Question

1) In what way was the seed such an important adaptation in the evolution of plants? 2) What could be the advantage having a free-living sporophyte as the dominant generational phase3) Discuss the role and evolutionary importance of the development of the pollen tube 4) Compare and contrast the origin and ploidy of the stored food that support the embryo in angiosperms and gymnosperms

Answer 1

1.The evolution of seeds allowed plants to reproduce independently of water; pollen allows them to disperse their gametes great distances.

Evolution of Seed Plants

The lush palms on tropical shorelines do not depend upon water for the dispersal of their pollen, fertilization, or the survival of the zygote, unlike mosses, liverworts, and ferns of the terrain. Seed plants, such as palms, have broken free from the need to rely on water for their reproductive needs. They play an integral role in all aspects of life on the planet, shaping the physical terrain, influencing the climate, and maintaining life as we know it. For millennia, human societies have depended upon seed plants for nutrition and medicinal compounds; and more recently, for industrial by-products, such as timber and paper, dyes, and textiles. Palms provide materials including rattans, oils, and dates. Wheat is grown to feed both human and animal populations. The fruit of the cotton boll flower is harvested as a boll, with its fibers transformed into clothing or pulp for paper. The showy opium poppy is valued both as an ornamental flower and as a source of potent opiate compounds.

Seeds and Pollen as an Evolutionary Adaptation to Dry Land

Unlike bryophyte and fern spores (which are haploid cells dependent on moisture for rapid development of gametophytes ), seeds contain a diploid embryo that will germinate into a sporophyte. Storage tissue to sustain growth and a protective coat give seeds their superior evolutionary advantage. Several layers of hardened tissue prevent desiccation, freeing reproduction from the need for a constant supply of water. Furthermore, seeds remain in a state of dormancy induced by desiccation and the hormone abscisic acid until conditions for growth become favorable. Whether blown by the wind, floating on water, or carried away by animals, seeds are scattered in an expanding geographic range, thus avoiding competition with the parent plant.

Pollen grains are male gametophytes carried by wind, water, or a pollinator. The whole structure is protected from desiccation and can reach the female organs without dependence on water. Male gametes reach female gametophyte and the egg cell gamete though a pollen tube: an extension of a cell within the pollen grain. The sperm of modern gymnosperms lack flagella, but in cycads and the Gingko, the sperm still possess flagella that allow them to swim down the pollen tube to the female gamete; however, they are enclosed in a pollen grain.

Evolutionary importance of the development of the pollen tube

Many transitional features have been identified that show correlation between the evolution of the pollen tube with that of a non-motile sperm. Early seed plants like ferns have spores and motile sperm that swim in a water medium, called zooidogamy.The angiosperm pollen tube is simple, unbranched, and fast growing, however this is not the case for ancestral plants.

In gymnosperms like Ginkgo biloba and cycadophyta, a haustorial pollen tube forms. The tube simply soaks up nutrients from the female nucellus and grows in two stages. The pollen tube is highly branched and grows on the female sporophyte tissues. First, it grows the main tube followed by a more spherical tip at the end to allow the sperm to burst near the archegonia. The binucleated, multiflagellated sperm can then swim to the egg. Cycads have a less branched structured and the tip end swells the same way as in the ginkgo. In cycads, however, various enzymes have been identified in the pollen tube that direct growth and the nucellus tissues are more damaged with the tube growth.

In other phyla of gymnosperms, coniferophyta and gnethophyta, the sperm is non motile, called siphonogamy and the pollen tube grows through the archegonia to help the sperm reach the egg more directly. Conifers can be branched or unbranched and they cause degeneration of the female tissue as it grows through more tissue. Pines, for instance discharge cytoplasm of the sperm and union of the one sperm occurs as the other sperm degenerates. Yet, in gnethophyta, there are features more similar to angiosperm pollen tubes where the tube reaches the egg with an early form of double fertilization. However, the endosperm does not form and the second fertilization is aborted.

In angiosperms, the mechanism has been studied more extensively as pollen tubes in flowering plants grow very fast through long styles to reach the well-protected egg. There is great variation in pollen tubes in angiosperms and many model plants like petunia, Arabidopsis, lily and tobacco plants have been studied for intraspecific variation and signaling mechanisms.In flowering plants, a phenomenon called polyambry can occur where many ovules are fertilized and overall fitness of the organism is yet to be studied with respect to rate of pollen tube growth.