give a higher mass increment (Lobanov, 1953, Reiner and Nelson-Jones, 1949).

  Keller (1948) and Lyubimenko (1923). when comparing the degree of symbiosis of different plants with fungi, and the degree of parasitism on the fungi, considered these phenomena as subsequent stages of evolution, as stages of transition from autotrophism to saprophitism and parasitism, i.e., to heterotrophy. In his book "The Fundamentals of Plant Evolution" Keller writes: "In the course of evolution the individual higher green plants of different families pass from this stage, and, under the pressure of appropriate natural conditions, change to nutrition at the expense of fungi. Thereby, the leaves lose their significance as organs of assimilation and become undeveloped, remaining only in the form of scales devoid of green coloration."

  Lyubimenko (1923) stressed that there is no sharp difference between autotrophic and heterotrophic plants. Saprophitism, according to him, is the natural consequence of the synthesis of organic compounds: "Organisms capable of synthesizing organic compounds from mineral ones, preferably use ready organic compounds and are capable of receiving their nutrition in the same way as saprophites. Saprophitism is not per se, a special specific property of a certain group of organisms; on the contrary, it is characteristic of all organisms with a possible exception of nitrifying bacteria, and therefore, it appears in nature in different degrees. On one hand we find plants which only accidentally assimilate organic compounds from the environment; these are the facultative saprophytes, for them the saprophytic nutrition is not indispensable." "On the other hand we find plants which are true saprophytes, which have completely lost the capacity of synthesizing organic from inorganic compounds. Between these two categories there is a gradual transition of forms lessening the division between them and in reality cancelling this division, (Lyublmenko, 1923, page 186).

  As can be seen from the above citation, Lyubimenko considers that all plants are capable of organic nutrition to a greater or lesser extent. The only exceptions are, according to him, the nitrifiers. We can add that, recently, material is being accumulated showing that even these organisms may utilize organic nutrients and grow on organic nutritious media In the same way as many bacteria-saprophytes.

  It to known that plants may be in close symbiosis, not only with fungi, but also with bacteria. The root-nodule bacteria which penetrate the roots of leguminous plants, with the formation of colonies in the form of nodules are well known. The nodules represent swollen roots filled with bacterial cells. These bacteria grow abundantly, multiply in the cells, and produce nitrogenous substances which are utilized by the plants.

  Analogous nodules are formed on roots of some other plants (nonleguminous). For example, nodules have been described on roots of Alnus Gaertn., Myrica L., Podocarpus L'Her., representatives of the family Elaeagnaceae Lindl.; E. multiflora Thunb., E. angustifolius L., on Shepherdia canadensis Nutt., on various species of Ceanothus L., on Coraria japonica Gray, and others. The nodules on these plants are formed by different representatives of the microbes such as bacteria, mycobacteria, actinomycetes and, according to our investigations, also by proactinomycetes.

  These organisms have a definite positive effect on plants much the same as the root-nodule bacteria have on leguminous plants. Under conditions where the bacteria and, consequently, the nodules are absent. the plants grow weakly and often they do not reach the stage of flowering and fruiting (Fred, Baldwin and McCoy, 1932; Jongh, 1938).

  There are many microbial symbionts which live on plant leaves, forming unique swellings--knots in the tissues. Such knots are formed by bacteria and mycobacteria. These organisms upon multiplying fill the cells of the knots in the same way as the root-nodule bacteria fill the nodules in the root tissues. These microorganisms, according to our investigations, exert a positive effect on the growth of plants. Without them the plants grow weakly and some species do not reach the stage of flowering and fruiting, as can be observed in Ardisia Thunb. The plants remain dwarfs, but after they are infected with the corresponding bacteria, knots appear on their leaves, the plants improve, they assume normal appearance, begin to flower and bear fruit (Jongh, 1938).

  About 400 species of plants with the above-described knots are described in the literature, including 30 species of the genus Ardisia Thunb., 244 species of the genus Pavetta L., 42 species of the genus Psychotri L., 5 species of the genera Amblyanthopsis and Amblyanthus D. C., 1 species of the genus Dioscorea L., and others. The bacteria forming the knots pass from plant to plant through the seeds,

  According to the data of many investigators, all these microbe-symbionts which live in the tissues of roots or in leaves of green plants affect the plants through their metabolic products. Some of them form and excrete biotic compounds which activate the growth of the plants, others, in addition, fix atmospheric nitrogen and pass it on to the plant in the fixed state.

  Having ascribed such a great importance to the above-listed bacteria and mycorhiza fungi which provide organic nutrition for the green plants, we must draw attention to other species of microorganisms which are closely linked with the root system of plants, although this link is of a different character. We have in mind the rhizosphere microflora.

  An immense number of microorganisms live around the plant roots. The most numerous among them are the bacteria. Have they any effect on the nutrition of the plants? What effect do their metabolic products have on the plant? Are they assimilated by plants and what role do they play in the growth of the latter? If plants utilize the metabolic products of microbial symbionts with such readiness why then can they not use analogous substances formed by the free-living organisms?

  Apparently, the difference lies in the fact that in the first case, symbiosis between the plant and the microbe, the metabolic products of the microbes remain inside the tissue, while, in the latter case, these compounds are being formed and accumulated outside the plant in the surrounding milieu, and, in order to become available to the plant, they must find their way inside the plant via the roots or some other way.

  Many authors think that the permeability of the root system for mineral substances differs from that for organic compounds. The suction power of the roots is different in these two cases. The suction power of the heterotroph plant parasite (Lathraea squamaria) is 22.7 atm, and the suction power of the root of the plant host (Prunus padus L. ) is only 3.7 atm (Kostychev, 1933).

  Many typical autotrophs, whose suction power is no more than average, readily assimilate organic compounds, Hutchinson and Miller (1912), studying nitrogen absorption by the pea seedling, found a phenomenon which, from the point of view of the prevalent opinion that they absorbed nitrogenous organic compounds more readily than nitrate nitrogen, was paradoxical. They tested many organic nitrogenous compounds, and in all cases the results were the same.

  These data were confirmed by numerous investigators (see later).

  As can be seen from the above, the subdivision of plants, according to their nutrition, to autotrophs and heterotrophs, should be considered as relative. Each of the so-called autotrophic plants is capable of assimilating organic compounds to a greater or lesser extent.

  Indeed the meaning of "nutrition" should be distinguished from the term "synthesis of organic substances." Nutrition in the real sense of the word, should be called all the processes which are directly involved in assimilation of substances by the living parts of the organism from the moment when the organic compound in prepared. (Lyubimenko, 1923, page 180). This process proceeds similarly in all organisms belonging either to the plant or animal kingdoms. They all feed on organic compounds. To build a living body, all cells, whether they are microbial, animal or plant, should possess ready-made elementary organic compounds or their building blocks--nitrogenous and nonnitrogenous compounds, which participate in the general process.

  The difference between plant and animal nutrition lies only in that the latter obtain ready-made synthesized organic compounds from the outside and the plants synthesize them for themselves.

  The process of plant nutrition consists therefore of two elements: a) synthesis of organic substance from inorganic elements; this process Lyubimenko considers as the preparative one, dispensable, from the point of view of nutrition in the narrow sense of the word; and b) nutrition proper, i.e., uptake or assimilation of elementary organic particles for the formation of a living organism; this process proceeds in the same way in plants and animals.

  The nutrition of plants and microorganisms may proceed according to the first or the second process, but each process may be manifested to a various degree in relation to the conditions of existence and to generic differences.

  Vinogradskii, in his brilliant studies on the chemotrophy of microorganisms, drew a sharp line between these two types of nutrition by dividing all the microorganisms into autotrophs and heterotrophs. Lebedev (1921). on the contrary, assumed that these two types of microorganisms differ from each other, not by their manner of nutrition, but by the way they obtain their energy. Lebedev thinks that both the autotrophs and heterotrophs assimilate CO₂, but while the former obtain their energy at the expense of the oxidation of inorganic substances the latter obtain their energy at the expense of the oxidation of organic compounds. This scientist was the first who showed experimentally that the heterotrophs utilize CO₂. His data were confirmed by many other investigators.

  At present, it has been confirmed that many microorganisms, even heterotrophs can assimilate CO₂, and Werkman even assumes that all the organisms in general assimilate CO₂ and that this is an important physiological function, insuring the synthesis of indispensable intermediate metabolic products (Werkman and Wilson, 1954). 

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