21.6 Angiosperms  (Page 19/3)

Introduction

The ginkgo tree is from the era of dinosaurs, but while the dinosaur has been extinguished, the modern ginkgo has not changed. After the atomic bomb in Hiroshima, the ginkgo was the first tree that came up. It’s amazing.

From their humble and still obscure beginning during the early Jurassic period, the angiosperms—or flowering plants—have evolved to dominate most terrestrial ecosystems ( [link] ). With more than 250,000 species, the angiosperm phylum (Anthophyta) is second only to insects in terms of diversification.

The success of angiosperms is due to two novel reproductive structures: flowers and fruit. The function of the flower is to ensure pollination. Flowers also provide protection for the ovule and developing embryo inside a receptacle. The function of the fruit is seed dispersal. They also protect the developing seed. Different fruit structures or tissues on fruit—such as sweet flesh, wings, parachutes, or spines that grab—reflect the dispersal strategies that help spread seeds.

Flowers

Flowers are modified leaves, or sporophylls, organized around a central stalk. Although they vary greatly in appearance, all flowers contain the same structures: sepals, petals, carpels, and stamens. The receptacle attaches the flower to the plant. A whorl of sepals (collectively called the calyx ) is located at the base of the peduncle (stem of the flower) and encloses the unopened floral bud. Sepals are usually photosynthetic organs, although there are some exceptions. For example, the corolla in lilies and tulips consists of three sepals and three petals that look virtually identical. Petals , collectively the corolla , are located inside the whorl of sepals and often display vivid colors to attract pollinators. Flowers pollinated by wind are usually small, feathery, and visually inconspicuous. The sexual organs (carpels and stamens) are located at the center of the flower.

As illustrated in [link] , styles, stigmas, and ovules constitute the female organ: the carpel . Flower structure is very diverse, and carpels may be singular, multiple, or fused. Multiple fused carpels comprise a pistil . The megaspores and the female gametophytes are produced and protected by the thick tissues of the carpel. A long, thin structure called a style leads from the sticky stigma , where pollen is deposited, to the ovary , enclosed in the carpel. The ovary houses one or more ovules, each of which will develop into a seed upon fertilization. The male reproductive organs, the stamens , surround the central carpel. Stamens are composed of a thin stalk called a filament and a sac-like structure called the anther. The filament supports the anther , where the microspores are produced by meiosis and develop into pollen grains.