Plant Embryology

the science of the origin and formation of new plants. In a broader sense, plant embryology studies not only embryonic development but also the formation of the generative sphere, the formation of sex cells in the generative sphere, and fertilization.

Plant embryology is one of the most important divisions of botany; its methods and goals are separate from those of plant morphology. General plant embryology explains the basic laws of the origin and development of generative and embryological structures (sporogenesis, gametogenesis, zygogenesis, endospermogenesis, embryogeny, apomixis). Comparative plant embryology studies embryological processes in various species in order to obtain data for solving problems of systematics and phytogeny. Specialized plant embryology is devoted to the study of embryological processes in the plants of individual systematic groups. Experimental plant embryology re-creates the course of development of plant organisms in order to reveal the functional, biochemical, and genetic nature of embryonic processes.

Brief history. The existence of sex in plants has been known since early times. Nevertheless, the ancient theories on the origin of plants represented far from a true understanding of embryonic processes. Only in the late 17th century, with the invention of the microscope, was it possible to examine the egg cell and the embryo in the ovule of flowering plants. Of great importance for the beginnings of plant embryology were J. Koelreuter’s works on the biology of flowering processes, the experiments on plant fertilization and hybridization conducted at the St. Petersburg Academy of Sciences from 1756 to 1761, and C. Wolffs research (Theoria generations, 1759 and 1764).

Plant embryology became an independent science in the 19th century, with the growing interest in the natural sciences that resulted from the formulation of the cell theory and Darwin’s theory of evolution. The development of plant embryology was encouraged by advances in optics and the preparation of microscopic slides. The study of sexual reproduction in higher plants, especially flowering plants, resulted in a more thorough investigation of male and female sexual elements.

The germination of pollen and the formation of the pollen tube on the stigma of the pistil were discovered in the early 19th century (Italian botanist G. Amici, 1823; French botanist A. Brongniart, 1827; German botanist F. Meyn, 1841). However, at that time it was incorrectly assumed that the embryo develops from the tip of the pollen tube during penetration of the embryo sac. One of the authors of the cell theory, the German botanist M. Schleiden, originally supported this theory (1836, 1838). He later (1856) admitted his error but still maintained, however, that pollen was the female organ and the embryo sac was the male organ. The Russian botanist N. I. Zheleznov (1840, 1842) correctly concluded that pollen is an element of the male organism.

In the mid-19th century the fundamental research of the German botanist W. Hofmeister presented extensive data on the ontogeny of flowering and spore-bearing plants. Hofmeister scientifically confirmed the alternation of sexual and asexual generations. The Russian botanist I. N. Gorozhankin (1880) established that during fertilization the tip of the pollen tube in gymnosperms penetrates the cavity of the egg cell, where the merging of male and female cytoplasm occurs. He also confirmed that the male sex nucleus joins with the nucleus of the egg cell upon emerging from the pollen tube. Thus, Gorozhankin presented the first true picture of fertilization in gymnosperms. V. I. Beliaev, who continued research in this area (1889, 1891), also studied the reduction of the male gametophytes (1894, 1901), the development of the antheridium, and the process of fertilization in heterosporous pteridophytes. His research played a large role in establishing phylogenetic ties among archegoniates.

The motile sperm in the cycadophytes and ginkgoes was discovered by the Japanese botanists S. Hirase (1896) and S. Ikeno (1898) and by the American scientist G. D. Chebber (1897). The works of the Russian botanists E. M. Sokolova (1890) and V. I. Arnol’di (1900, 1907) were very important for the development of the phytogeny of archegoniates. The German botanist J. B. Hanstein (1870) developed the concept of primary layers (dermatogen, periblem, plerome), from which the permanent tissue of the adult organism develops.

The Russian botanist S. M. Rozanov (1865, 1866) laid the foundation for the comparative embryology of Hydrophyllaceae, Boraginaceae, and other families. A. S. Famintsyn made a detailed study of embryogeny in both dicotyledonous and monocotyledonous plants. S. G. Navashin (1898) discovered the phenomenon of double fertilization in angiosperms, by which two sperms carried by the pollen tube into the embryo sac fertilize different bodies. One sperm fertilizes the nucleus of the egg cell, and the other fertilizes the secondary nucleus of the central cell. In the first case a diploid embryo results, and in the second a triploid endosperm.

Analogous fertilization processes were later confirmed by the French embryologist L. Guignard (1899) and by the German botanist E. Strasburger (1900). Further extensive study of embryological processes was devoted to the morphology and embryology of gymnosperms (American scientists J. Coulter and C. Chamberlain, 1910 and 1915; German morphologist K. Schnarf, 1933) and angiosperms (Schnarf, 1927–29 and 1931). The French scientist R. Souèges and his students (1934–39) developed a system of classification of embryonomic types in angiosperms and explained the basic laws of cell division and embryological segmentation and differentiation.

In the 1950’s, the comparative embryology of many tropical families of flowering plants was studied by a school of researchers (R. Kapil, B. Swamy, V. Tiagi, N. Bhandari, and others) under the leadership of the Indian botanist P. Maheshwari. Consequently, embryologists (especially the Indians N. Rangaswami, 1961; Maheshwari, and K. Kanta, 1962; B. Johri and C. Sengal, 1963; Johri, 1971 and 1975) directed their attention toward the artificial cultivation of ovaries, ovules, pollen, endosperm, and embryos, as well as toward the study of fertilization and apomixis. The Swedish embryologists H. Juel (1915), K. V. O. Dahlgren (1940), F. Fagerlind (1944) and A. Gustafson (1946) answered many questions concerning gametogenesis and apomixis.

The success of research on the comparative embryology of gymnosperms and angiosperms is reflected in many handbooks and reports on the subject, including those by the American botanist D. Johansen and the Soviet botanist Ia. S. Modilevskii. The British morphologist C. Wardlaw (1955) produced the first comprehensive work on the development of the embryo in all classes of the plant kingdom—from algae to flowering plants. Embryogeny is examined against a background of individual and historical development, taking into account the distinctive morphological and functional features of the groups under study.

Of great importance for the continued development of plant embryology was the monograph of the Soviet scientist P. A. Baranov (1955), which presents the history of concepts on the origin and development of organisms from ancient to modern times. A sequential representation of modern views on the development of generative and embryological structures is given in handbooks by V. A. Poddubnaia-Arnol’di (1964, 1976) and G. L. Davis (1966) and in textbooks by the Polish embryologist B. Rodkiewicz (1973) and the Italian embryologist E. Battaglia (1976). Extensive theoretical research has been done in France on the problem of the sexual process and embryogeny in gymnosperms and angiosperms (B. Vazart and J. Vazart, 1958–69; A. Kamer, 1958–69; Guignard, 1961–69; S.-C. Mestre, 1967; M. Favre-Duchartre, 1963–69). In the USA much research has been devoted to the ultrastructure of gametophytes, the embryo, the endosperm, and the process of fertilization in angiosperms (W. Jensen, 1965–75).

Soviet embryologists have developed a mitotic hypothesis of double fertilization in angiosperms (E. N. Gerasimova-Navashina, 1951–55). They have also discovered and thoroughly examined a new type of embryogeny in peonies (M. S. Iakovlev, 1951; M. S. Iakovlev and M. D. Ioffe, 1957–61). M. S. Iakovlev (1974) proposed a new concept in the evolution of gametogenesis in angiosperms, and I. D. Romanov (1971) developed a classification system of embryo sacs. Soviet embryologists have discovered cy-toembryological laws governing growth of pollen tubes and fertilization, including those pertaining to distant hybrids and polyploids (E. N. Gerasimova-Navashina, 1955–71; V. A. Poddubnaia-Arnol’di, 1964–76; G. V. Kandelaki, 1969; G. B. Batygina, 1974; V. P. Bannikova, 1975). The phenomena of apomixis and polyembryony are being studied in more detail (S. S. Khokhlov, 1946–73; D. F. Petrov, 1963–73; M. P. Solntseva, 1968–73), and extensive research is being conducted on ecological embryology (E. S. Terekhin, 1968–77), which will make it possible to relate the evolution of embryological structures to environmental factors (biotic relations, behavioral reactions).

In the USSR and abroad, experimental plant embryology is being developed with the use of cytochemical and histochemical methods for studying cultures of cells, tissues, and generative organs. New methods are being developed for live observations using phase-contrast, Anoptral, and ultraviolet microscopic techniques and cinemicrography. Data from electronic and scanning microscopy are becoming especially important, expanding our knowledge of embryonic structure immeasurably.

Modern problems. A basic feature of modern plant embryology is the interweaving of theory and practice. Embryologists are seeking laws that are the bases for complex phenomena, for example, microgametogenesis and macrogametogenesis, double fertilization, embryogeny, endospermatogenesis, polyembryony, and apomixis. The study of these processes is of great significance for working out problems of evolution, phylogeny, morphogenesis, and species formation. The solutions to many questions confronting geneticists and breeders are based on the research in plant embryology (distant hybridization, sterility, apomixis). The genesis of male and female gametes and their origin and evolution in different phyla in the plant kingdom are among the basic problems of plant embryology. The next question that must be dealt with is the reexamination of outdated concepts on the homology of the embryo sac and the pollen grain in angiosperms to the gametophytes of archegoniates. Many aspects of apomixis, polyembryony, and parthenocarpy also require further study.

Scientific centers and periodicals. In the USSR plant embryology is studied at the V. L. Komarov Botanical Institute of the Academy of Sciences of the USSR, the Central Botanical Gardens of the Academy of Sciences of the USSR, the Institute of Botany of the Academy of Sciences of the Ukrainian SSR, the Institute of Botany of the Academy of Sciences of the Georgian SSR, the Botanical Gardens of the Academy of Sciences of the Moldavian SSR, the Institute of Experimental Plant Biology and the Institute of Botany of the Academy of Sciences of the Uzbek SSR, and the N. I. Vavilov All-Union Institute of Horticulture of the V. I. Lenin All-Union Academy of Agricultural Sciences. There are also departments of plant embryology at Moscow University and the universities of Kazan and Yerevan, as well as at a number of other research institutions throughout the country.

International research centers are located mainly in the botany departments of universities in many countries, including Yugoslavia, the USA, Poland, Austria, India, France, Japan, and Czechoslovakia.

Articles on plant embryology appear in the journal Phytomorphology, which has been published by the International Society of Plant Morphologists in Delhi since 1951. Every two years there is an international symposium on basic problems in embryology.