Here is a gymnosperm embryo, in this case an avocado seed, surrounded by gametophyte tissue. Read more about this in the Reproduction section:. Angiosperms are flowering plants and their bright colors make them favorites in gardens, in bouquets and for corsages.
Angiosperms make flowers and fruits, so you can thank them for most of the food we eat in addition to making the garden look nice. Angiosperm sporophytes can be trees, shrubs, grasses, or forbs. One example of an angiosperm sporophyte is this lily:. Alternation of generations arose in green algae, and a green alga was the ancestor of land plants.
Just goes to show how much you inherit from your family. Parents Home Homeschool College Resources. Study Guide. Previous Next. Alternation of Generations Plants are really weird. A general form of alternation of generations looks like this: A general form of alternation of generations looks like this: The forms that the sporophytes, spores, gametophytes and gametes take on vary depending on the type of plant.
From earliest in the evolutionary order to latest, they are: 1. Bryophytes 2. Ferns 3. Gymnosperms 4. Angiosperms Starting at the bottom of the totem pole, we have bryophytes, which include mosses, liverworts and hornworts. Check it out yourself, in this old botanical drawing, done in by Ernst Haeckel: All of the above plants are bryophytes, with both generations shown.
Here is a picture of one with the sporophyte generation just starting to grow on top: Image from here. The zygote develops into the diploid sporophyte, which extends from the gametophyte and produces haploid spores through meiosis. Once the spores germinate, they produce new gametophyte plants and the cycle continues. Tracheophytes are plants that contain vascular tissue; two of the major classes of tracheophytes are gymnosperms conifers and angiosperms flowering plants.
Tracheophytes, unlike bryophytes, have developed seeds that encase and protect their embryos. The dominant phase in the tracheophyte life cycle is the diploid sporophyte stage. The gametophytes are very small and cannot exist independent of the parent plant. The reproductive structures of the sporophyte cones in gymnosperms and flowers in angiosperms , produce two different kinds of haploid spores: microspores male and megaspores female. Just as the evolution of spores was the key to the invasion of the land surface by bryophytes, the invention of complex vascular tissues let tracheophytes complete the conquest of dry land.
There are about , species of vascular plants, grouped in nine divisions. Tracheophytes all have a well developed root-shoot system, with highly specialized roots, stems, and leaves, and specialized vascular tissue xylem and phloem that function like miniature tubes to conduct food, water, and nutrients throughout the plant.
Because ferns and fern allies posses true vascular tissues, they can grow to be much larger and thicker than the bryophytes. The ferns and fern allies non-seed tracheophytes mark two major evolutionary strides. In these and in all more advanced plants, the leafy green diploid sporophyte now becomes the dominant stage. The tiny gametophyte may be either autotropophic like the fern prothallus or heterotrophic like the gametophytes of some lycopsids , and is generally free living and independent of the parental sporophyte.
Unlike the vascular sporophytes, the gametophytes have no vascular tissue at all. These gametophytes are therefore very small, and develop best in moist areas, where they can absorb water directly from their surroundings. Like the bryophytes, ferns and fern allies are still restricted to moist habitats. Their flagellated sperm need a thin film of water to swim between the antheridium and the archegonium.
And when the baby sporophyte grows up from the gametophyte, it is exposed to desiccation drying up. This basic strategy of a free-swimming sperm and a non-motile egg is shared by plants, animals, and algae. It makes sense, because it means only one set of gametes has to make the perilous journey outside of the organism. The ferns and fern allies germinate from spores.
These plants are mostly homosporous - their spores are identical and you can't differentiate which will grow into male or female plants. They are also monoecious - both the archegonia and antheridia male and female reproductive structures are borne on the same plant. Contrast these primitive vascular plants with the more advanced seed plants, the gymnosperms and angiosperms, which germinate from seeds rather than from spores. Seed plants are all heterosporous. It is easy to differentiate the larger female megaspore from the smaller male microspore.
The sperm of seed plants have no flagella. They lack antheridia, and only a few still have an archegonia.
Unlike the more primitive ferns and fern allies, seed plants are mostly dioecious , having separate male and female plants. In many of these primitive plants, certain leaves are specialized for reproduction.
These modified leaves, or sporophylls , bear the sporangia at their bases. These sporophylls usually branch out from a shortened stem, forming a club shaped structure called a strobilus. The pine cone and the flower are elaborate variations on these primitive strobili. There are four divisions of non-seed tracheophytes, vascular plants that reproduce by means of spores , the Psilophyta, Lycophyta, Sphenophyta, and Pterophyta.
Before these non-seed tracheophytes evolved, the bryophytes were the dominant form of plant life. The evolutionary edge of having a more efficient conducting system, and a well-developed root-shoot system enabled them to outcompete bryophytes. There are only two living genera of whisk ferns, sole survivors of a large and widespread group of early land plants. In addition to the living Division Psilophyta, the psilopsids, there are two extinct divisions of primeval vascular plants.
The primitive whisk ferns resemble these extinct pioneers in many ways. They are the only living vascular plants that lack a root-shoot system, a characteristic they share with both extinct Divisions of ancestral vascular plants.
Some recent molecular evidence suggests that one, or even both, of the living genera of psilopsids may actually be more closely related to ferns, like a fern that has reverted to more primitive traits. If this is true, then Psilophyta will join the ranks of the numerous extinct Divisions of plants. Psilopsids are found in tropical and subtropical areas, and occurs throughout the southern US. I once found one growing on my back porch under the leaves of a spider plant.
Whisk ferns are a common weed in greenhouses all over the world. They are simple green upright stems, with dichotomous branching. They have no leaves, and no true roots. The outer tissues of the stem do all the photosynthesizing. A portion of the stem called a rhizome runs along the ground, or just below it.
A rhizome is a horizontal stem that spreads the plant around. Roots grow out the bottom of the rhizome, and a new plant can arise at the same point from the top. The green stem-like plant is the diploid sporophyte, the dominant stage in the life cycle.
In the small sporangia bright yellow that form along the upper stems, the spore mother cell forms haploid spores by meiosis. Their gametophytes are tiny little thread-like underground plants that lack chlorophyll, and live as heterotrophs in the soil, looking and acting much like a tiny fungi. It actually contains a symbiotic fungi, the same mycorrhizae that live in the rhizomes of the adult sporophyte.
Division Lycophyta - 1, sp. Their are only five living genera of lycopsids, but at one time from the distant Devonian, about mya, well into the Carboniferous, they were the dominant form of vegetation on the face of the Earth. Now they are reduced to a shadow of their glorious past, inconspicuous little plants in the forest understory. The tropical species are small epiphytes plants that grow on other plants. Their roots grow from special underground stems called rhizomes, as do most of these primitive tracheophytes.
In some species the sporophylls are mixed in with the scale-like leaves. Fertilization occurs when sperm swim to an egg inside an archegonium. The resulting zygote develops into an embryo that becomes a new sporophyte plant. Then the cycle repeats. What's a fiddlehead? Life Cycle of Seedless Vascular Plants Unlike nonvascular plants, all vascular plants—including seedless vascular plants—have a dominant sporophyte generation.
In the life cycle of a fern, the sporophyte generation is dominant.
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