Psilotum nudum , finally bagged at the Oxford Botanic Gardens. They both have a horizontal stem called a rhizome, from which upright stems grow. These upright stems branch into two equal sub-branches again and again and again in both species. Both plants carry reproductive structures called sporangia below on these branches. They both have relatively simple plumbing inside their stems, and they lack true roots and leaves, although both plants appear to be associated with symbiotic fungi.
Interpreting fossils is not at all easy, and this is not a gripe at palaeontologists. However, this tendency to see some organisms as primitive relics, and others generally the ones we eat, or the ones we are as advanced, is as tempting as it is wrong-headed, and it needs to be resisted. All land-plants are descendents of the first algae that started made a living on land during the Devonian, this is not a singular characteristic of Psilotum , or whatever poor organism has been tarred with the primitive brush this week.
This phylogeny differs slightly from the Vegetable Empire I posted last week in the placement of the horsetails. Structure of synangium Each synangium is trilobed , stalked structure borne at the apex of short lateral branch.
A bilobed appendage is present at the base of each synangium that curve and surround the stalk of synangium Nature of synangium There are different views 1. The trilobed synangium is formed by fusion of two or more sporangia 2. One sporangium with 3 chambers trilocular sporangium 3. Synanium is cauline developed at the apex of stem in nature and it is actually modified trilocular sporangium present on lateral branches 4. This concept was suggested by Bierhorst according to him each unit in synangium represents a condensed fertile axis.
The synangium in Psilotum can be considered as homologus to fertile portion of Rhynia where one arm is fertile and other is sterile. The condensation of fertile arm is modified into synangium. Structure of synangium It coinsist if three chambers or locules. Wall of synangium is 3 — 4 layers 2. Thick outer wall forms the epidermis 3. Inner wall separates the three locules 4. Each locule is filled up with large number of spore. And these are homosporous in nature 5.
Synangium splits up from 3 lines along the epidermis and dehiscence occurs. Sexual reproduction gametophyte The gametophyte lives underground as a saprophyte, sometimes in a mycorrhizal association.
When the gametophyte is mature, it produces both egg and sperm cells. The gametophyte of Psilotum is unusual in that it branches dichotomously, lives underground and possesses vascular tissue. The gametophyte of Psilotum is called as Prothallus. It contains parenchyma cells and there is strand of tracheids extending back from the apex. Total views 21, On Slideshare 0. From embeds 0. Number of embeds Downloads Shares 0. Comments 0. Likes You just clipped your first slide!
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Visibility Others can see my Clipboard. Cancel Save. Exclusive 60 day trial to the world's largest digital library. Activate your free 60 day trial. Upright stems were able to overtop flat thalli and became dominant organisms. The production of photosynthetic branches further increased the surface area available for photosynthesis.
Branching was initially Dichotomous for stems and roots. Lateral branching developed later in Evolution. The ascent of aerial stems and their branches required the development of superior support tissues.
This was achieved with the Psilophyta. Xylem Tracheary Elements have thick walls and provide some structural support. Fibers are found in the Vascular Tissues or in close association with them. Sclerenchyma and Parenchyma are the two principal Ground Tissues. The final step in the evolution of our theoretical plant is the production of Leaves. These are highly specialized for Photosynthesis in many ways.
They usually have a wide thin Blade Lamina and a thicker Midrib. They may have only one Vascular Bundle Vein or a network of interconnected Veins. They are attached to the stem by a nonphotosynthetic petiole. The vein had a central location and was surrounded by Photosynthetic Parenchyma. The major functions of the stem are translocation of water and photosynthate and structural support for the leaves.
Species in the Lycophyta illustrate simple plants that have Microphylls. Reconstruction of Rhynia one of the first fossil land plants. Cross section of a Rhynia Stem: Note the concentric circles of primary tissues.
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