Chapter 21b-The Early Tracheophytes
Early Tracheophytes are the next level (after Bryophytes) of evolution to dry land.
Tracheophyte refers to plants with true vascular systems comprised of phloem and xylem.
The vascular system allowed plants to move water from the soil to aerial portions of the plant.
Rhynia, an extinct early tracheophyte from about 400 million years ago had a simple vascular
system. Plants rarely exceeded 20 cm tall.
The fossil record of these early Tracheophytes has enabled botanists to reconstruct the likely evolutionary path from
Bryophytes to Tracheophytes.
This process included the following innovations:
1. a dichotomously branching sporophyte with multiple terminal sporangia
2. a free-living, nutritionally independent sporophyte that is prominent in the life cycle
3. a reduced gametophyte
4. lignified vascular tissue (xylem)
5. development of a main stem with side branches
Relationships among Early Tracheophytes
Tracheophytes that release spores rather than seeds.
These two lineages are characterized
by several morphological featues,
but also a major molecular feature.
Lycophytes
These plants show a gradation in complexity
from the fairly simple Zosterophyllophyta to
the Lepidodendrids.
Zosterophyllophyta (extinct) were simple,
dichotomously branched plants that
lacked leaves and roots, much like Rhynia.
Pteridophytes
The Lycophytes
The next innovation in the Lycophytes was the microphyll.
Microphyll does not refer to size.
Instead, the name refers to the presence of only a single
vascular bundle in the leaf.
We will focus our discussion on the 3 extant groups of
Lycophytes:
-Lycopodium group
-Selaginella
-Isoetes.
The final group, the Lepidodendrids, are extinct.
These tree-like plants were the major plant type of the extensive coal-age forests.
The Lycopodium group.
The first land plants with microphylls.
Lycopodium and related genera are mostly trailing plants, with short upright
branches that resemble thick mosses or pine seedlings.
Most commonly called club moss.
How does Lycopodium differ from a moss?
1. Major vegetative plant is the sporophyte.
2.The leaves of Lycopodium are microphylls.
3. The vascular tissue includes true phloem and xylem, with
lignin present in the xylem.
4. Roots with vascular tissue are present
The Lycopodium group.
The cross-section of a microphyll reveals its simple construction.
The Lycophyte life cycle starts with the mature sporophyte, the portion of the life cycle we generally see.
The Lycopodium life cycle starts with the mature sporophyte, the portion of the life cycle we generally see.
The sporangia consist of modified leaves called sporophylls arranged in a cone or strobilus.
Numerous lateral sporangia produce haploid spores by meiosis.
Lycopodium is homosporous,
The gametophytes are small, longliving, and often subterranean.
They require an endosymbiotic
fungus to survive.
Each gametophyte produces egg
cells in archegonia and sperm in
antheridia.
The flagellate sperm swim to the
archegonium and fertilization
occurs.
The embryo develops and
ultimately grows into a mature
sporophyte.
Selaginella has another major innovation -- heterospory
Selaginella is the only living genus of this lineage, but it very
diverse, with about 700 species.
Most species produce a prostrate system of dichotomously branched stems covered with four rows of microphylls.
Selaginella life cycle.
The sporophyte produces stobuli somewhat
like those of Lycopodium.
However, two types of sporangia are present
in each strobulus.
The microsporangia produce microspores.
The megasporangia produce megaspores.
Fertilization occurs in the archegonium of the
megagametophyte.
The embryo develops, eventually becoming
the free-living sporophyte.
Isoetes
Isoetes is the last extant relative of the Lepidodendrids
There are about 125 species of Isoetes, most of which grow submerged in water.
They have the basic appearance of a grass or sedge, but the leaves are actually
microphylls (single vascular bundle)
Isoetes is linked to the extinct lepidodendrid group by several morphological features,
The (ferns and relatives)
This lineage is most closely related to the seed plants and
shares a number of features with that group.
Megaphylls
We will briefly examine the features of each of the lineages of Monilophytes
We will briefly examine the features of each of the lineages of Monilophytes
The Whisk Ferns
The plant consists of dichotomously
branched rhizomes and upright
dichotomously branched stems.
http://www.plantscience4u.com/2014/04/psilotum-sporophyte-plantbody.html#.Vi5cIrerTcs
The Ophioglossean ferns
https://www2.mcdaniel.edu/Biology/botf99/earlyplants/page3psi.html
The Ophioglossean ferns are closely related to the whisk ferns.
However, they have an appearance much more like true ferns.
A major feature is the unusual leaves divided into two segements:
The Horsetails
The Horsetails are very common fern-like plants
http://www.easttennesseewildflowers.com/galle
ry/index.php/ferns/Copy_of_Horsetails
In the Carboniferous period (about 300
million years ago)
Sporangia are borne at the
tips of many stems.
The cross-section of an Equisetum stem
shows a large central cavity surrounded
by several vascular bundles.
True Ferns are the major group of Pteridophytes, with over
12,000 described species.
Most true ferns are readily recognized by their complex fronds.
The real distinguishing feature of True Ferns
The leptosporangium has a distinctive stalk and a
covering referred to as an annulus.
Specific terminology is used for the vegetative structures of ferns.
Ferns also have an extensive underground portion, the
rhizome, which serves to vegetatively propogate the plant.
Fern life cycle
Most ferns are homosporous,
producing spores of a single type that
grow into a bisexual gametophyte.
Study outline for Chapter 21b-Early Tracheophytes (Lycophytes and Pteridophytes)
Define tracheophyte.
When did the tracheophytes first appear on dry land?
Describe Rhynia.
Name the evolutionary innovations that occurred in the evolutionary path from bryophytes to lycophytes.
Name the two major extant lineages of the early tracheophytes.
Name the five groups of Lycophytes.
Which groups are extinct?
Which groups are extant?
Lycophytes
Define microphyll.
Define strobilus.
How does Lycopodium differ from a moss?
Lycopodium is __________ (only one type of spore) and Selaginella is _____________ (two different ypes of spores).
Define the following terms and use these terms to label the following life cycles of Selaginella:
microsporangia
megasporangia
microspores
megaspores
strobilus
female gametophyte
(megagametophyte)
male gametophyte
(microgametophyte)
sporophyte
Lycophytes- Label the following: Zosterophyllaphyta; Lycopodium group; Selaginella group; Isoetes; and
Lepidodendrids. List 3 characteristics for each picture.
Pteridophytes
Define megaphyll.
Name the four groups of Pteridophytes that we discussed in class.
Define enation.
Define leptosporangium.
Define the following terms and label the figure.
frond
rachis
pinna
pinnule
fiddlehead
rhizome
Matching-Fern Life Cycle
D.
C.
___ sori
E.
B.
___ archegonium
F
___ gametophyte
___ leptosporangium
___ zygote
B.
___ antheridium
E.
A.
___ sporophyte
G.
G.
Pteridophytes- Label the following: Psilotum (whisk ferns); Ophioglossum ferns; Equisetum (horsetails); and True ferns.
List 3 characteristics for each picture.