PLANT MORPHOLOGY - T. A. Sautkina - 2012

CHAPTER 7. THE FLOWER AS A SPECIALIZED REPRODUCTIVE ORGAN OF ANGIOSPERMS

7.3. Hypotheses of Flower Origin

Because flowering plants play a vital role in human life, the question of their origin—and THE ORIGIN OF their specialized reproductive organ, the flower—remains of paramount importance in botanical research across various disciplines. Despite the vast body of factual material accumulated by morphologists, taxonomists, embryologists, and paleobotanists, the famous words of the great naturalist Charles Darwin regarding the "abominable mystery" surrounding flowering plants have lost none of their significance today.

Indeed, the question of the morphological nature of flower parts, which J. W. von Goethe attempted to resolve, failed to solve the broader problem of the Water/144.html">Origin of the flower as a distinct reproductive organ.

The desire to identify potential ancestors of angiosperms and, by studying their reproductive Organs, to discover an organ that could have served as a precursor to the flower led botanists to examine fossil gymnosperms. This gave rise to the pseudanthial hypothesis (from the Greek pseudo meaning false and anthos meaning flower) and the euanthial or strobilar hypothesis (from the Greek eu meaning true and anthos meaning flower) concerning the origin of the flower.

The pseudanthial hypothesis was formulated by the Austrian botanist R. Wettstein in 1901. He considered fossil fossilized Gnetales (such as Ephedra) to be the ancestral group of angiosperms; these plants lack typical cones but possess peculiar structures known as microstrobili and megastrobilis (Fig. 147). An individual microstrobilus outwardly resembles an angiosperm stamen, while a cluster of microstrobili surrounded by bracts resembles an inflorescence. A megastrobilus consists of a single ovule surrounded by an envelope (sterile scale-like leaves). Wettstein hypothesized that reduction of the bracts in a microstrobilus could give rise to a staminate flower with a simple perianth, while the megastrobilus could give rise to a naked (apetalous) pistillate flower. According to Wettstein's concept, the bisexual flower of angiosperms is homologous to an inflorescence (a collection of micro- and megastrobili) of the Ephedrales, though he failed to conclusively demonstrate how unisexual flowers could have evolved into bisexual ones. Botanists gradually abandoned this hypothesis. Nevertheless, the pseudanthial hypothesis played a notable historical role and formed the basis for several systems of angiosperm Classification. The best known of these is The system of A. Engler (1844–1930), the only classification in which phylogenetic relationships are traced down to the genus level. It is still used today in several major herbaria, including the Herbarium of the Komarov Botanical Institute of the Russian Academy of Sciences in St. Petersburg.

Class="center">Fig. 147. Reproductive structures of Ephedra (Ephedra sp.): A—microstrobili; B—external appearance of a microstrobilus; C—external appearance of a megastrobilus (ovule); D—Cytology/practical/54.html">Longitudinal section of an ovule: 1—cluster of microstrobili; 2—individual microstrobilus; 3—bract of the microstrobilus; 4—synangium (anther with microsporangia); 5—sterile scale-like leaves; 6—ovule (megasporangium); 7—nucellus of the ovule; 8—integument; 9—pollen chamber; 10—female gametophyte

By the late 19th and early 20th centuries, conditions were ripe for understanding the evolutionary pathways of flower origin. Drawing on paleobotanical and morphological data, the American botanist and phylogeneticist C. Bessey (1845–1915) noted that the angiosperm reproductive organ consists of a stem bearing sporogenous and sterile leaves, and later suggested that angiosperms originated from strobiliferous cycads.

Independently of Bessey, the German taxonomist G. Hallier in 1902, based on The Study of a greening, proliferating flower of columbine (Aquilegia vulgaris), interpreted the flower as a modified reproductive SHOOT. Thus, Bessey and Hallier essentially laid the foundations for the euanthial, or strobilar, hypothesis of flower origin. However, the hypothesis was definitively formulated somewhat later.

In 1906, the German plant physiologist G. Wieland (1877–1957) described a bisexual strobilus in fossil gymnosperms known as Bennettitales (Fig. 148).

Fig. 148. Reconstruction of a longitudinal section of the strobilus of Cycadeoidea dacotensis: 1—cluster of megasporophylls with megasporangia; 2—microsporophylls with microsporangia; 3—bracts of the strobilus

This discovery enabled E. Arber and J. Parkin to definitively formulate the strobilar hypothesis of flower origin. According to this hypothesis, the flower is a bisexual strobilus adapted for reproduction. In the view of Arber and Parkin, the flower represents a modern Modification of the strobilus—the anthostrobilus (Fig. 149). Arber and Parkin believed that Bennettitales were probably not direct ancestors of flowering plants, but might have given rise to some hypothetical semi-angiosperms that have not yet been discovered. In their paper "On the Origin of Angiosperms" (1907), the authors established criteria for flower primitiveness and specialization and presented their perspective on the phylogenetic system of angiosperms, which stands in stark contrast to Wettstein's view. While Wettstein considered the most primitive families to be those with small, unisexual, wind-pollinated flowers lacking a perianth or possessing a reduced perianth, Arber and Parkin maintained that the most primitive families comprise plants with large, bisexual, insect-pollinated flowers featuring a simple or double perianth.

Fig. 149. Proanthostrobilus—a hypothetical flower of an ancestral angiosperm type: 1—flower axis; 2—megasporophylls; 3—microsporophylls; 4—bracts

The discovery of psilophytes (Rhyniophyta), recognized as the most primitive vascular plants, provided a reason to revisit Structure/149.html">The problem of flower origin and put forward the telome theory. Its founder was W. Zimmermann (1930), who believed that all organs of higher plants evolved from simple, undifferentiated elements he termed telomes. According to Zimmermann's concepts, sterile telomes (phylloids) and fertile telomes bearing sporangia could fuse to form syntelomes. The flower, in Zimmermann's view, is a syntelome whose emergence occurred in parallel with The formation of the vegetative shoot. Although this theory was not elaborated in complete detail, it was of great significance for resolving certain questions regarding The Nature of floral organs. From the 1940s and 1950s—a period of particularly intense debate between proponents of the telome theory and adherents of classical Morphology—the flower ceased to be defined as a metamorphosed vegetative shoot.

According to V. N. Tikhomirov (1932–1998), a Corresponding Member of the Russian Academy of Sciences and professor at Moscow State University, it is fundamentally impossible to reduce the vast diversity of flowers to a single model. Taking floral polymorphism into account, he suggested that the flowers of modern angiosperms evolved along different pathways during the evolutionary diversification of reproductive structures from a common ancestral group.

With The Emergence of new scientific data, novel hypotheses regarding the origin of flower evolution continue to appear, yet practically none of them are sufficiently convincing. Summarizing all knowledge in this field, the renowned Soviet botanist B. M. Kozo-Polyansky (1890–1957) was compelled to state: "Neither the ancestor of flowering plants nor the specific historical precursor organ of the flower is known."