Systematics, Plant

a branch of systematics. Plant systematics has a long history. The first attempts at botanical classification were based on a few, very obvious external characters related by neither internal unity nor origin. Modern systems are based on a large number of facts and take into account genuine kinship ties. Theophrastus divided plants into four groups—trees, shrubs, subshrubs, and herbs. The only subsequent pre-Renaissance observations of interest are those of Albertus Magnus (Albert of Bollstädt), who was the first to note the difference between monocotyledonous and dicotyledonous plants. In 1583, Andrea Cesalpino published the first artificial classification of plants, which was based mainly on the structure of the fruits and seeds. In addition to the four plant groups proposed by Theophrastus, Cesalpino distinguished a group of seedless plants, which included ferns, mosses, fungi, and algae. At the end of the 16th century, G. Bauhin established the separate categories of genus and species and laid the foundations of binominal nomenclature. In 1693 the English naturalist J. Ray established the concept of the species, and in 1700 the French botanist Tournefort established the concept of genus. The third principal taxonomic category, the family, was defined earlier, in 1689, by the French botanist P. Magnol. Tournefort’s system, which was based on the structure of the corolla (class Labiatae, class of tetrapetalous flowers), received wide acceptance owing to its simplicity. More complex, but more natural, was Ray’s system (1686–1704), which included the groups Dicotyledones (dicots) and Monocotyledones (monocots). Ray divided these two groups into classes according to the type of fruit, and the classes were broken down into smaller categories according to the structure of the leaf and flower.

The best of the artificial systems was that of C. Linnaeus (1735), at the basis of which were the number of stamens, the means of stamen concrescence, and the distribution of unisexual flowers. Linnaeus arranged all seed plants into 23 classes; to a 24th class, Cryptogamia, he assigned algae, fungi, mosses, and ferns. Because of the extreme artificiality of Linnaeus’ classification, the most diverse genera were placed in the same class, whereas genera belonging indisputably to the same natural family (for example, cereals) often turned up in different classes. Nevertheless, Linnaeus’ system was widely adopted not only among specialists but also among amateur botanists, since it afforded a quick way of identifying plants. Linnaeus perfected and consolidated the binominal system of nomenclature in botany. The generic and specific names currently accepted by the International Code of Botanical Nomenclature for most groups of extant plants date back to 1753, when the first edition of Linnaeus’ Species plantarum was published.

A turning point in the establishment of a natural method of plant systematics was the publication of Families of Plants by the French scientist M. Adanson (1763–64), who considered it necessary to use in classification the maximum number of characters, assigning the same significance to each character. Of even greater importance in the development of plant systematics was the system proposed in 1789 by the French botanist A. L. de Jussieu, which divided plants into 15 classes, comprising 100 “natural orders” (ordines naturales). Jussieu named and described these orders, most of which have been preserved to this day as families (for example, Gramineae, Campanulaceae, Rosaceae, and Papaveraceae). Jussieu united fungi, algae, mosses, ferns, and naiads under the name Acotyledones. He also divided seed plants, except naiads, into Monocotyledones and Dicotyledones, assigning the Coniferae to the latter.

The most important classification system introduced in the 19th century was that of A. P. de Candolle (1813, 1819). De Candolle divided the plant world into two phyla: vascular and cellular (avascular) plants. To the vascular plants he assigned the dicotyledons and the monocotyledons; the latter contained the subclass Cryptogamae, which included horsetails, ferns, club mosses, and naiads. De Candolle’s system was subsequently modified by a number of botanists. In 1825 the English botanist R. Brown established the difference between gymnospermous and angiospermous (flowering plants). In the same year the Russian scientist M. A. Maksimovich (1804–73) set forth the theoretical principles of natural systematics in On Systems of the Plant Kingdom.

In Europe the classification system of the Austrian botanist S. Endlicher, which he elaborated from 1836 to 1840, was widely accepted. Endlicher divided all plants into two kingdoms: Thallophyta (algae, lichens, and fungi) and Cormophyta (higher plants). This system was incorporated into many subsequent systems, despite Endlicher’s failure to go beyond de Candolle in the description of the cormophytes. Hence, the conifers and joint firs remained among the dicotyledons, and the sago palms remained in the same group as the horsetails, ferns, club mosses, and lepidodendrons. Endlicher also included Balanophora, rafflesias, and certain other parasitic dicotyledons among the cormophytes.

In 1843 French botanist A. Brogniart divided plants into cryptogams (all seedless) and phanerogams. The latter were divided into monocotyledons and dicotyledons (angiosperms and gymnosperms).

A significantly improved variant of de Candolle’s system was the natural system proposed by the English botanists G. Bentham and J. Hooker (1862–83). All natural systems were not natural in the modern sense of the word. Almost all their authors believed in the fixity of species; however, the plants were united on the basis of affinity, that is, only resemblance, and not by kinship in the evolutionary sense.

The rudiments of evolutionary, or phylogenetic, plant systematics existed even before the revolution in biology produced by C. Darwin. For example, the Russian botanist P. F. Gorianinov as early as 1834 advanced the idea of a general evolution of nature—from simple forms to more perfect ones. The origin of modern evolutionary plant systematics dates to 1859, the year Darwin’s Origin of Species was published. One of the first systems influenced by Darwin’s theory was proposed in 1864 by the German scientist A. Braun. In 1875 the German botanist A. Eichler, also an evolutionist, proposed a botanical classification system, which, in contrast to Braun’s system, considered choripetalous plants to be more primitive than gamopetalous ones.

Eichler’s system was developed further by the German botanist A. Engler, who used it as the basis of his multivolume Natural Plant Families (1887–1909). Engler’s system, which was worked out down to genera and sections, received almost worldwide acceptance. Its organization, however, did not differ essentially from that of Eichler’s system. Moreover, Engler’s proposal of the independent (polyphyletic) origin of angiosperms from different groups of extinct gymnosperms was not confirmed. In 1901 the Austrian botanist R. von Wettstein modified and substantially improved Engler’s system. He placed monocotyledons after dicotyledons, and he considered the group Polycarpicae the ancestor of the most primitive representatives of monocots (Alismataceae, Butomaceae, Liliaceae) and families closely related to them. However, the artificial group Pteridophyta was still preserved in this system, and casuarinas and other Monochlamydeae were placed at the basis of the system of dicotyledons. Wettstein’s system was somewhat modified and improved by the Dutch botanist A. Pulle (1937 and later).

In the early 20th century the reclassification of the group Algae was undertaken, first by Wettstein and Engler and later by A. Pascher (1914, 1921, 1931) and many other investigators. Modern classification systems designate several independent phyla of algae. Taxonomic revision of another completely artificial group, Pteridophyta, was begun in 1889 by the American anatomist E. Jeffrey and was continued by the English paleobotanist D. Scott and many others. As a result, this group was divided into the independent phyla Psilophyta, Psilotophyta, Lycopodiophyta, Equisetales, and Polypodiophyta. Many 20th-century botanists have tried to come up with a modern classification of Bryophyta (mosses), which at present is usually divided into three independent classes: Anthocerotales, Hepaticae, and Musci.

In the late 19th century the revision of flowering plants on an evolutionary basis was begun. The American botanist C. Bessey proposed a theoretically new system, based on recognition of the strobiloid nature of the flower and the primitiveness of Magnoliaceae, Calycanthaceae, Annonaceae, Ranunculaceae, Berberidaceae, Lauraceae, Dilleniaceae, Winteraceae, and related families. Bessey believed that progressive evolution is realized through both complication and simplification and emphasized that the polymerous structures of flowers precede the oligomerous structures. In addition, he analyzed the lower and higher organization of flowers and vegetative organs and established criteria establishing the level of evolutionary development of certain groups of flowering plants.

Bessey’s work was further developed in the USA by J. Schaffner (1929, 1934) and later by A. Cronquist (1968). Almost simultaneously with and yet independently of Bessey, the German botanist H. Hallier, a student of E. Haeckel, revised the classification of flowering plants. Hallier proposed an innovative system (1896, 1912) based on the synthesis of an enormous amount of factual material from various botanical disciplines and from plant chemistry. He advanced theories concerning the convergence of Papaveraceae with Ranunculaceae, the origin of the order Caryophyllales from Berberidaceae, and the origin of Salicaceae from Flacourtiaceae. In Russia these theories were first set forth in K. S. Merezhkovskii’s Outline Course of General Botany (1910). Soon after the introduction of Hallier’s system, a number of systems combining Engler’s and Hallier’s principles were proposed, for example, N. I. Kuznetsov’s system (1914). Kh. la. Gobi went considerably further than his predecessors, creating a new system not only for flowering plants but for the entire plant world. Gobi independently arrived at many of the principles of phylogenetic systematics and, moreover, correctly understood the significance of reduction. His system as a whole was very advanced.

In the USSR the development of phylogenetic plant systematics is associated first and foremost with B. M. Kozo-Polianskii and his pupils and followers. In Introduction to the Phylogenetic Systematics of Higher Plants (1922), Kozo-Polianskii proposed an innovative but in many ways, very controversial system of higher plants. One of his system’s great merits was the use of extensive factual material on comparative morphology; a shortcoming, however, was its one-sided and often very subjective interpretation of the data.

The classification system proposed by the English botanist J. Hutchinson (1926, 1934), which was based almost exclusively on the study of external morphology, was widely adopted. The major weakness of the system was the division of angiosperms into two “phyla”—Lignosae and Herbaceae. Hutchinson assigned all “basically” ligneous groups to the first phylum and all “basically” herbaceous ones to the second.

A number of new classification systems based on the recognition of the monophyletic origin of flowering plants have been proposed. Such systems have been elaborated by the Soviet botanists M. I. Golenkin (1937), A. A. Grossgeim (1945), A. L. Takhtadzhian (1954, 1959, 1966, 1973), and I. S. Vinogradov (1958), as well as by the foreign botanists A. Gundersen (1950, dicots only), R. Soó (1953, 1961), F. Novák (1954, 1961), M. Deyl (1955, monocots only), F. Němeje (1956), Y. Kimura (1956, monocots only), A. Cronquist (1957, dicots only, 1968), R. Thorne (1963), M. Tamura (1974), and R. Dahlgren (1974).

Evolutionary plant systematics is becoming increasingly synthetic; that is, there is maximum use of the data of comparative phytochemistry, serology, and comparative and evolutionary morphology (including comparative embryology, palynology, and karyology). The successful development of evolutionary systematics has been fostered to a great degree by the development of the modern theory of evolution and the theory of systematics itself. Some contemporary systems, especially those of Takhtadzhian, Cronquist, and Dahlgren, differ from each other considerably less than, for example, the systems of Bessey and Hallier. This may be explained by the mutual influence and definite convergence of these systems, as well as by the significantly increased objectivity of the methods of evolutionary systematics. Despite all the achievements of contemporary plant systematics, the elaboration of a system for the entire plant world is still far from complete.

The development of plant systematics has been furthered by the compilation of floras of entire countries (for example, Flora of the USSR, continents (for example, Flora Europaea), and various other regions. The literature on systematic botany also includes surveys of the systematic composition of a given regional flora and monographs of certain more interesting and important taxa. Much research has been conducted on genera, subgenera, sections, and individual species.