Nonvascular plants and seedless vascular plants were presented in the last two tutorials. The next two tutorials will present the two categories of seed plants, also known as the gymnosperms and angiosperms. This tutorial will focus on the evolution of seed plants and, in particular, nonflowering seed plants.
By the end of this tutorial you should have a basic working understanding of:. By now you should be familiar with the alternation of generations present in all plants, and that the gametophyte 1n and the sporophyte 2n exist in multicellular forms.
The gametophyte is the dominant form in nonvascular plants. In fact, the sporophyte is smaller and is nourished by the gametophyte as it develops. In seedless vascular plants and seed plants the sporophyte is the dominant form. This first variation in the alternation of generations is clearly depicted in this figure on the right. The second variation in the alternation of generations came about with the appearance of vascular tissue. The seedless vascular plants are most commonly represented by "Seed plants all have flagellated sperm" ferns.
For example, the ferns that Seed plants all have flagellated sperm observed growing in the forest and the fern fronds leaves that are a part of floral arrangements are sporophytes 2n. The spores that are released from the undersides of fern fronds grow into gametophytes 1nwhich exist independently from the sporophyte. Note, the sporophyte is much larger than the gametophyte depicted above.
Seed plants represent the third variation of alternation of generations. Like the ferns, the sporophyte is the prominent generation. Unlike the ferns, however, seed plants have gametophytes that are surrounded by sporophytic tissue. This sporophytic tissue nourishes the gametophyte; therefore, the gametophyte does not live independently from the sporophyte and is even more reduced in size.
In Figure 1 above, the line to the gametophyte of the seed plant points to a structure composed of only eight cells. In addition to having reduced gametophytes, seed plants use a different method to disperse their offspring than seedless vascular plants. A seedless vascular plant e. If conditions are suitable, the spore grows into a mature gametophytic generation. If conditions are harsh, the spore will persist without germinating and will lie dormant until favorable conditions present themselves.
Many seeds are also known for lying dormant until conditions are optimal for germination and growth. For example, some pine seeds actually require the heat of a fire to trigger germination.
This is adaptive because just after a fire the seedling can grow quickly without competition from taller trees. Therefore, spores and seeds are similar because they both are resistant to harsh conditions. However, spores and seeds differ in their structure and composition. Spores are unicellular, haploid, and contain little storage tissue. They are also very small and relatively simple. Seeds are multicellular, larger, and can contain a large amount of storage material.
Another major difference between the spores of seedless plants and seeds is that the haploid spores "Seed plants all have flagellated sperm" seedless plants are released by the parent and they develop independently, whereas seeds develop within the parental sporophytic tissue.
Seed plants have female spores megaspores and male spores microspores. The male microspore of a seed plant produces sperm within pollenwhich are transported to the female megaspore.
Seed plants have a haploid megaspore Seed plants all have flagellated sperm is contained within a fleshy solid mass contained within an ovule. A seed is a fertilized mature ovule. There are also tissues in the ovule integuments that become the seed coat. The image on the right shows the location of these structures in a simple flower. Therefore, while seedless vascular Seed plants all have flagellated sperm disperse their offspring via haploid spores, seed plants disperse their offspring via diploid seeds.
The sperm from seedless plants have flagella that propel them through water to reach the egg cell. This mechanism works well for plants in moist environments, however, these plants have a difficult time reproducing in drier environments. The pollen grain is an important adaptation to dry environments. Pollen is a tough structure that contains the precursor to sperm cells. The tough outer coat of the pollen grain is able to survive very harsh conditions, therefore, it can protect the cells for years.
When the pollen grain finally lands on the female structure of a plant, it germinates and sperm cells travel to the egg cell through a pollen tube.
Thus, the male gamete is protected rather than open to the environment, as in seedless plants. This adaptation allows seed plants to live in such dry and harsh conditions as deserts. Note the differences in the life cycles shown below. The nonflowering seed plants sometimes called gymnosperms probably arose from a fern relative, appropriately named a progymnosperm, sometime between million years ago. The nonflowering seed plants had largely taken over as the predominant land plants about million years ago.
They were the primary vegetation for dinosaurs. Currently, the nonflowering seed plants are comprised of four major groups: Nonflowering seed plants are able to grow larger than seedless vascular plants because of their woody stems. The vascular tissue in most members is highly lignified, which adds strength to their cell walls. The strengthened wood allows them to achieve great heights. The members of this group are very diverse, however, they share one distinctive feature; they all have "naked" seeds.
This means that they lack ovaries. As you will learn, nonflowering seed plants and flowering seed plants have ovules in which their seeds develop.
However, flowering seed plant ovules are contained within an ovary, whereas nonflowering seed plant ovules are not. Cycads are slow-growing and long-lived perennials. They live and reproduce year after year. They are considered woody, even though their wood does not look like that of a pine or oak tree. The leaves of cycads are large and appear feather-like, much like those of palm leaves. These leaves are arranged spirally at the top of the stem.
Cycads are dioecious plants; that is, their male and female reproductive structures reside on separate plants. One feature retained in cycads is motile sperm. Remember, nonvascular plants and seedless vascular plants have sperm equipped with flagella for motility.
However, like all nonflowering seed plants, the cycads have "naked" seeds which are visible in the image at the right i. There is only one extant species of ginkgo, appropriately named Ginkgo biloba. The leaves are uniquely fan-shaped, with a split in the middle that makes them appear to have two lobes image on right. Like cycads, ginkgos are dioecious and have motile sperm.
Due to their broad leaves, ginkgos are often mistaken for a flowering seed plant. However, the of veins dichotomous venation in the leaves is unlike any found in flowering seed plants. Look closely and you will see that each vein splits in two as it passes across the leaf. The most common place in America to find a ginkgo tree is along the sidewalk. They are frequently used to add color and shade to urban settings. The ginkgo tree is particularly resistant to disease, insects, and air pollution.
In addition, the leaves turn a beautiful yellow in autumn, just before they fall. There are three genera of gnetophytes: Weltwitschia, Ephedra, and Gnetum examples of each are shown in the images below. These are probably the least familiar gymnosperms, however, they serve an important role in understanding the evolution of angiosperms.
Although the precise relationships are not known, it is generally accepted that the gnetophytes are more closely related to angiosperms than any other nonflowering plants because they are the only gymnosperms to undergo a process known as double fertilization.
In double fertilization, two sperm cells enter the ovule; one fertilizes the egg and the other fertilizes another cell within the ovary. Pine trees, firs, spruces, larches, yews, junipers, cedars, cypresses, and redwoods are all conifers. Most of these are evergreens, however, there are a few deciduous trees that drop their leaves each fall conifers e. The name conifer comes from the Latin word meaning cone bearing.
Conifers can be either monoecious or dioecious. That is, their male and female reproductive structures reside on the same or different plants, respectively, but unlike nonflowering seed plants, their sperm are not flagellated.
They are delivered directly via the pollen tube. Conifers date back to the Mesozoic period. Unlike the other tropical nonflowering seed plants, most conifers are found in the forested parts of the Northern Hemisphere. They are by far the most economically and ecologically important members of the group. You probably are familiar with the 2X4's used in construction. These boards, as well as many others, are made from pine trees. The wood of pine trees is softer than that of flowering seed plant trees, therefore, it is easier to hammer nails into this wood.
We will discuss the life cycle of nonflowering seed plants, using pines as an example shown here on the right. The male cones produce haploid pollen grains by meiosis. The pollen grains are immature microgametophytes male gametophytes.
The female cones have scales that each contain two ovules. -unlike most seed plants, cycads have flagellated sperm -all angiosperms are classified in a single phylum, Anthophyta, from the greek anthos for flower. Like all seed plants, gymnosperms are heterosporous.
They have flagellated sperm, but the male gametophyte grows a pollen tube, a long filament through. The only extant seed plants that have flagellated sperm are Ginkgo and Cycadales. dynamic microtubule arrays, occurred in all microtubules at all stages of.
- Nonvascular plants and seedless vascular plants were presented in the last two tutorials.
- Cycads, like all seed-producing plants, have a dominant diploid sporophyte phase in...
- Note - Many of the photographs below are linked to larger photographs.
- Cyanohydrins to Departments of philosophy:
- Gymnosperms and Angiosperms
- The gymnosperms , also known as Acrogymnospermae ,  are a group of seed-producing plants that includes conifers...
Why do I want to get married?The sperm from seedless plants have flagella that propel them through However, like all nonflowering seed plants, the cycads have "naked". With such evolutionary advantages, seed plants have become the most The sperm of modern gymnosperms and all angiosperms lack flagella, but in cycads, ..
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The two wing-like structures on the pollen grain aid in enabling the pollen to be carried by the wind. All angiosperms produce flowers , reproductive structures that are formed from four whorls of modified leaves. Just as the evolution of the amniotic egg enabled reptiles to become the first truly terrestrial vertebrates, to break that final link with their aquatic heritage, so did the evolution of the seed allow plants to escape the limitation of growing in very moist environments. Moth-pollinated flowers are white and bloom at night.
During pollination, pollen grains are physically transferred between plants from the pollen cone to the ovule.
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- The sperm from seedless plants have flagella that propel them through However, like all nonflowering seed plants, the cycads have "naked".
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