28
Learning Goals
By the end of this reading you should be able to:
- Explain when seed plants first appeared and when gymnosperms became the dominant plant group
- Describe the two major innovations that allowed seed plants to reproduce in the absence of water
- Discuss the purpose of pollen grains and seeds
- Discuss the type of seeds produced by gymnosperms, as well as other characteristics of gymnosperms
Introduction
In seed plants (Fig. 1), the evolutionary trend led to a dominant sporophyte generation, and at the same time, a systematic reduction in the size of the gametophyte: from a conspicuous structure to a microscopic cluster of cells enclosed in the tissues of the sporophyte. Whereas lower vascular plants, such as club mosses and ferns, are mostly homosporous (produce only one type of spore), all seed plants, or spermatophytes, are heterosporous. They form two types of spores: megaspores (female) and microspores (male). Megaspores develop into female gametophytes that produce eggs, and microspores mature into male gametophytes that generate sperm. Because the gametophytes mature within the spores, they are not free-living, as are the gametophytes of other seedless vascular plants. Heterosporous seedless plants are seen as the evolutionary forerunners of seed plants.
The first naked seed plants (progmnosperms), arose about 380 million years ago and were a transitional group of plants that superficially resembled conifers (cone bearers) because they produced wood from the secondary growth of the vascular tissues. However, the progymnosperms still reproduced like ferns, releasing spores into the environment.
Two critical adaptations to dry land
Pollen and seed were innovative structures that allowed seed plants to break their dependence on water for reproduction and development of the embryo, and to conquer dry land.
Pollen: The pollen grains are male gametophytes that contain the sperm (gametes) of the plant and are carried by the wind, water, or a pollinator. The small haploid (1n) cells are encased in a protective coat that prevents desiccation (drying out) and mechanical damage. Pollen grains can travel far from their original sporophyte, spreading the plant’s genes and can reach the female organs without dependence on water. Male gametes reach the female gametophyte and the egg cell gamete through a pollen tube: an extension of a cell within the pollen grain. The sperm of ost modern gymnosperms lack flagella, except in cycads and the Gingko, where the sperm still possess flagella that allow them to swim down the pollen tube to the female gamete.
Seeds: Unlike bryophyte and fern spores (which are single haploid cells dependent on moisture for the rapid development of gametophytes), seeds contain a multicellular diploid embryo that will germinate into a sporophyte (Fig. 2). The seed offers the embryo protection, nourishment, and a mechanism to maintain dormancy for tens or even thousands of years, ensuring germination can occur when growth conditions are optimal.
The seed coat provides several layers of hardened tissue around the embryo that prevents desiccation and frees 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. Within the seed and surrounding the embryo are storage tissues that provide the embryo with needed nutrients when it begins to germinate and before it becomes photosynthetically active.
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. Seeds, therefore, allow plants to disperse the next generation through both space and time. With such evolutionary advantages, seed plants(Gymnosperms and Angiosperms) have become the most successful and familiar group of plants, in part because of their size and their vast diversity and distribution.
Review Question:
What are some of the advantages of seeds over spores? (Multiple Answers)A) they have mechanisms to prevent desiccation and spores do not
B) they are multicellular while spores are single cells
C) they contain nutrient stores which spores do not have
D) they are composed of haploid cells and spores are diploid
Key Characteristics of Gymnosperms
Gymnosperms, meaning “naked seeds,” are a diverse group of seed plants and are paraphyletic. Paraphyletic groups are those in which not all members are descendants of a single common ancestor. The shared characteristics of this group include naked seeds, separate female and male gametes, pollination by wind, and tracheids (which transport water and solutes in the vascular system).
Most Gymnosperms are adapted to live where freshwater is scarce during part of the year, or in the nitrogen-poor soil of a bog. Therefore, they are still the prominent phylum in the coniferous biome or taiga, where the evergreen conifers have a selective advantage in cold and dry weather. Many evergreen conifers are capable of low levels of photosynthesis during the cold months, and because of this, they can take advantage of the first sunny days of spring. However, evergreen conifers can be more susceptible than deciduous trees to infestations because these conifers do not lose their leaves all at once. They cannot, therefore, shed parasites and restart with a fresh supply of leaves in spring.
Gymnosperm seeds are not enclosed in an ovary like seeds of Angiosperms; rather, they are exposed on cones or modified leaves. These cones are composed of specialized sporophylls tightly arranged around a central stalk.
Life Cycle of a Conifer
The life cycle of a conifer can be seen in Figure 3. Pine trees are conifers (cone-bearing) and carry both male and female sporophylls on the same mature sporophyte. Therefore, they are monoecious plants. Like all gymnosperms, pines are heterosporous and generate two different types of spores: male microspores and female megaspores. In the male staminate cones, the microsporocytes give rise to pollen grains (microspores) by meiosis. These cones release large amounts of yellow pollen that is readily carried by the wind. Some of the pollen grains, containing the male gametophytes will land on a female cone. When the initiation of a pollen tube begins then pollination has occurred. The pollen tube develops slowly, and a specialized generative cell in the pollen grain divides into two haploid sperm cells by mitosis. At fertilization, one of the sperm cells will finally unite its haploid nucleus with the haploid nucleus of a haploid egg cell to create the diploid zygote.
Female ovulate cones, or, contain two ovules per scale. One megaspore mother cell, the megasporocyte, undergoes meiosis in each ovule producing four spore cells. Three of the four cells break down and the single surviving cell develops into a female multicellular gametophyte which encloses the archegonium. Upon fertilization, the diploid zygote will give rise to the embryo enclosed in a seed coat of tissue from the parent plant. Fertilization and seed development is a long process in pine trees: it may take up to two years after pollination. The seed that is formed contains three generations of tissues: the seed coat that originates from the sporophyte tissue, the gametophyte that will provide nutrients, and the embryo itself.
Review Question:
At what stage does the diploid zygote form?
A) When the female cone begins to bud from the tree
B) At fertilization
C) When the seeds drop from the tree
D) When the pollen tube begins to grow
Diversity of Gymnosperms
Modern gymnosperms are classified into four phyla. Coniferophyta, Cycadophyta, and Ginkgophyta are similar in their production of secondary cambium (cells that generate the vascular system of the trunk or stem and are partially specialized for water transportation) and their pattern of seed development. However, the three phyla are not closely related phylogenetically to each other. Gnetophyta is considered the closest group to angiosperms because they produce true xylem tissue.
Conifers (Coniferophyta)
Conifers are the dominant phylum of gymnosperms, with the most variety of species (Fig. 4). Most are typically tall trees that usually bear scale-like or needle-like leaves. Water evaporation from leaves is reduced by their thin shape and the thick cuticle. Snow slides easily off needle-shaped leaves, keeping the load light and decreasing the breaking of branches. Adaptations to cold and dry weather explain the predominance of conifers at high altitudes and in cold climates. Conifers include familiar evergreen trees such as pines, spruces, firs, cedars, sequoias, and yews. A few species are deciduous and lose their leaves in the fall. The European larch and the tamarack are examples of deciduous conifers. Many coniferous trees are harvested for paper pulp and timber. The wood of conifers is more primitive than the wood of angiosperms; it contains tracheids, but no vessel elements, and is therefore referred to as “softwood.”
Cycads
Cycads thrive in mild climates and are often mistaken for palms because of the shape of their large, compound leaves (Fig. 5). Cycads bear large cones and may be pollinated by beetles rather than wind: unusual for a gymnosperm. They dominated the landscape during the age of dinosaurs in the Mesozoic, but only a hundred or so species persisted to modern times. They face possible extinction, and several species are protected through international conventions. Because of their attractive shape, they are often used as ornamental plants in gardens in the tropics and subtropics.
Gingkophytes
The single surviving species of the gingkophytes group is the Gingko biloba (Fig. 6). Its fan-shaped leaves—unique among seed plants because they feature a dichotomous venation pattern—turn yellow in autumn and fall from the tree. For centuries, G. biloba was cultivated by Chinese Buddhist monks in monasteries, which ensured its preservation. It is planted in public spaces because it is unusually resistant to pollution. Male and female organs are produced on separate plants. Typically, gardeners plant only male trees because the seeds produced by the female plant have an off-putting smell of rancid butter.
Gnetophytes
Gnetophytes are the closest relative to modern angiosperms and include three dissimilar genera of plants: Ephedra, Gnetum, and Welwitschia (Fig. 7). Like angiosperms, they have broad leaves. In tropical and subtropical zones, gnetophytes are vines or small shrubs. Ephedra occurs in dry areas of the West Coast of the United States and Mexico. Ephedra’s small, scale-like leaves are the source of the compound ephedrine, which is used in medicine as a potent decongestant. Because ephedrine is similar to amphetamines, both in chemical structure and neurological effects, its use is restricted to prescription drugs. Like angiosperms, but unlike other gymnosperms, all gnetophytes possess vessel elements in their xylem.
Summary
Seed plants appeared about one million years ago, during the Carboniferous period. Two major innovations—seed and pollen—allowed seed plants to reproduce in the absence of water. The gametophytes of seed plants shrank, while the sporophytes became prominent structures and the diploid stage became the longest phase of the lifecycle. Gymnosperms became the dominant group during the Triassic. In these, pollen grains and seeds protect against desiccation. The seed, unlike a spore, is a diploid embryo surrounded by storage tissue and protective layers. It is equipped to delay germination until growth conditions are optimal.
Gymnosperms are heterosporous seed plants that produce naked seeds. They appeared in the Paleozoic period and were the dominant plant life during the Mesozoic. Modern-day gymnosperms belong to four phyla. The largest phylum, Coniferophyta, is represented by conifers, the predominant plants at high altitudes and latitudes. Cycads (phylum Cycadophyta) resemble palm trees and grow in tropical climates. Gingko Biloba is the only representative of the phylum Gingkophyta. The last phylum, Gnetophyta, is a diverse group of shrubs that produce vessel elements in their wood.
Review Questions
Review: Seed Plant Characteristics
Review: Gymnosperm Characteristics
2) Which of the following traits characterizes gymnosperms?
Review: Life Cycle Features
3) Megasporocytes will eventually produce which of the following?
Image Attributions
Figure 1. Image courtesy of Laurenprue216 [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]. Modified by D. Jennings
Figure 2. Image created and provided by D. Jennings
Figure 3. Image courtesy of http://leschampsdemi-nuit.weebly.com/blog/dis-comment-les-plantes-font-des-bbs [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]. Modified by D. Jennings
Figure 4. Image courtesy of CNX OpenStax [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)]
Figure 5. Image courtesy of JMK [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
Figure 6. Left Image courtesy of CameliaTWU [CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/)]. Right Image courtesy of Nicole Acosta on Flickr [CC BY 2.0 (https://creativecommons.org/licenses/by/2.0)]
Figure 7. Image courtesy of James Anderson [CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/)]