Botany - B.E. Yakubenko 2017

Part Three. Kingdom Plantae
Chapter IX. Division Angiosperms, or Flowering Plants (Angiospermae, Magnoliophyta)

Flowering plants, or angiosperms, constitute the largest division of the plant kingdom, encompassing over 240,000 species that inhabit every climatic zone and diverse ecological niches. They form the foundation of the biosphere's plant biomass and represent the most vital group of plants for humanity. Emerging during the Cretaceous period of the Mesozoic era, flowering plants rapidly spread and assumed a dominant position on the planet.

A defining characteristic of angiosperms is the flower, which develops from a bud as a shortened SHOOT performing a reproductive function. A typical flower consists of a pedicel, receptacle, perianth, gynoecium, and androecium (Fig. 100).

Class="center">Fig. 100

The pedicel is the stalk connecting the flower to the stem. Flowers lacking a pedicel are termed sessile. The receptacle is the expanded upper portion of the pedicel to which the other floral Organs are typically attached; its shape varies significantly. A convex or sometimes elongated receptacle, a spiral arrangement of floral parts on it, and an indefinite number of elements are considered primitive floral traits. A flat or concave receptacle and a definite number of floral elements are characteristic of more highly evolved angiosperms.

The perianth is the floral envelope that serves a protective function and attracts insect pollinators. A simple perianth consists solely of a calyx, as in herb-paris, or a corolla (usually colored), as in a tulip. A double perianth consists of both a calyx and a corolla (e.g., in cherries). The calyx is composed of green sepals, which may be free or fused, while the corolla similarly consists of free or fused petals. Petals vary greatly in shape and color. A polypetalous (free-petalled) corolla can be papilionaceous, as in legumes (Fig. 101), where its petals are referred to as the standard (banner), wings, and keel. A gamopetalous (sympetalous) corolla may be tubular, ligulate, biligulate, funnel-shaped, rotate, or bilabiate. In some plants, the perianth is reduced to scales or entirely absent, in which case the flower is termed naked.

Figure 101. Types of corollas:

a — actinomorphic; b — zygomorphic; c — papilionaceous; d — bilabiate;

e — tubular; f — funnel-shaped; g — rotate; h — campanulate

A corolla and a flower through which multiple planes of Symmetry can be passed are termed regular (actinomorphic), whereas a flower through which only a single plane of symmetry can pass is called zygomorphic, or irregular (see Fig. 101).

The most essential elements of the flower are the gynoecium and the androecium. The gynoecium is the aggregate of carpels forming one or more pistils. If a pistil is formed by a single carpel or multiple unfused carpels, the gynoecium is termed apocarpous. A pistil composed of fused carpels forms a syncarpous gynoecium, which can be unilocular or multilocular. A pistil consists of an Ovary, style, and stigma. Inside the ovary are ovules, within which the female gametophyte (the eight-nucleate embryo sac) develops from a single megaspore.

The androecium is the aggregate of stamens. A stamen consists of a filament, an anther, and a connective (see Fig. 91). Microspores are formed within the anthers and develop into highly reduced male gametophytes (two-celled pollen grains).

An exceptionally crucial distinguishing feature of flowering plants is double Fertilization, which results in The formation of not only the embryo but also the secondary endosperm. While in gymnosperms the endosperm develops prior to fertilization and Functions as a female gametophyte (haploid), in angiosperms it is formed As a result of fertilization (triploid) and carries both maternal and paternal hereditary traits. This circumstance allows double fertilization to be viewed as an advanced evolutionary adaptation that enhances the vitality of offspring as well as their morphological and ecological plasticity. In angiosperms, as in the vast majority of vascular plants, The life cycle involves the alternation of two generations—the sporophyte and the gametophyte—with the sporophyte being dominant. A greater reduction of gametophytes occurs compared to gymnosperms, along with their accelerated development due to a minimal number of mitotic divisions. The gametophytes of flowering plants have lost antheridia and archegonia.