Cones, Flowers, and Seeds

<h1>Text Preview</h1> <p>slide 1</p> <p>Seed plants are at the extreme in the evolutionary trend of plants toward smaller and more dependent gametophytes. Spores are never released from the sporophyte.</p> <p>Instead, they develop into gametophytes while still contained within the sporophyte.</p> <p>Seed plants possess both male and female sporangia. A female sporangium produces a female spore, which develops into a female gametophyte. This gametophyte generates an egg cell, while remaining enclosed with an organ of the sporophyte called an ovule. Meanwhile, a male sporangium on another plant of the same species produces male spores that develop into tiny, immature male gametophytes. Each of these gametophytes is a grain of pollen.</p> <p>A pollen grain is carried by wind or an animal to the ovule. It lands, germinates, and develops into a mature male gametophyte that releases its sperm and fertilizes the egg cell. The resulting zygote develops into a sporophyte embryo. The entire ovule becomes a seed, the protective container for this embryo.</p> <p>slide 2</p> <p>Gymnosperms are a diverse group of plants, distinguished by the lack of a protective cover around their seeds. The seeds are often carried on clusters of specialized leaves having the shape of a cone. Gymnosperms were the dominant plant life during the age of the dinosaurs.</p> <p>The best known gymnosperms of today are the conifers, a group that includes trees like pines, spruces, firs, and redwoods. These plants form vast forests in parts of the Northern Hemisphere. Their needle-like leaves are adapted to prevent the water loss that might occur in cold, windy climates and at high altitudes. Lesser-known groups of gymnosperms are the palmlike cycads, the gnetophytes like Ephedra, and the ginkgo. The ginkgoes were a diverse group of plants during the Mesozoic era; only a single species, Ginkgo biloba, has survived till today. You may recognize Ephedra and Ginkgo biloba as medicinal plants.</p> <p>slide 3</p> <p>Let’s consider the life cycle of a well-known gymnosperm: the pine tree. Pines and other conifers have both male cones and female cones, which contain male and female sporangia. The sporangia produce spores that develop into female and immature male gametophytes within the cones. The male cones release their immature male gametophytes as pollen, which is carried by the wind to a female cone. When a pollen grain reaches an ovule within a female cone, it develops further and begins to form a tube that will conduct its sperm to the egg within the ovule.</p> <p>Fertilization may occur a year after pollination—these are two distinct events in the life cycle. The zygote develops into a sporophyte embryo within the ovule. When the ovule has matured into a seed, it’s released. If the seed finds suitable ground and germinates, it begins its life as a pine tree.</p> <p>slide 4</p> <p>Angiosperms are the most diverse and widespread of all plants. Their defining feature is their reproductive organ, the flower. Although many angiosperms have conspicuous flowers, in some, like certain trees and grasses, the flowers aren’t obvious.</p> <p>Animals and humans alike depend on flowering plants and their seeds and fruits for food. Humans also make use of these plants for building materials, fuel, fibers, and medicines. </p> <p>Angiosperms form a monophyletic taxon whose most recent common ancestor was a gymnosperm. The earliest known fossil angiosperms date to the Cretaceous period. At the end of the Cretaceous, when many species of gymnosperms and nonvascular plants disappeared along with the dinosaurs, the angiosperms began to expand and diversify. They came to dominate the terrestrial habitats of the Cenozoic era.</p> <p>slide 5</p> <p>Flowers have both male and female reproductive structures. The male structure is the stamen. The female structure is the carpel. One or more ovules are contained within a protective structure called an ovary, at the base of the carpel. The ovary distinguishes angiosperms from gymnosperms.</p> <p>Pollen is produced in the anther of the stamen. Animals such as insects are commonly the vehicles by which the pollen is delivered to the carpel of another flower. If a pollen grain lands on the sticky stigma of a carpel, it germinates into a male gametophyte and produces a tube that grows down the style. The style is a column of tissue that connects the stigma to the ovary.</p> <p>When the tube reaches the ovule, the male gametophyte releases two sperm cells. One sperm cell fertilizes the egg cell. The other sperm cell combines with a special diploid cell of the female gametophyte, which develops to form a triploid tissue called the endosperm. Endosperm acts as a food supply for the developing sporophyte embryo. This process of double fertilization is typical of angiosperms. After double fertilization, the ovule becomes a seed, and the ovary ripens into a fruit.</p> <p id="TextCopyright">Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education</p>