Angiosperms

The flowering plants or angiosperms, (also known as Angiospermae or Magnoliophyta), are the most diverse group of land plants. They are are seed-producing plants - like the gymnosperms - but can be distinguished from the gymnosperms by a series of synapomorphies (derived characteristics). These characteristics include flowers, endosperm within the seeds, and the production of fruits that contain the seeds. The ancestors of flowering plants probably diverged from gymnosperms around 245–200 million years ago, and the first flowering plants known to exist are from 140 million years ago. They diversified enormously during the Lower Cretaceous and became widespread around 100 million years ago, but replaced conifers as the dominant trees only around 60–100 million years ago. Flowers are the reproductive organs of flowering plants, and are the most remarkable feature distinguishing them from other seed plants and endow angiosperms by enabling a wider range of adaptability and broadening the ecological niches open to them. This has allowed flowering plants to largely dominate most terrestrial ecosystems. Stamens are the pollen-producing reproductive organs of a flower, typically consisting of a stalk and an anther, which contains microsporangia - typically four microsporangia - which form sacs (locules) in the anther. Each microsporangium is lined with a nutritive tissue layer called the tapetum and initially contains diploid pollen mother cells. These undergo meiosis to form haploid spores. Each microspore then divides mitotically to form an immature microgametophyte called a pollen grain. The pollen is eventually released by the opening - or dehiscence - of the anther. When ready, the pollen is carried by some external agent (wind, water or some member of the animal kingdom) to the receptive surface of the carpel of the same or another flower. This process is known as pollination. After successful pollination, the pollen grain (immature microgametophyte) completes its development by growing a pollen tube and undergoing mitosis. A flower is called an androecium. The androecium forms a whorl surrounding the gynoecium (carpels) and inside the perianth (the petals and sepals together) if there is one. Stamens can be free or fused in various ways. A column formed from the fusion of multiple filaments is known as an androphore. Stamens have become modified through time which prevents self-fertilization, producing further diversification, allowing angiosperms eventually to fill more niches. The closed carpel of angiosperms also allows adaptations to specialized pollination syndromes and controls. This helps to prevent self-fertilization, thereby maintaining increased diversity. Once the ovary is fertilized, the carpel and some surrounding tissues develop into a fruit. This fruit often serves as an attractant to seed-dispersing animals. The resulting cooperative relationship presents another advantage to angiosperms in the process of dispersal. Generally endosperm formation begins after fertilization and before the first division of the zygote. Endosperm is a highly nutritive tissue that can provide food for the developing embryo, the cotyledons, and sometimes the seedling when it first appears. Petala are usually accompanied by another set of special leaves called sepals lying just beneath the corolla. Petals - are modified leaves that surround the reproductive parts of flowers. They are often brightly colored or unusually shaped to attract pollinators. NB - both sepals and petals are modified leaves. Petala are usually accompanied by another set of special leaves called sepals lying just beneath the corolla. Corolla - together, all of the petals/sepals of a flower are called a corolla. Tepals - petals and sepals of a flower that look similar Calyx (plural, calices, the sepals) and the corolla (the petals) are the outer sterile whorls of the flower, which together form what is known as the calyx. Perianth - The term has two similar but separate meanings in botany: In flowering plants, the perianth is composed of the outer, sterile whorls of a flower - either consist of the calyx (all sepals) and the corolla (all petals), or, if sepals and petals are not differentiated, of the perigone or tepals. In liverworts or the Marchantiophyta** it’s the sterile tubelike tissue that surrounds the female reproductive structure or developing sporophyte. Carpel or Gynoecium (from Ancient Greek γυνή, gyne, meaning woman, and οἶκος, oikos, meaning house) is most commonly used as a collective term for all carpels in a flower. A carpel is the ovule and seed producing reproductive organ in flowering plants. Carpels are derived from ovule-bearing leaves, which evolved to form a closed structure containing the ovules. They did this by folding and fusing at their edges to form a chamber in which the ovules develop. In many flowers, several to many carpels are fused into a structure that resembles a single carpel. The term gynoecium is useful because it refers to the ovule producing structure in a flower, whether it is a single carpel, multiple un-fused carpels or multiple fused carpels. In a typical flower, the gynoecium is the innermost whorl of structures and is surrounded by the androecium (stamens) and then by the perianth (all the petals and sepals). In imperfect or incomplete flowers the androecium and perianth, respectively, may be absent. The gynoecium is often referred to as female because it gives rise to female (egg-producing) gametophytes, however, strictly speaking sporophytes do not have sex, only gametophytes do. Flowers that bear a gynoecium but no androecium are called carpellate. Flowers lacking a gynoecium are called staminate. A gynoecium may consist of a single carpel, multiple distinct (un-fused) carpels or multiple connate (fused) carpels. Each carpel typically contains one or more ovules. During pollination, pollen is deposited on the gynoecium (typically on a stigma). Successful germination of pollen and growth of pollen tubes results in fertilization of ova. There is typically one ovum in each ovule. After fertilization, ovules develop into seeds, and the gynoecium forms the pericarp of the associated fruit. Notes - Undifferentiated tepals are thought to be the ancestral condition in flowering plants. Amborella, which is thought to have separated earliest in the evolution of flowering plants, has flowers with undifferentiated tepals. Distinct petals and sepals would therefore have arisen by differentiation, probably in response to animal pollination. In typical modern flowers, the outer or enclosing whorl of organs forms sepals, specialised for protection of the flower bud as it develops, while the inner whorl forms petals, which attract pollinators. In some plants the flowers have no petals, and all the tepals are sepals modified to look like petals. These organs are described as petaloid, e.g. the sepals of Hellebore. Undifferentiated tepals are common in Monocotyledons. In tulips, for example, the first and second whorls both contain structures that look like petals. These are fused at the base to form one large, showy, six-parted structure. In lilies the organs in the first whorl are separate from the second, but all look similar, thus all the showy parts are often called tepals. Usage of the term 'tepal' is inconsistent - some authors refer to 'sepals and petals' where others use the word 'tepals' in the same context.