BSC 314 Lecture Notes - Lecture 57: Phloem, Cellulose, Lignin
27 Jun 2018
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The First Vascular Plants
Scientists widely believe that the first land plants evolved during the late Ordovician to
early Silurian, although fossils from this time are incomplete and difficult to interpret. By
the end of the Silurian a land flora had evolved that throughout the next 50 million years
of the Devonian (410 to 360 mya [million years ago]) continued to change, adapt to life
on land exposed to air, and spread across a landscape previously devoid of vegetation.
By the end of the period, small plants had given way to tree size, well diversified ‐
vascular plants.
The lycophytes separated from the rest of the early land plants, evolved adequate
reproductive, supportive, and transport systems, and, by the Carboniferous, were large
swamp forest trees. Three groups of now extinct vascular plants were prevalent in Devonian
times: the rhyniophytes, zosterophylls, and trimerophytes. The oldest known vascular plant
is Cooksonia, a 6.5-centimeter-tall plant with dichotomously branched (forking into two)
leafless stems with sporangia at their tips. Only bits and pieces have so far been recovered
and no rhizomes or below ground parts have been found. It is a rhyniophyte and it and its
relatives were extinct by mid-Devonian time.
The trimerophytes are the basal group of the lineage that gave rise to the flowering plants
and are also the ancestors of the horsetails, ferns, and progymnosperms. Superficially, the
trimerophytes resembled their rhyniophyte ancestors and the zosterophylls, but differed
from them in bearing terminal sporangia on branch tips. At a meter or slightly less in height,
these were the largest of the three groups of early land plants.
The Ferns and Their Allies
The seedless vascular plants are intermediate in their structural and reproductive
adaptations between the more “primitive” bryophytes and the “advanced” seed plants.
They often are called the amphibians of the plant world for although their sporophytes
are well adapted to life on dry land, their gametophytes require a moist habitat to grow ‐
vegetatively and to reproduce sexually. The group includes the ferns and the “fern
allies,” the latter a collection of plants whose relatives were the dominant plants in
Paleozoic landscapes for 60 million or more years. Today, the members are a few
remnant species reduced to an exceedingly minor role in the flora. The fossilized
remains of the early vascular plants exhibit a variety of ways of coping with the
terrestrial environment—only some of which were successful.
The relationships among and within the groups remain unclear for three major reasons:
1.) fossils form an incomplete record, but are the basis for many of the conclusions, 2.)
data from molecular RNA sequencing of living species are incomplete, and 3.) opinions
vary among botanists concerning how the available morphological and molecular data
fit together in proposed phylogenies. The prevalent hypothesis concerning the
placement of the groups on the large tree of life distinguishes two main lineages of
vascular plants that diverge very early in the development of a land flora. One line
includes the most primitive taxa and the lycophytes, the other the ferns, horsetails, and
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seed plants. The “which goes where” disagreements primarily concern extinct groups, ‐ ‐
but some extant ferns are problematical in their relationships also.
On one level the possession of vascular tissue—xylem and phloem—separates the
ferns and their allies from the bryophytes and the lack of seeds from the gymnosperms
and angiosperms. Other character istics they share in common are more varied and ‐
include:
An alternation of a haploid gametophyte phase with a diploid sporophyte, a
sporic meiosis. The gametophyte and sporophyte are nutritionally independent of
one another.
The sporophyte is the dominant, often branched, long lived phase (the leafy fern ‐
plant is the sporophyte, for example). Many are perennial and vegetative
(asexual) reproduction is common.
The gametophyte is smaller and either photosynthetic or saprophytic. Because
the flagellated sperm need water in which to swim to the egg (like bryophyte
sperm), the gametophyte is restricted in distribution by habitat. The plants are
oogamous.
Eggs are produced in archegonia, one per archegonium; sperm in antheridia,
many per antheridium. The gametangia are multicellular with a protective coat of
sterile cells and borne in nearby areas on one gametophyte or on separate ones.
Haploid spores are produced by meiosis in sporangia and in some of the
seedless vascular plants are of two different kinds: microspores and
megaspores. The sporangia develop on specialized leaves called sporophylls.
Some members of the group have strobili (singular strobilus), cones in which the
sporophylls are clustered.
A cell plate separates the new daughter nuclei during cell division.
Cell walls are cutinized (unlike the bryophytes in which a cuticle is lacking).
Xylem and phloem are well developed and transport water, minerals, and
carbohydrates throughout the large sporophytes.
Cellulose is the common wall material; a secondary wall of lignin strengthens the
cells of most of the group.
The living members of seedless vascular plants belong to four different phyla whose
general characteristics are summarized in Table . Each group is described separately in
the following pages.
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Document Summary
Scientists widely believe that the first land plants evolved during the late ordovician to early silurian, although fossils from this time are incomplete and difficult to interpret. By the end of the period, small plants had given way to tree size, well diversified vascular plants. The lycophytes separated from the rest of the early land plants, evolved adequate reproductive, supportive, and transport systems, and, by the carboniferous, were large swamp forest trees. Three groups of now extinct vascular plants were prevalent in devonian times: the rhyniophytes, zosterophylls, and trimerophytes. The oldest known vascular plant is cooksonia, a 6. 5-centimeter-tall plant with dichotomously branched (forking into two) leafless stems with sporangia at their tips. Only bits and pieces have so far been recovered and no rhizomes or below ground parts have been found. It is a rhyniophyte and it and its relatives were extinct by mid-devonian time.
