Phyla Pterophyta and Lycophyta: Ferns and Their Relatives


Sarah Fleming

external image Ferns1.JPG

external image 150px-Fractal_fern1.png

Introduction

Pictures of the world during the Mesozoic Era show dinosaurs wandering through forests made up of ferns and other closely related plants. Fossil evidence reveals that plants similar to the modern fern appeared during the Devonian period more than 350 years ago, before the dawn of the dinosaurs. Ferns and their relatives (all seedless vascular plants) are known as pteridophytes. Plants known as lycophytes belong to the phylum Lycophyta, while ferns, whisk ferns, and horsetails belong to the phylum Pterophyta.

Diagnostic characteristics

Pteridophytes are seedless vascular plants. They primarily use spores, reproductive cells that develop into multicellular organisms without fusing with another cell, to reproduce. The spores are produced from structures called sporangia. As with most plants, pteridophytes cycle between gametophyte and sporophyte generations, but the sporophyte generation is dominant. (This alteration of generations is further explained in the section on reproduction.) Pteridophytes were the first plants in evolutionary history that used vascul
Fern Structure (LS)
Fern Structure (LS)
ar tissue to transport materials. This vascular system consists of tube-like cells that make up xylem tissue, which transports water, and phloem tissue, which transports nutrients and sugar throughout the plant.

Habitats

Most pteridophytes prefer damp environments where their flaggellated sperm (flaggellated = having tails) can easily swim to the eggs. Ferns, however, are the most versatile and widespread of the seedless vascular plants, and they are found in the tropics, temporal forests, and sometimes in deserts. Ferns can live up to 3,000 meters above sea level. Some species can inhabit almost any type of soil as long as they obtain enough moisture and are in good climatic conditions (JM).
Club moss
Club moss

Major types

All seedless plants belong to either the phylum Lycophyta or Pterophyta. Major examples of these plants include:
Lycophyta
  • Lycopodium: Although these plants are commonly referred to as "club mosses," they are not actually classified as mosses. Their sporophylls are clustered into characteristic oblong structures called strobili on the ends of their branches. They are native to forests in the northeastern US. [These are flowerless, vaslcular, terrestrial or epiphytic plants, with widely-branched stems and needle-like leaves that cover the branches and stems thickly. RS http://en.wikipedia.org/wiki/Lycopodium]

Pterophyta
  • Polypodium vulgare: This plant is known as the fern, the most common and widespread of all the seedless vascular plants. [Rhizome: creeping, branching, whitish waxy, rather thick, with phylopodia, scales lanceolate, base and margins light brown, sometimes with dark central stripe.
    . RS http://hardyfernlibrary.com/ferns/listSpecies.cfm?Auto=107]
  • Psilotum: These plants, commonly known as whisk ferns, have neither vascular tissue nor true leaves. Their sporangia are arranged in small separate clusters on the stems. Because they lack roots, they are anchored by a horizontal stem known as a rhizome, which are found on the surface instead of underground (LD).
  • Equisetum: Also called horsetails, these plants have hollow, jointed stems with small leaves and branches extending from the joints. Their stems have a rough texture because of the silica contained in their epidermis, or outer layer of cells.
    Closeup of a whisk fern
    Closeup of a whisk fern

The common horsetail
The common horsetail

I10-22a-ferns.jpg
Examples of different types of ferns (Whisk fern, club moss, tree fern, and horsetail) (SW)

Basic anatomy

Pteridophytes have stems with extensively branched vascular systems and leaves that may be large or small and are sometimes made up of many smaller leaflets. Fern leaves are known as fronds. A frond begins as a coiled mass near the bottom of the stem. [Fronds have two distinct parts: the stipe (leaf part) and the blade (leafy expanded portion on the frond). The size of fronds can vary between 1/16 of an inch in mosquito ferns to 12 feet in tree ferns. (BY)] The tip of the coil is called the fiddlehead, and as the plant grows the fiddlehead unfurls and the frond emerges. Ferns have structures that produce spores on the undersides of specialized leaves called sporophylls. These spore-producing structures are known as sporangia, and they are arranged in clusters called sori (singular: sorus). [There are two types of sporangia: eusporangia and leptosporangia. Eusporangia are thick walled and release thousands of spores. They open by splitting transversely. Leptosporangia are delicate and thin walled, no more than a few layers thick. The annulus is an area where these cell walls are thickened, and as the fern matures annulus cells dry out, causing the sporangium to split, releasing spores. (In leptosporangia, it is rare for more than 100 spores to be released) DG]. In addition, pteridophytes have horizontal stems called rhizomes which grow along the surface of the ground. Most pteridophytes have roots which extend from the rhizome into the ground.
Basic anatomy of a fern
Basic anatomy of a fern


Transport of materials

Ferns were the first plants to develop vascular systems that transport materials through the plant body. The vascular tissue is made up of phloem, which transports nutrients and sugar, and xylem, which moves water to different parts of the plant. The types of vascular systems vary among the pteridophytes. Ferns and most other members of the phylum Pterophyta have large leaves known as megaphylls attached to an extensively branched vascular system, while lycophytes have smaller leaves called microphylls and a vascular system consisting of single unbranched veins. The major veins go laterally out from the rachis, while the minor veins follow the pattern. In the leaves, the veins form loops and the space in between is called the areole (JL).


Diagram of Xylem and Phloem (KTD)
Diagram of Xylem and Phloem (KTD)




external image Fern_life_cycle2copy.jpg

Reproduction

Life Cycle

Pteridophytes have a complex life cycle that includes both haploid gametophyte and diploid sporophyte generations. The term "haploid" refers to having one set of chromosomes, while "diploid" means having two sets of chromosomes. The sporophytes are the fern plants that we recognize; gametophytes are tiny, fragile plants that we usually can't see. The sporophytes produce spores, which are haploid reproductive cells that form into new organisms without having to fuse with another cell. Spores, the "seeds" of ferns, fall from the parent fern onto the ground, and hopefully they will find sufficient moisture and sunlight. The single celled organism then begins to mature by cell division. (AK)
Spores contain oil droplets and sometimes chlorophyll in addition to their nucleus. Ferns drop millions, often times billions of spores during their lifetime but very few ever land in a spot suitable for growth.(JJF) These haploid spores grow into gametophytes, which are so small that they may even grow below the surface of the soil. These mature gametophytes produce gametes--eggs and sperm--which fuse together to form a diploid zygote, or fertilized egg, which grows into a sporophyte. This sporophyte produces more spores, and the cycle continues.

Sporophytes produce spores in structures called sporangia, which are arranged in clusters on special leaves called sporophylls. Gametophytes produce their eggs and sperm within structures called gametangia. Female gametangia that produce eggs are called the archegonia (singular: archegonium), while male gametangia that produce sperm are antheridia (singular: antheridium).

Sporophytes may be either homosporus or heterosporus. Homosporus sporophytes produce one type of spore which matures into a gametophyte that contains both male and female gametangia. In contrast, heterosporus sporophytes produce two types of spores; megaspores develop into female gametophytes, and microspores develop into male gametophytes. Almost all pteridophytes are homosporus (only water ferns are heterosporus).

Environmental adaptations

Pteridophytes were the first plants to evolve vascular tissue and true roots. Prior to this adaptation, plants had recently begun living on land; having vascular tissue made it much easier for plants to survive on land because they could easily supply water to the different parts of their plant bodies. Since then, pteridophytes have adapted to their environments in different ways. For example, horsetails evolved air canals in their stems which allow easy flow of oxygen into the plant roots. This is helpful because horsetails frequently grow in swamps that have low concentrations of oxygen in the soil. Ferns also developed megaphylls, large leaves with branched vascular systems, in order to have greater photosynthetic productivity and produce more sugar to support the plant.

The horsetails have air canals which take in oxygen and carries it to the roots.(JS)
The horsetails have air canals which take in oxygen and carries it to the roots.(JS)



Review Questions:
1) What is the common characteristic of lycophodiom? (APS)
2) Explain the roles of xylem and phloem in a fern. (KNS)
3)Explain why ferns need to live in damp environments.(Walker K.)
4) Compare and contrast the three major types of ferns in the phylum Pterophyta. (JAF)
5) How have ferns adapted to living on land? (AZ)
6) What are the major types of ferns?
7) Compare Lycophyta and bryophyte. How are they morphologically different? (DRM)

Sources
http://www.nilesbio.com/subcat293.html (KTD)
http://amerfernsoc.org/ [BY]
http://www.cliffsnotes.com/WileyCDA/CliffsReviewTopic/Phylum-Pterophyta-Ferns.topicArticleId-23791,articleId-23768.html(DG)
http://tolweb.org/Filicopsida (Jesse Landy)
http://amerfernsoc.org/ (Ali Kirsch)
http://universe-review.ca/R10-34-anatomy2.htm (Sara Waugh)
http://nefern.info/jpgs/notaxa/frndprts.BMP(Leo Schwartz)
http://www.inetours.com/images/Snglimgs/SABG/Horsetails.jpg (Jake Schwartz)
http://amerfernsoc.org/ (JJF)
http://www.vumba-nature.com/about_ferns.htm#Habitats (Jackson Murphy)
http://www.ucmp.berkeley.edu/plants/pterophyta/psilotales.html (Liz Daley)