Kyffin, Southern Victoria Land. The dominant stage in the life cycle of bryophytes is the sexually reproducing gametophyte plant stage. But because of the extreme environmental conditions under which these plants exist, most reproduction occurs asexually. The plants can reproduce from a deciduous shoot apex or other specialised asexual reproductive structures, called gemmae. Sexual reproductive structures may be present on the gametophyte, but male and female organs are rarely found on the same plant.
In many cases the sporophyte stage, produced after sexual reproduction, is completely absent amongst Antarctic bryophytes. Many of the mosses in Antarctica have tightly packed stems and shoots to minimise water loss. Some mosses have orange carotenoid pigments, which may help prevent photosystem damage during the growing season. Snow cover protects the plants from wind, windblown ice and sand particles and temperature extremes. 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. The sperm swim down the strobilus to the archegonia, and the zygote that forms is retained in the cone, which ripens and falls to the ground. The gametophytes are independent and free-living, They are curious creatures that look and act nothing like their sporophyte parents.
They can be either heterotrophic or autotrophic, and usually have a symbiotic fungi associated with them. Many of the lycopsids are heterosporous. Selaginella is a good example of a heterosporous plant. Division Sphenophyta - 15 sp. In waste places, disturbed areas like trails and railroad beds, and in odd corners of fields and forests you might find another small plant quietly dreaming of its former splendor, the horsetail.
Horsetails appeared in the late Devonian, and were among the dominant forest trees for hundreds of millions of years. Only one genus of Sphenophyta still exists, the genus Equisetum , and it may be the oldest living genus of plants on earth. Horsetails towered among the Carboniferous forests, reaching heights of feet.
Much of the coal deposits we exploit for fuel today were formed from horsetails and other trees during the Carboniferous, toward the end of the Paleozoic. Horsetails have true roots, stems, and leaves, though the leaves are little more than flattened stems.
Their hollow, ribbed stems are jointed, kind of like a stalk of bamboo, and a whorl of leaves arises at each joint. The plants are spread vegetatively by rhizomes.
The stems feel very rough, because the epidermal tissues are impregnated with tiny grains of silica sand. This probably helps protect the plant against herbivores. The green plant we see is the diploid sporophyte generation. The stalks can be highly branched vegetative stalks, which actually look like horse tails, or straight unbranched reproductive stalks, which are tipped with a large strobilus containing the sporangia.
The homosporous spores develop into a teeny-tiny green gametophyte, just a few mm long, that looks like the gametophyte of a fern. The gametophyte is haploid, free-living, and autotrophic. Ferns probably evolved from the psilopsids, sometime in the Devonian, relatively early on in land plant evolution. They are very abundant and diverse, ranging in size from a single centimeter to trees 24 meters tall with 5 meter fronds.
Ferns are relatively advanced plants, with true roots, stems and leaves. The blade of the fern is called a frond, and the little individual leaflets are called pinnae. Ferns have true leaves, what botanists call macrophylls. While the leaves of more primitive plants, which are called microphylls, are simply extensions of the epidermis of the stem, the leaves of ferns and higher plants were formed as a web of tissue stretched between small terminal branches.
The leaves of higher plants, as well as the modified leaves that make up the pine cone and the flower. The life cycle of the fern is typical of other non-seed vascular plants. The leafy green plant is the sporophyte. Fertile fronds develops clusters of small sporangia on the underside of the frond. These clusters of sporangia are called sori sing. Sori are often protected by a tiny umbrella-like cap called an indusium -ia.
Ferns are mostly homosporous, though some are heterosporous. The heterosporous state is a more advanced condition, that seems to have evolved independently in several groups of plants. The haploid spores are formed by meiosis inside the sporangium. They are ejected in a miniature explosion caused by the unequal drying of the alternate thick and thin-walled cells that line the outer surface.
The top pulls slowly back until it reaches a critical point and then snaps forward at an incredible speed. At that size scale, the expulsion of fern spores is one of the most explosive events in nature. The spores germinate into tiny gametophytes. Its small size lets it rely entirely on diffusion.
Its tiny rhizoids are associated with mycorrhizal fungi. The little prothallus is green, and photosynthetic, and bears either antheridia and archegonia, or sometimes both together, on its upper surface lab slides have both on same prothallus.
The archegonia are always found at the arch of the heart, and the antheridia are tucked away among the tiny rhizoids at the other end. The sperm swims to the egg to fuse into a diploid zygote. The new sporophyte grows directly out of the top of the gametophyte. When it first begins to uncurl, the frond looks like the scrolled neck of a violin or fiddle, and this stage of development is called a fiddlehead.
Examine the living lycopsids on display. Why are they called club mosses? Notice that quillworts and Selaginella are very different in appearance from the club mosses. Examine slides of Selaginella's strobilus. Identify megaspores and microspores. Examine the living horsetails on display.
Moss plants are usually very small. They have leaf-like structures or phyllids that are usually only a single cell layer thick. The phyllids spiral around a stem-like structure called the caulid. Mosses reproduce in an unusual way. There is a first generation moss, the gametophyte.
The gametophyte produces a sperm and an egg. They come together and grow into the next generation, the sporophyte.
The sporophyte usually grows on a stalk or seta. The sporophyte has no chlorophyl and lives on a gametophyte. The sporophyte dries out and releases spores that grow into a new generation of gametophytes.
0コメント