BOTANY. PLANT MORPHOLOGY - O. A. Shevchuk - 2014

ORIGIN OF THE FLOWER

The question of the Water/144.html">Origin of the flower remains ultimately unresolved. Various hypotheses or theories exist, each with its own shortcomings.

Pseudanthial Theory (pseudo — false, anthos — flower)

Richard Wettstein proposed the pseudanthial theory. According to this concept, floral evolution progressed toward gradual improvement from a more primitive, monochlamydeous, anemophilous type to a more highly developed entomophilous type with a double perianth and a more constant, regular number of floral parts. Under this theory, the bisexual flower of angiosperms is a simplified inflorescence (a pseudanthium) formed by the aggregation of unisexual male and female "flowers" of higher gymnosperms, particularly Ephedra; here, each stamen and pistil is homologous to a simple flower.

Strobilar and Euanthial Theories of Floral Origin (The Theory of the True Flower)

The authors of this theory are English scientists A. Arber and J. Parkin. The views of A. Engler and R. Wettstein regarding the primitiveness of monochlamydeous PLANTS AND THEIR taxonomic systems were subjected to harsh criticism in the late 19th and early 20th centuries by the American botanist Bessey, English scientists A. Arber and J. Parkin, and especially the German botanist Hallier. A new theory of floral origin was developed and proposed—the euanthial theory, or the theory of the true flower. A convincing argument for The Development of the strobilar theory was the discovery of Mesozoic gymnosperms—bennettitaleans, which possessed bisexual strobili and were entomophilous.

According to this theory, the angiosperm flower is not an inflorescence, as Wettstein believed, but a true flower that evolved from the primitive strobili of bennettitaleans (hence this theory is also called strobilar).

The flower of ancestral angiosperms is viewed as a modified strobilus, its receptacle representing a shortened and metamorphosed strobilus axis, while stamens and carpels are modified sporophylls. The ancestral, primitive floral type of angiosperms is recognized as the Magnolia-type flower with an indefinite (large) number of stamens (microsporophylls) and free carpels (macrosporophylls), a convex receptacle, a well-developed perianth, and a spiral arrangement of floral elements. From this ancestral floral type, other types subsequently arose, primarily As a result of a reduction in the number of floral parts, a transition from acyclic to hemicyclic and cyclic arrangements, the fusion of floral elements, The Emergence of zygomorphy, Asymmetry, and other lines of specialization.

In light of the euanthial theory and the recognition of Magnoliaceae and other polycarpous taxa as the ancestral primitive type of angiosperms, the "simplicity" of the flower Structure in monochlamydeous plants is considered a secondary phenomenon—a consequence of reduction associated with the transition to wind pollination.

The euanthial and pseudanthial theories of floral origin are fundamentally "foliar" theories (from the Latin "foliaris" — pertaining to a leaf). According to J. Goethe, the flower is interpreted as a metamorphosed SHOOT comprising a stem and leaves, all "parts" of which (except the receptacle) represent modified leaves.

Telomome Theory

The classical foliar theory was succeeded by a new "telome" theory of floral origin. The prerequisite for the telome theory was the discovery between 1917 and 1920 of the most primitive group of vascular plants—psilophytes. The German botanist and phylogeneticist W. Zimmermann was a particularly vigorous proponent and developer of the telome theory. He believed that all Organs of vascular plants, including the flower, evolved over a long period from the telomes of psilophytes. Terminal telomes bearing sporangia could have given rise to stamens and pistils without passing through a leaf stage.

In W. Zimmermann's view, a stamen can be conceptualized, through simplification and schematization, as a system of four fused telomes bearing terminal sporangia; by fusing together, the sporangia formed an anther—a synangium composed of four sporangia. Similarly, but through even more complex modifications and fusions, carpels bearing ovules—which gave rise to the pistil—could have formed from sporangial telomes. This scientist regards petals as modified stamens, whereas only sepals possess a true leaf nature, originating from apical leaves. The telome theory of THE ORIGIN OF vegetative organs and the angiosperm flower is quite interesting and has many supporters, yet it remains insufficiently substantiated by empirical data. The classical foliar euanthial theory is more convincing.

Other Theories of the Origin of the Flower and Its Parts

The English scientist H. Thomas believes that the angiosperm carpel has a complex nature. The walls of the carpel are formed by two fused cupules, between which lie remnants of axial tissue, while the placentas represent the apices of fertile branchlets.