Structure and Reproduction of Ferns and Others
The Form and Reproductive Patterns of Vascular Spore Plants
Aug 15, 2009
Plants may be informally grouped into three categories based on structure and mode of reproduction: nonvascular spore plants which are simple in structure (no vascular system) and reproduce by spores, vascular spore plants which are advanced in structure (possess a vascular system) but reproduce by spores, and vascular seed plants which are advanced in structure and reproduce by seeds.
The General Body Structure of Vascular Spore Plants
Vascular spore plants – ferns and their taxonomic relatives – are successful on land because of the complexity of their bodies. Unlike the simpler nonvascular spore plants (bryophytes), they possess true roots, stems, and leaves and a vascular system of tubes connecting these complex body organs.
Inside the stem of a vascular spore plant you will find two kinds of vascular tissue: xylem tubes carrying water up to the leaves from the roots and phloem tubes carrying food molecules down to the roots from the leaves.
Furthermore, vascular spore plants also possess a cuticle (a waxy outer covering that reduces dehydration and stomata (tiny pores), mainly on the leaves, through which gases and water are exchanged with the environment. Properly equipped for land life, vascular spore plants grow both larger than bryophytes and are found in drier areas than bryophytes can endure.
Specific Body Form of the Various Types of Vascular Spore Plants
Whisk Ferns have a green branching stem with tiny scale-like appendages and they produce their spores in yellow globular structures. They are unique among the vascular plants in that they lack both complete roots and leaves, although they do possess rhizomes (horizontal underground stems) like other vascular spore plants
Horsetails have ribbed, jointed green stems with whorls of thin wire-like leaves that soon become brown and no longer undergo photosynthesis. To some, this gives them the appearance of a horse’s tail while others compare them to the wire-handled brushes used to clean glassware in the laboratory.
There are two types of stems in horsetails. The stems displaying the leaves are known as vegetative stems. The spore cases of this group form in cone-like structures at the tip of bare vertical growths known as fertile stems. Some species may grow as tall as 2 meters (over 6 feet) or more. Their ancient ancestors grew as tall as modern trees and contributed significantly to the coal deposits we mine today.
Club Mosses are not true mosses and only superficially resemble them with small overlapping scale-like leaves. Their spore cases are arranged in club-like clumps at the ends of erect stems.
True Ferns grace woodlands and gardens with their large divided leaves called fronds. The fronds arise from a horizontal underground stem or rhizome. When they are just beginning to grow, young fern fronds are tightly coiled and look very much like the top part of a violin. For this reason, young fern fronds are known as fiddle-heads.
The Reproductive Patterns of Vascular Spore Plants
Like the life cycles of all other plants, those of vascular spore plants involve alternation of generations. Nevertheless, there is more than one way to do things in GreenWorld. The green, fuzzy moss (a nonvascular spore plant) growing on the side of a tree in a forest is the gametophyte stage and the sporophyte stage grows from it. On the other hand, the tree (a vascular seed plant) is the sporophyte stage and the gametophyte stage grows from it.
The fern, a vascular spore plant, growing beneath that moss-covered tree is similar in some aspects to both of the others yet totally different from either in other ways. Like the moss, the fern produces spores and the sporophyte grows out of the gametophyte yet like the tree, the body of the fern is the sporophyte stage. However, in ferns, the sporophyte stage and the gametophyte stage occur on two totally separate plant bodies, a strange reproductive twist not found in any other group in the plant world.
The familiar bushy fern is the sporophyte and it produces haploid (n = half set of genes) spores in tiny containers called sporangia.
When spores are ripe, they are released from the sporangia and may be carried by wind or water over long distances. If the spores land in a suitable nursery, they will germinate into haploid gametophytes. The gametophyte first grows a set of root-like rhizoids. Then it flattens out into a thin heart-shaped green structure called a prothallium.
Antheridia and archegonia, which produce the male and female gametes, are found on the underside of the prothallium. When the antheridia are mature, sperm are released. If the ground and prothallia are covered with a thin film of water, fern sperm swim to the archegonia and fertilize the eggs therein.
The diploid zygote (2n = full set of genes) produced by fertilization immediately begins to grow into a new sporophyte plant.
Equipped with vascular systems vascular spore plants – ferns and their taxonomic relatives – are able to range over a wider diversity of habitats than are the more primitive nonvascular spore plants (bryophytes)
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